# Python 3.7 有什么新变化

编者: | Elvis Pranskevichus <elvis@magic.io>

本文解释了 Python 3.7 相比 3.6 的新增特性。Python 3.7 于 2018 年 6 月 27 日发布。完整的详情可参阅 更新日志。

## 摘要 - 发布重点

新的语法特性：

- PEP 563, 类型注解延迟评估.

向后不兼容的语法更改：

- `async` 和 `await` 现在是保留的关键字。

新的库模块：

- `contextvars`: PEP 567 -- 上下文变量
- `dataclasses`: PEP 557 -- 数据类
- importlib.resources

新的内置特性：

- PEP 553, 新的 `breakpoint()` 函数。

对 Python 数据模型的改进：

- PEP 562, 自定义可访问的模块属性。
- PEP 560, typing模块和泛型类型的核心支持。
- dict 对象会保持插入时的顺序这个特性 正式宣布 成为 Python 语言官方规范的一部分。

标准库中的重大改进：

- `asyncio` 模块添加了新的功能，重大改进请参阅 可用性与性能提升。
- `time` 模块现在提供 纳秒级精度函数 的支持。

CPython 实现的改进：

- 避免使用 ASCII 作为默认的文本编码：
  - PEP 538，传统 C 区域强制转换
  - PEP 540，强制 UTF-8 运行时模式
- PEP 552，确定性的 .pycs
- 新的开发运行时模式
- PEP 565，改进的 `DeprecationWarning` 处理

C API 的改进：

- PEP 539，用于线程本地存储的新 C API

文档的改进：

- PEP 545, Python 文档翻译
- 新的文档翻译：Japanese，French 和 Korean。

此版本在诸多方面有显著的性能改进。Optimizations 章节详细列出了它们。

和之前的 Python 版本存在兼容性的更改列表，请参阅 Porting to Python 3.7 章节。

## 新的特性

### PEP 563：延迟的标注求值

随着 PEP 3107 加入标注功能并在 PEP 526 进一步细化，Python 中类型提示的出现揭示了两个明显的可用性问题：

- 标注只能使用在当前作用域中已经存在的名称，也就是说，它们不支持任何形式的前向引用；而且——
- 标注源码对 Python 程序的启动时间有不利的影响。

这两个问题都可以通过延迟标注求值来解决。在定义标注的时候，编译器并不会编译执行相应表达式的代码，而是保存与相应表达式的 AST 等价的字符串形式。如果有需要，标注可以在运行时使用 `typing.get_type_hints()` 进行解析。在不需要这种解析的通常情况下，标注的存储成本更低（因为解析器只需处理较短的字符串）且启动时间更短。

在可用性方面，标注现在支持向前引用，以使以下句法有效:

    class C:
        @classmethod
        def from_string(cls, source: str) -> C:
            ...

        def validate_b(self, obj: B) -> bool:
            ...

    class B:
        ...

由于此修改会破坏兼容性，在 Python 3.7 中此种新的行为需要在每个模块层级上使用 `__future__` 导入来启用:

    from __future__ import annotations

它将在 Python 4.0 中成为默认行为。

参见

PEP 563 -- 延迟的标注求值
  PEP 由 Łukasz Langa 撰写并实现。

### PEP 538: 传统 C 区域强制转换

Python 3 系列有一个持续的挑战就是为处理 7 比特位 ASCII 文本的假定编码确定合理的默认策略，目前的设定是在非 Windows 平台上使用默认的 C 或 POSIX 区域设置。

PEP 538 更新了默认的解释器命令行接口，以自动将上述区域强制转换为可用的基于 UTF-8 的区域，具体描述可参见有关新增环境变量 `PYTHONCOERCECLOCALE` 的文档。 以这种方式自动设置 `LC_CTYPE` 意味着核心解释器和能感知区域的 C 扩展 (例如 `readline`) 都将会假定 UTF-8 已被用作默认的文本编码，而不再是 ASCII。

PEP 11 中的平台支持定义也已被更新以限制完整文本处理支持适当配置的基于非 ASCII 的语言区域。

作为此更改的一部分，当使用任何已定义的强制转换目标区域时 (目前为 `C.UTF-8`, `C.utf8` 和 `UTF-8`) `stdin` 和 `stdout` 默认的处理器现在将为 `surrogateescape` (而不是 `strict`)。 而无论是什么区域，`stderr` 默认的处理器仍为 `backslashreplace`。

默认情况下区域强制转换会静默进行，但为了辅助调试潜在的区域相关集成问题，可以通过设置 `PYTHONCOERCECLOCALE=warn` 来请求显式地启用警告信息（直接在 `stderr` 上发出）。 此设置还会使得 Python 运行时在核心解释器初始化时如果传统 C 区域仍然处于激活状态时发出警告。

虽然 PEP 538 的区域强制转换的好处在于它还会同时影响扩展模块 (例如 GNU `readline`) 以及子进程 (包括运行非 Python 应用和旧版本 Python 的子进程)，但它也存在需要所运行系统必须存在适合的目标区域的缺点。 为了更好地处理没有可用适合的目标区域的情况 (例如在 RHEL/CentOS 7 上就会出现此情况)，Python 3.7 还实现了 PEP 540: 强制 UTF-8 运行时模式。

参见

PEP 538 -- 强制转换传统 C 区域到基于 UTF-8 的区域
  PEP 由 Nick Coghlan 撰写并实现。

### PEP 540: 强制 UTF-8 运行时模式

新的 `-X` `utf8` 命令行选项和 `PYTHONUTF8` 环境变量可被用来启用 CPython 的 UTF-8 模式。

当处于 UTF-8 模式时，CPython 会忽略区域设置，并默认使用 UTF-8 编码。 用于 `sys.stdin` 和 `sys.stdout` 流的错误处理器将设置为 `surrogateescape`。

强制 UTF-8 模式可被用来在嵌入的 Python 解释器中改变文本处理行为，而不会改变嵌入方应用的区域设置。

PEP 540 的 UTF-8 模式的好处是不必关心运行所在系统中有哪些可用区域即可工作，但它也存在对扩展模块 (例如 GNU `readline`)、运行非 Python 应用的子进程以及运行旧版本 Python 的子进程不起作用的缺点。 为了减小与这些组件通信时破坏文本数据的风险，Python 3.7 还实现了 PEP 540: 强制 UTF-8 运行时模式)。

UTF-8 模式在语言区域为 `C` 或 `POSIX` 并且 PEP 538 区域强制转换特性无法将其修改为某种基于 UTF-8 的替代项时会被默认启用（无论修改失败是由于设置了 `PYTHONCOERCECLOCALE=0`, `LC_ALL` 还是由于缺少适合的目标区域）。

参见

PEP 540 -- 增加了新的 UTF-8 模式
  PEP 由 Victor Stinner 撰写并实现

### PEP 553: 内置的 `breakpoint()`

Python 3.7 包含了新的内置 `breakpoint()` 函数，作为一种简单方便地进入 Python 调试器的方式。

内置 `breakpoint()` 会调用 `sys.breakpointhook()`。 在默认情况下后者会导入 `pdb` 然后再调用 `pdb.set_trace()`，但是通过将 `sys.breakpointhook()` 绑定到你选定的函数，`breakpoint()` 可以进入任何调试器。 此外，环境变量 `PYTHONBREAKPOINT` 可被设置为你选定的调试器的可调用对象。 设置 `PYTHONBREAKPOINT=0` 会完全禁用内置 `breakpoint()`。

参见

PEP 553 -- 内置的 breakpoint()
  PEP 由 Barry Warsaw 撰写并实现

### PEP 539: 用于线程局部存储的新 C API

虽然 Python 已提供了用于线程局部存储支持的 C API；但原有的 线程局部存储 (TLS) API 使用 `int` 来表示所有平台上的 TLS 密钥。 对于官方支持的平台而言这通常不是问题，但这既不符合 POSIX 标准，也不具备任何实际意义上的可移植性。

PEP 539 通过向 CPython 提供了一个新的 线程特定存储 (TSS) API 来改变这一点，它取代了原有的 CPython 内部 TLS API 的使用，并且原有 API 已弃用。 TSS API 使用一种新类型 `Py_tss_t` 而非 `int` 来表示 TSS 密钥 -- 这是一种不透明类型，其定义可能依赖于下层的 TLS 实现。 因此，这将允许在以无法安全地转换为 `int` 的方式定义原生 TLS 密钥的平台上构建 CPython。

请注意在原生 TLS 密钥定义方式无法被安全地转换为 `int` 的平台上，原有 TLS API 中的全部函数将无法执行并会立即返回失败信息。 这样能清楚地表明原有 API 在无法可靠使用的平台上不受支持，并且不会再尝试添加此类支持。

参见

PEP 539 -- 在 CPython 中用于线程局部存储的新 C-API
  PEP 由 Erik M. Bray 撰写；由 Masayuki Yamamoto 实现。

### PEP 562: 定制对模块属性的访问

Python 3.7 允许在模块上定义 `__getattr__()` 并且当以其他方式找不到某个模块属性时将会调用它。 在模块上定义 `__dir__()` 现在也是允许的。

一个典型的可能有用的例子是已弃用模块属性和惰性加载。

参见

PEP 562 -- 模块的 `__getattr__` 和 `__dir__`
  PEP 由 Ivan Levkivskyi 撰写并实现

### PEP 564: 具有纳秒级精度的新时间函数

现代系统的时钟精度可以超过由 `time.time()` 函数及其变化形式所返回的浮点数的有限精度。 为了避免精度损失，PEP 564 在 `time` 模块中增加了原有计时器函数的六个新“纳秒版”变化形式:

- `time.clock_gettime_ns()`
- `time.clock_settime_ns()`
- `time.monotonic_ns()`
- `time.perf_counter_ns()`
- `time.process_time_ns()`
- `time.time_ns()`

这些新函数会以整数值的形式返回纳秒数。

测量 表明在 Linux 和 Windows 上 `time.time_ns()` 的精度大约比 `time.time()` 要高 3 倍。

参见

PEP 564 -- 增加具有纳秒级精度的新时间函数
  PEP 由 Victor Stinner 撰写并实现

### PEP 565: 在 `__main__` 中显示 DeprecationWarning

`DeprecationWarning` 的默认处理方式已经被更改，这此警告默认只显示一次，仅有当直接在 `__main__` 模块中运行的代码触发它们时才会再次显示。 因此，单文件脚本开发者以及 Python 交互模式使用者应该会再次开始看到针对他们所使用 API 的已弃用警告，但被导入应用、库和框架模块所触发的已弃用警告默认将继续隐藏。

作为此项更改的结果，标准库现在允许开发者在三种不同的已弃用警告行为之间进行选择:

- `FutureWarning`: 默认情况下总是会显示，建议用于应用程序最终用户应该看到的警告信息（例如对于已弃用的应用程序配置的设置选项）。
- `DeprecationWarning`: 默认情况下仅在 `__main__` 中以及当运行测试时会显示，建议用于其他 Python 开发者应该看到的警告信息，提示版本升级可能导致行为改变或者错误。
- `PendingDeprecationWarning`: 默认情况下仅在运行测试时会显示，可用于提示未来版本升级将会改变警告类别为 `DeprecationWarning` 或 `FutureWarning` 的情况。

在此之前 `DeprecationWarning` 和 `PendingDeprecationWarning` 都仅在运行测试时可见，这意味着主要编写单文件脚本或使用 Python 交互模式的开发者可能会因他们所用 API 突然出现的改变而感到惊讶。

参见

PEP 565 -- 在 `__main__` 中显示 DeprecationWarning
  PEP 由 Nick Coghlan 撰写并实现

### PEP 560: 对 `typing` 模块和泛型类型的核心支持

PEP 484 最初的设计方式使其不会向核心 CPython 解释器引入 任何 更改。 现在类型提示和 `typing` 模块已被社区广泛使用，因此这个限制已被取消。 这个 PEP 引入了两个特殊方法 `__class_getitem__()` 和 `__mro_entries__`，这些方法现在被 `typing` 中的大多数类和特殊构造所使用。 结果就是与类型相关的各类操作的速度提升了 7 倍，泛型类型可以在没有元类冲突的情况下被使用，而 `typing` 模块中几个长期存在的错误也已被修正。

参见

PEP 560 -- 对 typing 模块和泛型类型的核心支持
  PEP 由 Ivan Levkivskyi 撰写并实现

### PEP 552: 基于哈希值的 .pyc 文件

传统上 Python 检查字节码缓存文件 (即 `.pyc` 文件) 是否最新的方式是通过对源码元数据 (最后更改的时间戳和大小）和生成缓存时保存在其文件头中的源码元数据进行比较。 这种检查方法虽然有效，但也存在缺点。 当文件系统的时间戳太粗糙时，Python 有可能错过源码更新，导致用户感到困惑。 此外，在缓存文件中存在时间戳对于 构建可再现 并且基于内容的构建系统来说是有问题的。

PEP 552 扩展了 pyc 格式以允许使用源文件的哈希值而非源文件的时间戳来检查有效性。 这种 `.pyc` 文件就称为“基于哈希值的”。 默认情况下，Python 仍然使用基于时间戳的有效性检查，不会在运行时生成基于哈希值的 `.pyc` 文件。 基于哈希值的 `.pyc` 文件可以使用 `py_compile` 或 `compileall` 来生成。

基于哈希值的 `.pyc` 文件包含两种变体：已选定和未选定。 Python 会在运行时针对相应源码文件验证已选定基于哈希值的 `.pyc` 文件，但对未选定基于哈希值的 pyc 文件则不会这样做。 未选定基于哈希值的 `.pyc` 文件对于需要由 Python 外部的系统（例如构建系统）负责使 `.pyc` 文件保持最新的环境来说是一种有用的性能优化。

请参阅 已缓存字节码的失效 了解更多信息。

参见

PEP 552 -- 确定性的 pyc 文件
  PEP 由 Benjamin Peterson 撰写并实现

### PEP 545: Python 文档翻译

PEP 545 描述了创建和维护 Python 文档翻译的整个过程。

新增了三个新的翻译版本:

- 日语: https://docs.python.org/ja/
- 法语: https://docs.python.org/fr/
- 韩语: https://docs.python.org/ko/

参见

PEP 545 -- Python 文档翻译
  PEP 由 Julien Palard, Inada Naoki 和 Victor Stinner 撰写并实现。

### 开发运行时模式: -X dev

新的 `-X` `dev` 命令行选项或新的 `PYTHONDEVMODE` 环境变量可被用来启用 CPython 的 开发模式。 在开发模式下，CPython 会执行额外的在默认情况下开销过大的运行时检查。 请参阅 `-X` `dev` 文档查看对于此模式效果的完整描述。

## 其他语言特性修改

- 现在可以将超过 255 个参数传递给一个函数，而现在一个函数也可以拥有超过 255 个形参。 （由 Serhiy Storchaka 在 bpo-12844 和 bpo-18896 中贡献。）
- 现在 `bytes.fromhex()` 和 `bytearray.fromhex()` 会忽略所有 ASCII 空白符，而非仅是空格符. （由 Robert Xiao 在 bpo-28927 中贡献。）
- `str`, `bytes` 和 `bytearray` 获得了对新 `isascii()` 方法的支持，该方法可被用来测试是个字符串或字节串是否仅包含 ASCII 字符。 （由 INADA Naoki 在 bpo-32677 中贡献。）
- 现在当 `from ... import ...` 失败时 `ImportError` 会显示模块名称和模块 `__file__` 路径。 （由 Matthias Bussonnier 在 bpo-29546 中贡献。）
- 现在已支持涉及将子模块绑定到一个名称的绝对导入的循环导入。 （由 Serhiy Storchaka 在 bpo-30024 中贡献。）
- 现在 `object.__format__(x, '')` 等价于 `str(x)` 而非 `format(str(self), '')`。 （由 Serhiy Storchaka d bpo-28974 中贡献。）
- 为更好地支持栈跟踪的动态创建，现在 `types.TracebackType` 可以从 Python 代码中被实例化，并且 回溯对象 的 `tb_next` 属性现在是可写的。 （由 Nathaniel J. Smith 在 bpo-30579 中贡献。）
- 当使用 `-m` 开关时，现在 `sys.path[0]` 会主动扩展为完整的起始目录路径，而不是保持为空目录（这将允许在发生导入时从 当前 工作目录导入） （由 Nick Coghlan 在 bpo-33053 中贡献。）
- 新的 `-X` `importtime` 选项或 `PYTHONPROFILEIMPORTTIME` 环境变量可被用来显示每次模块导入的时间。 （由 Victor Stinner 在 bpo-31415 中贡献。）

## 新增模块

### contextvars

新的 `contextvars` 模块和一组 新的 C API 引入了对 上下文变量 的支持。 上下文变量在概念上类似于线程局部变量。 与 TLS 不同，上下文变量能正确地支持异步代码。

`asyncio` 和 `decimal` 已得到更新以使用和支持开箱即用的上下文变量。 特别是激活的 decimal 上下文现在将存储在上下文变量中，它允许十进制运算在异步代码中使用正确的上下文。

参见

PEP 567 -- 上下文变量
  PEP 由 Yury Selivanov 撰写并实现

### dataclasses

新的 `dataclass()` 装饰器提供了一种声明 数据类 的方式。 数据类使用变量标注来描述其属性。 它的构造器和其他魔术方法例如 `__repr__()`, `__eq__()` 以及 `__hash__()` 会自动地生成。

示例:

    @dataclass
    class Point:
        x: float
        y: float
        z: float = 0.0

    p = Point(1.5, 2.5)
    print(p)   # produces "Point(x=1.5, y=2.5, z=0.0)"

参见

PEP 557 -- 数据类
  PEP 由 Eric V. Smith 撰写并实现

### importlib.resources

新的 `importlib.resources` 模块提供了一些新的 API 和一个新的 ABC 用于访问、打开和读取包内的 资源。 资源基本上类似于包内的文件，但它们不一定是物理文件系统中实际的文件。 模块加载器可以提供 `get_resource_reader()` 函数，它会返回一个 `importlib.abc.ResourceReader` 实例来支持这个新 API。 内置的文件路径加载器和 zip 文件加载器都支持此特性。

由 Barry Warsaw 和 Brett Cannon 在 bpo-32248 中贡献。

参见

importlib_resources -- 用于先前 Python 版本的 PyPI 下层接口。

## 改进的模块

### argparse

新的 `ArgumentParser.parse_intermixed_args()` 方法允许混合选项与位置参数。 （由 paul.j3 在 bpo-14191 中提供。）

### asyncio

`asyncio` 模块获得了许多新的特性、可用性和 性能提升。 重要的改变包括:

- 新的 暂定 `asyncio.run()` 函数可被用于通过自动创建和销毁事件循环以基于同步代码运行协程。 （由 Yury Selivanov 在 bpo-32314 中贡献。）

- asyncio gained support for `contextvars`. `loop.call_soon()`, `loop.call_soon_threadsafe()`, `loop.call_later()`, `loop.call_at()`, and `Future.add_done_callback()` have a new optional keyword-only context parameter. `Tasks` now track their context automatically. See PEP 567 for more details. (Contributed by Yury Selivanov in bpo-32436.)

- The new `asyncio.create_task()` function has been added as a shortcut to `asyncio.get_event_loop().create_task()`. (Contributed by Andrew Svetlov in bpo-32311.)

- The new `loop.start_tls()` method can be used to upgrade an existing connection to TLS. (Contributed by Yury Selivanov in bpo-23749.)

- The new `loop.sock_recv_into()` method allows reading data from a socket directly into a provided buffer making it possible to reduce data copies. (Contributed by Antoine Pitrou in bpo-31819.)

- The new `asyncio.current_task()` function returns the currently running `Task` instance, and the new `asyncio.all_tasks()` function returns a set of all existing `Task` instances in a given loop. The `Task.current_task()` and `Task.all_tasks()` methods have been deprecated. (Contributed by Andrew Svetlov in bpo-32250.)

- The new provisional `BufferedProtocol` class allows implementing streaming protocols with manual control over the receive buffer. (Contributed by Yury Selivanov in bpo-32251.)

- The new `asyncio.get_running_loop()` function returns the currently running loop, and raises a `RuntimeError` if no loop is running. This is in contrast with `asyncio.get_event_loop()`, which will create a new event loop if none is running. (Contributed by Yury Selivanov in bpo-32269.)

- The new `StreamWriter.wait_closed()` coroutine method allows waiting until the stream writer is closed. The new `StreamWriter.is_closing()` method can be used to determine if the writer is closing. (Contributed by Andrew Svetlov in bpo-32391.)

- The new `loop.sock_sendfile()` coroutine method allows sending files using `os.sendfile` when possible. (Contributed by Andrew Svetlov in bpo-32410.)

- The new `Future.get_loop()` and `Task.get_loop()` methods return the instance of the loop on which a task or a future were created. `Server.get_loop()` allows doing the same for `asyncio.Server` objects. (Contributed by Yury Selivanov in bpo-32415 and Srinivas Reddy Thatiparthy in bpo-32418.)

- It is now possible to control how instances of `asyncio.Server` begin serving. Previously, the server would start serving immediately when created. The new start_serving keyword argument to `loop.create_server()` and `loop.create_unix_server()`, as well as `Server.start_serving()`, and `Server.serve_forever()` can be used to decouple server instantiation and serving. The new `Server.is_serving()` method returns `True` if the server is serving. `Server` objects are now asynchronous context managers:

      srv = await loop.create_server(...)

      async with srv:
          # some code

      # At this point, srv is closed and no longer accepts new connections.

  (Contributed by Yury Selivanov in bpo-32662.)

- Callback objects returned by `loop.call_later()` gained the new `when()` method which returns an absolute scheduled callback timestamp. (Contributed by Andrew Svetlov in bpo-32741.)

- The `loop.create_datagram_endpoint()` method gained support for Unix sockets. (Contributed by Quentin Dawans in bpo-31245.)

- The `asyncio.open_connection()`, `asyncio.start_server()` functions, `loop.create_connection()`, `loop.create_server()`, `loop.create_accepted_socket()` methods and their corresponding UNIX socket variants now accept the ssl_handshake_timeout keyword argument. (Contributed by Neil Aspinall in bpo-29970.)

- The new `Handle.cancelled()` method returns `True` if the callback was cancelled. (Contributed by Marat Sharafutdinov in bpo-31943.)

- The asyncio source has been converted to use the `async`/`await` syntax. (Contributed by Andrew Svetlov in bpo-32193.)

- The new `ReadTransport.is_reading()` method can be used to determine the reading state of the transport. Additionally, calls to `ReadTransport.resume_reading()` and `ReadTransport.pause_reading()` are now idempotent. (Contributed by Yury Selivanov in bpo-32356.)

- Loop methods which accept socket paths now support passing path-like objects. (Contributed by Yury Selivanov in bpo-32066.)

- In `asyncio` TCP sockets on Linux are now created with `TCP_NODELAY` flag set by default. (Contributed by Yury Selivanov and Victor Stinner in bpo-27456.)

- Exceptions occurring in cancelled tasks are no longer logged. (Contributed by Yury Selivanov in bpo-30508.)

- New `WindowsSelectorEventLoopPolicy` and `WindowsProactorEventLoopPolicy` classes. (Contributed by Yury Selivanov in bpo-33792.)

Several `asyncio` APIs have been deprecated.

### binascii

The `b2a_uu()` function now accepts an optional backtick keyword argument. When it's true, zeros are represented by `'`'` instead of spaces. (Contributed by Xiang Zhang in bpo-30103.)

### calendar

The `HTMLCalendar` class has new class attributes which ease the customization of CSS classes in the produced HTML calendar. (Contributed by Oz Tiram in bpo-30095.)

### collections

`collections.namedtuple()` now supports default values. (Contributed by Raymond Hettinger in bpo-32320.)

### compileall

`compileall.compile_dir()` learned the new invalidation_mode parameter, which can be used to enable hash-based .pyc invalidation. The invalidation mode can also be specified on the command line using the new `--invalidation-mode` argument. (Contributed by Benjamin Peterson in bpo-31650.)

### concurrent.futures

`ProcessPoolExecutor` and `ThreadPoolExecutor` now support the new initializer and initargs constructor arguments. (Contributed by Antoine Pitrou in bpo-21423.)

The `ProcessPoolExecutor` can now take the multiprocessing context via the new mp_context argument. (Contributed by Thomas Moreau in bpo-31540.)

### contextlib

The new `nullcontext()` is a simpler and faster no-op context manager than `ExitStack`. (Contributed by Jesse-Bakker in bpo-10049.)

The new `asynccontextmanager()`, `AbstractAsyncContextManager`, and `AsyncExitStack` have been added to complement their synchronous counterparts. (Contributed by Jelle Zijlstra in bpo-29679 and bpo-30241, and by Alexander Mohr and Ilya Kulakov in bpo-29302.)

### cProfile

The `cProfile` command line now accepts `-m module_name` as an alternative to script path. (Contributed by Sanyam Khurana in bpo-21862.)

### crypt

The `crypt` module now supports the Blowfish hashing method. (Contributed by Serhiy Storchaka in bpo-31664.)

The `mksalt()` function now allows specifying the number of rounds for hashing. (Contributed by Serhiy Storchaka in bpo-31702.)

### datetime

The new `datetime.fromisoformat()` method constructs a `datetime` object from a string in one of the formats output by `datetime.isoformat()`. (Contributed by Paul Ganssle in bpo-15873.)

The `tzinfo` class now supports sub-minute offsets. (Contributed by Alexander Belopolsky in bpo-5288.)

### dbm

`dbm.dumb` now supports reading read-only files and no longer writes the index file when it is not changed.

### decimal

The `decimal` module now uses context variables to store the decimal context. (Contributed by Yury Selivanov in bpo-32630.)

### dis

The `dis()` function is now able to disassemble nested code objects (the code of comprehensions, generator expressions and nested functions, and the code used for building nested classes). The maximum depth of disassembly recursion is controlled by the new depth parameter. (Contributed by Serhiy Storchaka in bpo-11822.)

### distutils

`README.rst` is now included in the list of distutils standard READMEs and therefore included in source distributions. (Contributed by Ryan Gonzalez in bpo-11913.)

### enum

The `Enum` learned the new `_ignore_` class property, which allows listing the names of properties which should not become enum members. (Contributed by Ethan Furman in bpo-31801.)

In Python 3.8, attempting to check for non-Enum objects in `Enum` classes will raise a `TypeError` (e.g. `1 in Color`); similarly, attempting to check for non-Flag objects in a `Flag` member will raise `TypeError` (e.g. `1 in Perm.RW`); currently, both operations return `False` instead and are deprecated. (Contributed by Ethan Furman in bpo-33217.)

### functools

`functools.singledispatch()` now supports registering implementations using type annotations. (Contributed by Łukasz Langa in bpo-32227.)

### gc

The new `gc.freeze()` function allows freezing all objects tracked by the garbage collector and excluding them from future collections. This can be used before a POSIX `fork()` call to make the GC copy-on-write friendly or to speed up collection. The new `gc.unfreeze()` functions reverses this operation. Additionally, `gc.get_freeze_count()` can be used to obtain the number of frozen objects. (Contributed by Li Zekun in bpo-31558.)

### hmac

The `hmac` module now has an optimized one-shot `digest()` function, which is up to three times faster than `HMAC()`. (Contributed by Christian Heimes in bpo-32433.)

### http.client

`HTTPConnection` and `HTTPSConnection` now support the new blocksize argument for improved upload throughput. (Contributed by Nir Soffer in bpo-31945.)

### http.server

`SimpleHTTPRequestHandler` now supports the HTTP `If-Modified-Since` header. The server returns the 304 response status if the target file was not modified after the time specified in the header. (Contributed by Pierre Quentel in bpo-29654.)

`SimpleHTTPRequestHandler` accepts the new directory argument, in addition to the new `--directory` command line argument. With this parameter, the server serves the specified directory, by default it uses the current working directory. (Contributed by Stéphane Wirtel and Julien Palard in bpo-28707.)

The new `ThreadingHTTPServer` class uses threads to handle requests using `ThreadingMixin`. It is used when `http.server` is run with `-m`. (Contributed by Julien Palard in bpo-31639.)

### idlelib and IDLE

Multiple fixes for autocompletion. (Contributed by Louie Lu in bpo-15786.)

Module Browser (on the File menu, formerly called Class Browser), now displays nested functions and classes in addition to top-level functions and classes. (Contributed by Guilherme Polo, Cheryl Sabella, and Terry Jan Reedy in bpo-1612262.)

The Settings dialog (Options, Configure IDLE) has been partly rewritten to improve both appearance and function. (Contributed by Cheryl Sabella and Terry Jan Reedy in multiple issues.)

The font sample now includes a selection of non-Latin characters so that users can better see the effect of selecting a particular font. (Contributed by Terry Jan Reedy in bpo-13802.) The sample can be edited to include other characters. (Contributed by Serhiy Storchaka in bpo-31860.)

The IDLE features formerly implemented as extensions have been reimplemented as normal features. Their settings have been moved from the Extensions tab to other dialog tabs. (Contributed by Charles Wohlganger and Terry Jan Reedy in bpo-27099.)

Editor code context option revised. Box displays all context lines up to maxlines. Clicking on a context line jumps the editor to that line. Context colors for custom themes is added to Highlights tab of Settings dialog. (Contributed by Cheryl Sabella and Terry Jan Reedy in bpo-33642, bpo-33768, and bpo-33679.)

On Windows, a new API call tells Windows that tk scales for DPI. On Windows 8.1+ or 10, with DPI compatibility properties of the Python binary unchanged, and a monitor resolution greater than 96 DPI, this should make text and lines sharper. It should otherwise have no effect. (Contributed by Terry Jan Reedy in bpo-33656.)

New in 3.7.1:

Output over N lines (50 by default) is squeezed down to a button. N can be changed in the PyShell section of the General page of the Settings dialog. Fewer, but possibly extra long, lines can be squeezed by right clicking on the output. Squeezed output can be expanded in place by double-clicking the button or into the clipboard or a separate window by right-clicking the button. (Contributed by Tal Einat in bpo-1529353.)

The changes above have been backported to 3.6 maintenance releases.

### importlib

The `importlib.abc.ResourceReader` ABC was introduced to support the loading of resources from packages. See also importlib.resources. (Contributed by Barry Warsaw, Brett Cannon in bpo-32248.)

`importlib.reload()` now raises `ModuleNotFoundError` if the module lacks a spec. (Contributed by Garvit Khatri in bpo-29851.)

`importlib.find_spec()` now raises `ModuleNotFoundError` instead of `AttributeError` if the specified parent module is not a package (i.e. lacks a `__path__` attribute). (Contributed by Milan Oberkirch in bpo-30436.)

The new `importlib.source_hash()` can be used to compute the hash of the passed source. A hash-based .pyc file embeds the value returned by this function.

### io

The new `TextIOWrapper.reconfigure()` method can be used to reconfigure the text stream with the new settings. (Contributed by Antoine Pitrou in bpo-30526 and INADA Naoki in bpo-15216.)

### ipaddress

The new `subnet_of()` and `supernet_of()` methods of `ipaddress.IPv6Network` and `ipaddress.IPv4Network` can be used for network containment tests. (Contributed by Michel Albert and Cheryl Sabella in bpo-20825.)

### itertools

`itertools.islice()` now accepts `integer-like objects` as start, stop, and slice arguments. (Contributed by Will Roberts in bpo-30537.)

### locale

The new monetary argument to `locale.format_string()` can be used to make the conversion use monetary thousands separators and grouping strings. (Contributed by Garvit in bpo-10379.)

The `locale.getpreferredencoding()` function now always returns `'UTF-8'` on Android or when in the forced UTF-8 mode.

### logging

`Logger` instances can now be pickled. (Contributed by Vinay Sajip in bpo-30520.)

The new `StreamHandler.setStream()` method can be used to replace the logger stream after handler creation. (Contributed by Vinay Sajip in bpo-30522.)

It is now possible to specify keyword arguments to handler constructors in configuration passed to `logging.config.fileConfig()`. (Contributed by Preston Landers in bpo-31080.)

### math

The new `math.remainder()` function implements the IEEE 754-style remainder operation. (Contributed by Mark Dickinson in bpo-29962.)

### mimetypes

The MIME type of .bmp has been changed from `'image/x-ms-bmp'` to `'image/bmp'`. (Contributed by Nitish Chandra in bpo-22589.)

### msilib

The new `Database.Close()` method can be used to close the MSI database. (Contributed by Berker Peksag in bpo-20486.)

### multiprocessing

The new `Process.close()` method explicitly closes the process object and releases all resources associated with it. `ValueError` is raised if the underlying process is still running. (Contributed by Antoine Pitrou in bpo-30596.)

The new `Process.kill()` method can be used to terminate the process using the `SIGKILL` signal on Unix. (Contributed by Vitor Pereira in bpo-30794.)

Non-daemonic threads created by `Process` are now joined on process exit. (Contributed by Antoine Pitrou in bpo-18966.)

### os

`os.fwalk()` now accepts the path argument as `bytes`. (Contributed by Serhiy Storchaka in bpo-28682.)

`os.scandir()` gained support for file descriptors. (Contributed by Serhiy Storchaka in bpo-25996.)

The new `register_at_fork()` function allows registering Python callbacks to be executed at process fork. (Contributed by Antoine Pitrou in bpo-16500.)

Added `os.preadv()` (combine the functionality of `os.readv()` and `os.pread()`) and `os.pwritev()` functions (combine the functionality of `os.writev()` and `os.pwrite()`). (Contributed by Pablo Galindo in bpo-31368.)

The mode argument of `os.makedirs()` no longer affects the file permission bits of newly-created intermediate-level directories. (Contributed by Serhiy Storchaka in bpo-19930.)

`os.dup2()` now returns the new file descriptor. Previously, `None` was always returned. (Contributed by Benjamin Peterson in bpo-32441.)

The structure returned by `os.stat()` now contains the `st_fstype` attribute on Solaris and its derivatives. (Contributed by Jesús Cea Avión in bpo-32659.)

### pathlib

The new `Path.is_mount()` method is now available on POSIX systems and can be used to determine whether a path is a mount point. (Contributed by Cooper Ry Lees in bpo-30897.)

### pdb

`pdb.set_trace()` now takes an optional header keyword-only argument. If given, it is printed to the console just before debugging begins. (Contributed by Barry Warsaw in bpo-31389.)

`pdb` command line now accepts `-m module_name` as an alternative to script file. (Contributed by Mario Corchero in bpo-32206.)

### py_compile

`py_compile.compile()` -- and by extension, `compileall` -- now respects the `SOURCE_DATE_EPOCH` environment variable by unconditionally creating `.pyc` files for hash-based validation. This allows for guaranteeing reproducible builds of `.pyc` files when they are created eagerly. (Contributed by Bernhard M. Wiedemann in bpo-29708.)

### pydoc

The pydoc server can now bind to an arbitrary hostname specified by the new `-n` command-line argument. (Contributed by Feanil Patel in bpo-31128.)

### queue

The new `SimpleQueue` class is an unbounded FIFO queue. (Contributed by Antoine Pitrou in bpo-14976.)

### re

The flags `re.ASCII`, `re.LOCALE` and `re.UNICODE` can be set within the scope of a group. (Contributed by Serhiy Storchaka in bpo-31690.)

`re.split()` now supports splitting on a pattern like `r'\b'`, `'^$'` or `(?=-)` that matches an empty string. (Contributed by Serhiy Storchaka in bpo-25054.)

Regular expressions compiled with the `re.LOCALE` flag no longer depend on the locale at compile time. Locale settings are applied only when the compiled regular expression is used. (Contributed by Serhiy Storchaka in bpo-30215.)

`FutureWarning` is now emitted if a regular expression contains character set constructs that will change semantically in the future, such as nested sets and set operations. (Contributed by Serhiy Storchaka in bpo-30349.)

Compiled regular expression and match objects can now be copied using `copy.copy()` and `copy.deepcopy()`. (Contributed by Serhiy Storchaka in bpo-10076.)

### signal

The new warn_on_full_buffer argument to the `signal.set_wakeup_fd()` function makes it possible to specify whether Python prints a warning on stderr when the wakeup buffer overflows. (Contributed by Nathaniel J. Smith in bpo-30050.)

### socket

The new `socket.getblocking()` method returns `True` if the socket is in blocking mode and `False` otherwise. (Contributed by Yury Selivanov in bpo-32373.)

The new `socket.close()` function closes the passed socket file descriptor. This function should be used instead of `os.close()` for better compatibility across platforms. (Contributed by Christian Heimes in bpo-32454.)

The `socket` module now exposes the `socket.TCP_CONGESTION` (Linux 2.6.13), `socket.TCP_USER_TIMEOUT` (Linux 2.6.37), and `socket.TCP_NOTSENT_LOWAT` (Linux 3.12) constants. (Contributed by Omar Sandoval in bpo-26273 and Nathaniel J. Smith in bpo-29728.)

Support for `socket.AF_VSOCK` sockets has been added to allow communication between virtual machines and their hosts. (Contributed by Cathy Avery in bpo-27584.)

Sockets now auto-detect family, type and protocol from file descriptor by default. (Contributed by Christian Heimes in bpo-28134.)

### socketserver

`socketserver.ThreadingMixIn.server_close()` now waits until all non-daemon threads complete. `socketserver.ForkingMixIn.server_close()` now waits until all child processes complete.

Add a new `socketserver.ForkingMixIn.block_on_close` class attribute to `socketserver.ForkingMixIn` and `socketserver.ThreadingMixIn` classes. Set the class attribute to `False` to get the pre-3.7 behaviour.

### sqlite3

`sqlite3.Connection` now exposes the `backup()` method when the underlying SQLite library is at version 3.6.11 or higher. (Contributed by Lele Gaifax in bpo-27645.)

The database argument of `sqlite3.connect()` now accepts any path-like object, instead of just a string. (Contributed by Anders Lorentsen in bpo-31843.)

### ssl

The `ssl` module now uses OpenSSL's builtin API instead of `match_hostname()` to check a host name or an IP address. Values are validated during TLS handshake. Any certificate validation error including failing the host name check now raises `SSLCertVerificationError` and aborts the handshake with a proper TLS Alert message. The new exception contains additional information. Host name validation can be customized with `SSLContext.host_flags`. (Contributed by Christian Heimes in bpo-31399.)

注解

The improved host name check requires a libssl implementation compatible with OpenSSL 1.0.2 or 1.1. Consequently, OpenSSL 0.9.8 and 1.0.1 are no longer supported (see Platform Support Removals for more details). The ssl module is mostly compatible with LibreSSL 2.7.2 and newer.

The `ssl` module no longer sends IP addresses in SNI TLS extension. (Contributed by Christian Heimes in bpo-32185.)

`match_hostname()` no longer supports partial wildcards like `www*.example.org`. `SSLContext.host_flags` has partial wildcard matching disabled by default. (Contributed by Mandeep Singh in bpo-23033 and Christian Heimes in bpo-31399.)

The default cipher suite selection of the `ssl` module now uses a blacklist approach rather than a hard-coded whitelist. Python no longer re-enables ciphers that have been blocked by OpenSSL security updates. Default cipher suite selection can be configured at compile time. (Contributed by Christian Heimes in bpo-31429.)

Validation of server certificates containing internationalized domain names (IDNs) is now supported. As part of this change, the `SSLSocket.server_hostname` attribute now stores the expected hostname in A-label form (`"xn--pythn-mua.org"`), rather than the U-label form (`"pythön.org"`). (Contributed by Nathaniel J. Smith and Christian Heimes in bpo-28414.)

The `ssl` module has preliminary and experimental support for TLS 1.3 and OpenSSL 1.1.1. At the time of Python 3.7.0 release, OpenSSL 1.1.1 is still under development and TLS 1.3 hasn't been finalized yet. The TLS 1.3 handshake and protocol behaves slightly differently than TLS 1.2 and earlier, see TLS 1.3. (Contributed by Christian Heimes in bpo-32947, bpo-20995, bpo-29136, bpo-30622 and bpo-33618)

`SSLSocket` and `SSLObject` no longer have a public constructor. Direct instantiation was never a documented and supported feature. Instances must be created with `SSLContext` methods `wrap_socket()` and `wrap_bio()`. (Contributed by Christian Heimes in bpo-32951)

OpenSSL 1.1 APIs for setting the minimum and maximum TLS protocol version are available as `SSLContext.minimum_version` and `SSLContext.maximum_version`. Supported protocols are indicated by several new flags, such as `HAS_TLSv1_1`. (Contributed by Christian Heimes in bpo-32609.)

### 字符串

`string.Template` now lets you to optionally modify the regular expression pattern for braced placeholders and non-braced placeholders separately. (Contributed by Barry Warsaw in bpo-1198569.)

### subprocess

The `subprocess.run()` function accepts the new capture_output keyword argument. When true, stdout and stderr will be captured. This is equivalent to passing `subprocess.PIPE` as stdout and stderr arguments. (Contributed by Bo Bayles in bpo-32102.)

The `subprocess.run` function and the `subprocess.Popen` constructor now accept the text keyword argument as an alias to universal_newlines. (Contributed by Andrew Clegg in bpo-31756.)

On Windows the default for close_fds was changed from `False` to `True` when redirecting the standard handles. It's now possible to set close_fds to true when redirecting the standard handles. See `subprocess.Popen`. This means that close_fds now defaults to `True` on all supported platforms. (Contributed by Segev Finer in bpo-19764.)

The subprocess module is now more graceful when handling `KeyboardInterrupt` during `subprocess.call()`, `subprocess.run()`, or in a `Popen` context manager. It now waits a short amount of time for the child to exit, before continuing the handling of the `KeyboardInterrupt` exception. (Contributed by Gregory P. Smith in bpo-25942.)

### sys

The new `sys.breakpointhook()` hook function is called by the built-in `breakpoint()`. (Contributed by Barry Warsaw in bpo-31353.)

On Android, the new `sys.getandroidapilevel()` returns the build-time Android API version. (Contributed by Victor Stinner in bpo-28740.)

The new `sys.get_coroutine_origin_tracking_depth()` function returns the current coroutine origin tracking depth, as set by the new `sys.set_coroutine_origin_tracking_depth()`. `asyncio` has been converted to use this new API instead of the deprecated `sys.set_coroutine_wrapper()`. (Contributed by Nathaniel J. Smith in bpo-32591.)

### time

PEP 564 adds six new functions with nanosecond resolution to the `time` module:

- `time.clock_gettime_ns()`
- `time.clock_settime_ns()`
- `time.monotonic_ns()`
- `time.perf_counter_ns()`
- `time.process_time_ns()`
- `time.time_ns()`

New clock identifiers have been added:

- `time.CLOCK_BOOTTIME` (Linux): Identical to `time.CLOCK_MONOTONIC`, except it also includes any time that the system is suspended.
- `time.CLOCK_PROF` (FreeBSD, NetBSD and OpenBSD): High-resolution per-process CPU timer.
- `time.CLOCK_UPTIME` (FreeBSD, OpenBSD): Time whose absolute value is the time the system has been running and not suspended, providing accurate uptime measurement.

The new `time.thread_time()` and `time.thread_time_ns()` functions can be used to get per-thread CPU time measurements. (Contributed by Antoine Pitrou in bpo-32025.)

The new `time.pthread_getcpuclockid()` function returns the clock ID of the thread-specific CPU-time clock.

### tkinter

The new `tkinter.ttk.Spinbox` class is now available. (Contributed by Alan Moore in bpo-32585.)

### tracemalloc

`tracemalloc.Traceback` behaves more like regular tracebacks, sorting the frames from oldest to most recent. `Traceback.format()` now accepts negative limit, truncating the result to the `abs(limit)` oldest frames. To get the old behaviour, use the new most_recent_first argument to `Traceback.format()`. (Contributed by Jesse Bakker in bpo-32121.)

### types

The new `WrapperDescriptorType`, `MethodWrapperType`, `MethodDescriptorType`, and `ClassMethodDescriptorType` classes are now available. (Contributed by Manuel Krebber and Guido van Rossum in bpo-29377, and Serhiy Storchaka in bpo-32265.)

The new `types.resolve_bases()` function resolves MRO entries dynamically as specified by PEP 560. (Contributed by Ivan Levkivskyi in bpo-32717.)

### unicodedata

The internal `unicodedata` database has been upgraded to use Unicode 11. (Contributed by Benjamin Peterson.)

### unittest

The new `-k` command-line option allows filtering tests by a name substring or a Unix shell-like pattern. For example, `python -m unittest -k foo` runs `foo_tests.SomeTest.test_something`, `bar_tests.SomeTest.test_foo`, but not `bar_tests.FooTest.test_something`. (Contributed by Jonas Haag in bpo-32071.)

### unittest.mock

The `sentinel` attributes now preserve their identity when they are `copied` or `pickled`. (Contributed by Serhiy Storchaka in bpo-20804.)

The new `seal()` function allows sealing `Mock` instances, which will disallow further creation of attribute mocks. The seal is applied recursively to all attributes that are themselves mocks. (Contributed by Mario Corchero in bpo-30541.)

### urllib.parse

`urllib.parse.quote()` has been updated from RFC 2396 to RFC 3986, adding `~` to the set of characters that are never quoted by default. (Contributed by Christian Theune and Ratnadeep Debnath in bpo-16285.)

### uu

The `uu.encode()` function now accepts an optional backtick keyword argument. When it's true, zeros are represented by `'`'` instead of spaces. (Contributed by Xiang Zhang in bpo-30103.)

### uuid

The new `UUID.is_safe` attribute relays information from the platform about whether generated UUIDs are generated with a multiprocessing-safe method. (Contributed by Barry Warsaw in bpo-22807.)

`uuid.getnode()` now prefers universally administered MAC addresses over locally administered MAC addresses. This makes a better guarantee for global uniqueness of UUIDs returned from `uuid.uuid1()`. If only locally administered MAC addresses are available, the first such one found is returned. (Contributed by Barry Warsaw in bpo-32107.)

### warnings

The initialization of the default warnings filters has changed as follows:

- warnings enabled via command line options (including those for `-b` and the new CPython-specific `-X` `dev` option) are always passed to the warnings machinery via the `sys.warnoptions` attribute.

- warnings filters enabled via the command line or the environment now have the following order of precedence:

  > - the `BytesWarning` filter for `-b` (or `-bb`)
  > - any filters specified with the `-W` option
  > - any filters specified with the `PYTHONWARNINGS` environment variable
  > - any other CPython specific filters (e.g. the `default` filter added for the new `-X dev` mode)
  > - any implicit filters defined directly by the warnings machinery

- in CPython debug builds, all warnings are now displayed by default (the implicit filter list is empty)

(Contributed by Nick Coghlan and Victor Stinner in bpo-20361, bpo-32043, and bpo-32230.)

Deprecation warnings are once again shown by default in single-file scripts and at the interactive prompt. See PEP 565: 在 __main__ 中显示 DeprecationWarning for details. (Contributed by Nick Coghlan in bpo-31975.)

### xml.etree

ElementPath predicates in the `find()` methods can now compare text of the current node with `[. = "text"]`, not only text in children. Predicates also allow adding spaces for better readability. (Contributed by Stefan Behnel in bpo-31648.)

### xmlrpc.server

`SimpleXMLRPCDispatcher.register_function` can now be used as a decorator. (Contributed by Xiang Zhang in bpo-7769.)

### zipapp

Function `create_archive()` now accepts an optional filter argument to allow the user to select which files should be included in the archive. (Contributed by Irmen de Jong in bpo-31072.)

Function `create_archive()` now accepts an optional compressed argument to generate a compressed archive. A command line option `--compress` has also been added to support compression. (Contributed by Zhiming Wang in bpo-31638.)

### zipfile

`ZipFile` now accepts the new compresslevel parameter to control the compression level. (Contributed by Bo Bayles in bpo-21417.)

Subdirectories in archives created by `ZipFile` are now stored in alphabetical order. (Contributed by Bernhard M. Wiedemann in bpo-30693.)

## C API Changes

A new API for thread-local storage has been implemented. See PEP 539: 用于线程局部存储的新 C API for an overview and Thread Specific Storage (TSS) API for a complete reference. (Contributed by Masayuki Yamamoto in bpo-25658.)

The new context variables functionality exposes a number of new C APIs.

The new `PyImport_GetModule()` function returns the previously imported module with the given name. (Contributed by Eric Snow in bpo-28411.)

The new `Py_RETURN_RICHCOMPARE` macro eases writing rich comparison functions. (Contributed by Petr Victorin in bpo-23699.)

The new `Py_UNREACHABLE` macro can be used to mark unreachable code paths. (Contributed by Barry Warsaw in bpo-31338.)

The `tracemalloc` now exposes a C API through the new `PyTraceMalloc_Track()` and `PyTraceMalloc_Untrack()` functions. (Contributed by Victor Stinner in bpo-30054.)

The new `import__find__load__start()` and `import__find__load__done()` static markers can be used to trace module imports. (Contributed by Christian Heimes in bpo-31574.)

The fields `name` and `doc` of structures `PyMemberDef`, `PyGetSetDef`, `PyStructSequence_Field`, `PyStructSequence_Desc`, and `wrapperbase` are now of type `const char *` rather of `char *`. (Contributed by Serhiy Storchaka in bpo-28761.)

The result of `PyUnicode_AsUTF8AndSize()` and `PyUnicode_AsUTF8()` is now of type `const char *` rather of `char *`. (Contributed by Serhiy Storchaka in bpo-28769.)

The result of `PyMapping_Keys()`, `PyMapping_Values()` and `PyMapping_Items()` is now always a list, rather than a list or a tuple. (Contributed by Oren Milman in bpo-28280.)

Added functions `PySlice_Unpack()` and `PySlice_AdjustIndices()`. (Contributed by Serhiy Storchaka in bpo-27867.)

`PyOS_AfterFork()` is deprecated in favour of the new functions `PyOS_BeforeFork()`, `PyOS_AfterFork_Parent()` and `PyOS_AfterFork_Child()`. (Contributed by Antoine Pitrou in bpo-16500.)

The `PyExc_RecursionErrorInst` singleton that was part of the public API has been removed as its members being never cleared may cause a segfault during finalization of the interpreter. Contributed by Xavier de Gaye in bpo-22898 and bpo-30697.

Added C API support for timezones with timezone constructors `PyTimeZone_FromOffset()` and `PyTimeZone_FromOffsetAndName()`, and access to the UTC singleton with `PyDateTime_TimeZone_UTC`. Contributed by Paul Ganssle in bpo-10381.

The type of results of `PyThread_start_new_thread()` and `PyThread_get_thread_ident()`, and the id parameter of `PyThreadState_SetAsyncExc()` changed from `long` to `unsigned long`. (Contributed by Serhiy Storchaka in bpo-6532.)

`PyUnicode_AsWideCharString()` now raises a `ValueError` if the second argument is NULL and the `wchar_t*` string contains null characters. (Contributed by Serhiy Storchaka in bpo-30708.)

Changes to the startup sequence and the management of dynamic memory allocators mean that the long documented requirement to call `Py_Initialize()` before calling most C API functions is now relied on more heavily, and failing to abide by it may lead to segfaults in embedding applications. See the Porting to Python 3.7 section in this document and the Before Python Initialization section in the C API documentation for more details.

The new `PyInterpreterState_GetID()` returns the unique ID for a given interpreter. (Contributed by Eric Snow in bpo-29102.)

`Py_DecodeLocale()`, `Py_EncodeLocale()` now use the UTF-8 encoding when the UTF-8 mode is enabled. (Contributed by Victor Stinner in bpo-29240.)

`PyUnicode_DecodeLocaleAndSize()` and `PyUnicode_EncodeLocale()` now use the current locale encoding for `surrogateescape` error handler. (Contributed by Victor Stinner in bpo-29240.)

The start and end parameters of `PyUnicode_FindChar()` are now adjusted to behave like string slices. (Contributed by Xiang Zhang in bpo-28822.)

## Build Changes

Support for building `--without-threads` has been removed. The `threading` module is now always available. (Contributed by Antoine Pitrou in bpo-31370.).

A full copy of libffi is no longer bundled for use when building the `_ctypes` module on non-OSX UNIX platforms. An installed copy of libffi is now required when building `_ctypes` on such platforms. (Contributed by Zachary Ware in bpo-27979.)

The Windows build process no longer depends on Subversion to pull in external sources, a Python script is used to download zipfiles from GitHub instead. If Python 3.6 is not found on the system (via `py -3.6`), NuGet is used to download a copy of 32-bit Python for this purpose. (Contributed by Zachary Ware in bpo-30450.)

The `ssl` module requires OpenSSL 1.0.2 or 1.1 compatible libssl. OpenSSL 1.0.1 has reached end of lifetime on 2016-12-31 and is no longer supported. LibreSSL is temporarily not supported as well. LibreSSL releases up to version 2.6.4 are missing required OpenSSL 1.0.2 APIs.

## Optimizations

The overhead of calling many methods of various standard library classes implemented in C has been significantly reduced by porting more code to use the `METH_FASTCALL` convention. (Contributed by Victor Stinner in bpo-29300, bpo-29507, bpo-29452, and bpo-29286.)

Various optimizations have reduced Python startup time by 10% on Linux and up to 30% on macOS. (Contributed by Victor Stinner, INADA Naoki in bpo-29585, and Ivan Levkivskyi in bpo-31333.)

Method calls are now up to 20% faster due to the bytecode changes which avoid creating bound method instances. (Contributed by Yury Selivanov and INADA Naoki in bpo-26110.)

The `asyncio` module received a number of notable optimizations for commonly used functions:

- The `asyncio.get_event_loop()` function has been reimplemented in C to make it up to 15 times faster. (Contributed by Yury Selivanov in bpo-32296.)
- `asyncio.Future` callback management has been optimized. (Contributed by Yury Selivanov in bpo-32348.)
- `asyncio.gather()` is now up to 15% faster. (Contributed by Yury Selivanov in bpo-32355.)
- `asyncio.sleep()` is now up to 2 times faster when the delay argument is zero or negative. (Contributed by Andrew Svetlov in bpo-32351.)
- The performance overhead of asyncio debug mode has been reduced. (Contributed by Antoine Pitrou in bpo-31970.)

As a result of PEP 560 work, the import time of `typing` has been reduced by a factor of 7, and many typing operations are now faster. (Contributed by Ivan Levkivskyi in bpo-32226.)

`sorted()` and `list.sort()` have been optimized for common cases to be up to 40-75% faster. (Contributed by Elliot Gorokhovsky in bpo-28685.)

`dict.copy()` is now up to 5.5 times faster. (Contributed by Yury Selivanov in bpo-31179.)

`hasattr()` and `getattr()` are now about 4 times faster when name is not found and obj does not override `object.__getattr__()` or `object.__getattribute__()`. (Contributed by INADA Naoki in bpo-32544.)

Searching for certain Unicode characters (like Ukrainian capital "Є") in a string was up to 25 times slower than searching for other characters. It is now only 3 times slower in the worst case. (Contributed by Serhiy Storchaka in bpo-24821.)

The `collections.namedtuple()` factory has been reimplemented to make the creation of named tuples 4 to 6 times faster. (Contributed by Jelle Zijlstra with further improvements by INADA Naoki, Serhiy Storchaka, and Raymond Hettinger in bpo-28638.)

`date.fromordinal()` and `date.fromtimestamp()` are now up to 30% faster in the common case. (Contributed by Paul Ganssle in bpo-32403.)

The `os.fwalk()` function is now up to 2 times faster thanks to the use of `os.scandir()`. (Contributed by Serhiy Storchaka in bpo-25996.)

The speed of the `shutil.rmtree()` function has been improved by 20--40% thanks to the use of the `os.scandir()` function. (Contributed by Serhiy Storchaka in bpo-28564.)

Optimized case-insensitive matching and searching of `regular expressions`. Searching some patterns can now be up to 20 times faster. (Contributed by Serhiy Storchaka in bpo-30285.)

`re.compile()` now converts `flags` parameter to int object if it is `RegexFlag`. It is now as fast as Python 3.5, and faster than Python 3.6 by about 10% depending on the pattern. (Contributed by INADA Naoki in bpo-31671.)

The `modify()` methods of classes `selectors.EpollSelector`, `selectors.PollSelector` and `selectors.DevpollSelector` may be around 10% faster under heavy loads. (Contributed by Giampaolo Rodola' in bpo-30014)

Constant folding has been moved from the peephole optimizer to the new AST optimizer, which is able perform optimizations more consistently. (Contributed by Eugene Toder and INADA Naoki in bpo-29469 and bpo-11549.)

Most functions and methods in `abc` have been rewritten in C. This makes creation of abstract base classes, and calling `isinstance()` and `issubclass()` on them 1.5x faster. This also reduces Python start-up time by up to 10%. (Contributed by Ivan Levkivskyi and INADA Naoki in bpo-31333)

Significant speed improvements to alternate constructors for `datetime.date` and `datetime.datetime` by using fast-path constructors when not constructing subclasses. (Contributed by Paul Ganssle in bpo-32403)

The speed of comparison of `array.array` instances has been improved considerably in certain cases. It is now from 10x to 70x faster when comparing arrays holding values of the same integer type. (Contributed by Adrian Wielgosik in bpo-24700.)

The `math.erf()` and `math.erfc()` functions now use the (faster) C library implementation on most platforms. (Contributed by Serhiy Storchaka in bpo-26121.)

## Other CPython Implementation Changes

- Trace hooks may now opt out of receiving the `line` and opt into receiving the `opcode` events from the interpreter by setting the corresponding new `f_trace_lines` and `f_trace_opcodes` attributes on the frame being traced. (Contributed by Nick Coghlan in bpo-31344.)
- Fixed some consistency problems with namespace package module attributes. Namespace module objects now have an `__file__` that is set to `None` (previously unset), and their `__spec__.origin` is also set to `None` (previously the string `"namespace"`). See bpo-32305. Also, the namespace module object's `__spec__.loader` is set to the same value as `__loader__` (previously, the former was set to `None`). See bpo-32303.
- The `locals()` dictionary now displays in the lexical order that variables were defined. Previously, the order was undefined. (Contributed by Raymond Hettinger in bpo-32690.)
- The `distutils` `upload` command no longer tries to change CR end-of-line characters to CRLF. This fixes a corruption issue with sdists that ended with a byte equivalent to CR. (Contributed by Bo Bayles in bpo-32304.)

## Deprecated Python Behavior

Yield expressions (both `yield` and `yield from` clauses) are now deprecated in comprehensions and generator expressions (aside from the iterable expression in the leftmost `for` clause). This ensures that comprehensions always immediately return a container of the appropriate type (rather than potentially returning a generator iterator object), while generator expressions won't attempt to interleave their implicit output with the output from any explicit yield expressions. In Python 3.7, such expressions emit `DeprecationWarning` when compiled, in Python 3.8 this will be a `SyntaxError`. (Contributed by Serhiy Storchaka in bpo-10544.)

Returning a subclass of `complex` from `object.__complex__()` is deprecated and will be an error in future Python versions. This makes `__complex__()` consistent with `object.__int__()` and `object.__float__()`. (Contributed by Serhiy Storchaka in bpo-28894.)

## Deprecated Python modules, functions and methods

### aifc

`aifc.openfp()` has been deprecated and will be removed in Python 3.9. Use `aifc.open()` instead. (Contributed by Brian Curtin in bpo-31985.)

### asyncio

Support for directly `await`-ing instances of `asyncio.Lock` and other asyncio synchronization primitives has been deprecated. An asynchronous context manager must be used in order to acquire and release the synchronization resource. (Contributed by Andrew Svetlov in bpo-32253.)

The `asyncio.Task.current_task()` and `asyncio.Task.all_tasks()` methods have been deprecated. (Contributed by Andrew Svetlov in bpo-32250.)

### collections

In Python 3.8, the abstract base classes in `collections.abc` will no longer be exposed in the regular `collections` module. This will help create a clearer distinction between the concrete classes and the abstract base classes. (Contributed by Serhiy Storchaka in bpo-25988.)

### dbm

`dbm.dumb` now supports reading read-only files and no longer writes the index file when it is not changed. A deprecation warning is now emitted if the index file is missing and recreated in the `'r'` and `'w'` modes (this will be an error in future Python releases). (Contributed by Serhiy Storchaka in bpo-28847.)

### enum

In Python 3.8, attempting to check for non-Enum objects in `Enum` classes will raise a `TypeError` (e.g. `1 in Color`); similarly, attempting to check for non-Flag objects in a `Flag` member will raise `TypeError` (e.g. `1 in Perm.RW`); currently, both operations return `False` instead. (Contributed by Ethan Furman in bpo-33217.)

### gettext

Using non-integer value for selecting a plural form in `gettext` is now deprecated. It never correctly worked. (Contributed by Serhiy Storchaka in bpo-28692.)

### importlib

Methods `MetaPathFinder.find_module()` (replaced by `MetaPathFinder.find_spec()`) and `PathEntryFinder.find_loader()` (replaced by `PathEntryFinder.find_spec()`) both deprecated in Python 3.4 now emit `DeprecationWarning`. (Contributed by Matthias Bussonnier in bpo-29576)

The `importlib.abc.ResourceLoader` ABC has been deprecated in favour of `importlib.abc.ResourceReader`.

### locale

`locale.format()` has been deprecated, use `locale.format_string()` instead. (Contributed by Garvit in bpo-10379.)

### macpath

The `macpath` is now deprecated and will be removed in Python 3.8. (Contributed by Chi Hsuan Yen in bpo-9850.)

### threading

`dummy_threading` and `_dummy_thread` have been deprecated. It is no longer possible to build Python with threading disabled. Use `threading` instead. (Contributed by Antoine Pitrou in bpo-31370.)

### socket

The silent argument value truncation in `socket.htons()` and `socket.ntohs()` has been deprecated. In future versions of Python, if the passed argument is larger than 16 bits, an exception will be raised. (Contributed by Oren Milman in bpo-28332.)

### ssl

`ssl.wrap_socket()` is deprecated. Use `ssl.SSLContext.wrap_socket()` instead. (Contributed by Christian Heimes in bpo-28124.)

### sunau

`sunau.openfp()` has been deprecated and will be removed in Python 3.9. Use `sunau.open()` instead. (Contributed by Brian Curtin in bpo-31985.)

### sys

Deprecated `sys.set_coroutine_wrapper()` and `sys.get_coroutine_wrapper()`.

The undocumented `sys.callstats()` function has been deprecated and will be removed in a future Python version. (Contributed by Victor Stinner in bpo-28799.)

### wave

`wave.openfp()` has been deprecated and will be removed in Python 3.9. Use `wave.open()` instead. (Contributed by Brian Curtin in bpo-31985.)

## Deprecated functions and types of the C API

Function `PySlice_GetIndicesEx()` is deprecated and replaced with a macro if `Py_LIMITED_API` is not set or set to a value in the range between `0x03050400` and `0x03060000` (not inclusive), or is `0x03060100` or higher. (Contributed by Serhiy Storchaka in bpo-27867.)

`PyOS_AfterFork()` has been deprecated. Use `PyOS_BeforeFork()`, `PyOS_AfterFork_Parent()` or `PyOS_AfterFork_Child()` instead. (Contributed by Antoine Pitrou in bpo-16500.)

## Platform Support Removals

- FreeBSD 9 and older are no longer officially supported.

- For full Unicode support, including within extension modules, *nix platforms are now expected to provide at least one of `C.UTF-8` (full locale), `C.utf8` (full locale) or `UTF-8` (`LC_CTYPE`-only locale) as an alternative to the legacy `ASCII`-based `C` locale.

- OpenSSL 0.9.8 and 1.0.1 are no longer supported, which means building CPython 3.7 with SSL/TLS support on older platforms still using these versions requires custom build options that link to a more recent version of OpenSSL.

  Notably, this issue affects the Debian 8 (aka "jessie") and Ubuntu 14.04 (aka "Trusty") LTS Linux distributions, as they still use OpenSSL 1.0.1 by default.

  Debian 9 ("stretch") and Ubuntu 16.04 ("xenial"), as well as recent releases of other LTS Linux releases (e.g. RHEL/CentOS 7.5, SLES 12-SP3), use OpenSSL 1.0.2 or later, and remain supported in the default build configuration.

  CPython's own CI configuration file provides an example of using the SSL compatibility testing infrastructure in CPython's test suite to build and link against OpenSSL 1.1.0 rather than an outdated system provided OpenSSL.

## API and Feature Removals

The following features and APIs have been removed from Python 3.7:

- The `os.stat_float_times()` function has been removed. It was introduced in Python 2.3 for backward compatibility with Python 2.2, and was deprecated since Python 3.1.
- Unknown escapes consisting of `'\'` and an ASCII letter in replacement templates for `re.sub()` were deprecated in Python 3.5, and will now cause an error.
- Removed support of the exclude argument in `tarfile.TarFile.add()`. It was deprecated in Python 2.7 and 3.2. Use the filter argument instead.
- The `splitunc()` function in the `ntpath` module was deprecated in Python 3.1, and has now been removed. Use the `splitdrive()` function instead.
- `collections.namedtuple()` no longer supports the verbose parameter or `_source` attribute which showed the generated source code for the named tuple class. This was part of an optimization designed to speed-up class creation. (Contributed by Jelle Zijlstra with further improvements by INADA Naoki, Serhiy Storchaka, and Raymond Hettinger in bpo-28638.)
- Functions `bool()`, `float()`, `list()` and `tuple()` no longer take keyword arguments. The first argument of `int()` can now be passed only as positional argument.
- Removed previously deprecated in Python 2.4 classes `Plist`, `Dict` and `_InternalDict` in the `plistlib` module. Dict values in the result of functions `readPlist()` and `readPlistFromBytes()` are now normal dicts. You no longer can use attribute access to access items of these dictionaries.
- The `asyncio.windows_utils.socketpair()` function has been removed. Use the `socket.socketpair()` function instead, it is available on all platforms since Python 3.5. `asyncio.windows_utils.socketpair` was just an alias to `socket.socketpair` on Python 3.5 and newer.
- `asyncio` no longer exports the `selectors` and `_overlapped` modules as `asyncio.selectors` and `asyncio._overlapped`. Replace `from asyncio import selectors` with `import selectors`.
- Direct instantiation of `ssl.SSLSocket` and `ssl.SSLObject` objects is now prohibited. The constructors were never documented, tested, or designed as public constructors. Users were supposed to use `ssl.wrap_socket()` or `ssl.SSLContext`. (Contributed by Christian Heimes in bpo-32951.)
- The unused `distutils` `install_misc` command has been removed. (Contributed by Eric N. Vander Weele in bpo-29218.)

## Module Removals

The `fpectl` module has been removed. It was never enabled by default, never worked correctly on x86-64, and it changed the Python ABI in ways that caused unexpected breakage of C extensions. (Contributed by Nathaniel J. Smith in bpo-29137.)

## Windows-only Changes

The python launcher, (py.exe), can accept 32 & 64 bit specifiers without having to specify a minor version as well. So `py -3-32` and `py -3-64` become valid as well as `py -3.7-32`, also the -m-64 and -m.n-64 forms are now accepted to force 64 bit python even if 32 bit would have otherwise been used. If the specified version is not available py.exe will error exit. (Contributed by Steve Barnes in bpo-30291.)

The launcher can be run as `py -0` to produce a list of the installed pythons, with default marked with an asterisk. Running `py -0p` will include the paths. If py is run with a version specifier that cannot be matched it will also print the short form list of available specifiers. (Contributed by Steve Barnes in bpo-30362.)

## Porting to Python 3.7

This section lists previously described changes and other bugfixes that may require changes to your code.

### Changes in Python Behavior

- `async` and `await` names are now reserved keywords. Code using these names as identifiers will now raise a `SyntaxError`. (Contributed by Jelle Zijlstra in bpo-30406.)

- PEP 479 is enabled for all code in Python 3.7, meaning that `StopIteration` exceptions raised directly or indirectly in coroutines and generators are transformed into `RuntimeError` exceptions. (Contributed by Yury Selivanov in bpo-32670.)

- `object.__aiter__()` methods can no longer be declared as asynchronous. (Contributed by Yury Selivanov in bpo-31709.)

- Due to an oversight, earlier Python versions erroneously accepted the following syntax:

      f(1 for x in [1],)

      class C(1 for x in [1]):
          pass

  Python 3.7 now correctly raises a `SyntaxError`, as a generator expression always needs to be directly inside a set of parentheses and cannot have a comma on either side, and the duplication of the parentheses can be omitted only on calls. (Contributed by Serhiy Storchaka in bpo-32012 and bpo-32023.)

- When using the `-m` switch, the initial working directory is now added to `sys.path`, rather than an empty string (which dynamically denoted the current working directory at the time of each import). Any programs that are checking for the empty string, or otherwise relying on the previous behaviour, will need to be updated accordingly (e.g. by also checking for `os.getcwd()` or `os.path.dirname(__main__.__file__)`, depending on why the code was checking for the empty string in the first place).

### Changes in the Python API

- `socketserver.ThreadingMixIn.server_close()` now waits until all non-daemon threads complete. Set the new `socketserver.ThreadingMixIn.block_on_close` class attribute to `False` to get the pre-3.7 behaviour. (Contributed by Victor Stinner in bpo-31233 and bpo-33540.)

- `socketserver.ForkingMixIn.server_close()` now waits until all child processes complete. Set the new `socketserver.ForkingMixIn.block_on_close` class attribute to `False` to get the pre-3.7 behaviour. (Contributed by Victor Stinner in bpo-31151 and bpo-33540.)

- The `locale.localeconv()` function now temporarily sets the `LC_CTYPE` locale to the value of `LC_NUMERIC` in some cases. (Contributed by Victor Stinner in bpo-31900.)

- `pkgutil.walk_packages()` now raises a `ValueError` if path is a string. Previously an empty list was returned. (Contributed by Sanyam Khurana in bpo-24744.)

- A format string argument for `string.Formatter.format()` is now positional-only. Passing it as a keyword argument was deprecated in Python 3.5. (Contributed by Serhiy Storchaka in bpo-29193.)

- Attributes `key`, `value` and `coded_value` of class `http.cookies.Morsel` are now read-only. Assigning to them was deprecated in Python 3.5. Use the `set()` method for setting them. (Contributed by Serhiy Storchaka in bpo-29192.)

- The mode argument of `os.makedirs()` no longer affects the file permission bits of newly-created intermediate-level directories. To set their file permission bits you can set the umask before invoking `makedirs()`. (Contributed by Serhiy Storchaka in bpo-19930.)

- The `struct.Struct.format` type is now `str` instead of `bytes`. (Contributed by Victor Stinner in bpo-21071.)

- `parse_multipart()` now accepts the encoding and errors arguments and returns the same results as `FieldStorage`: for non-file fields, the value associated to a key is a list of strings, not bytes. (Contributed by Pierre Quentel in bpo-29979.)

- Due to internal changes in `socket`, calling `socket.fromshare()` on a socket created by `socket.share` in older Python versions is not supported.

- `repr` for `BaseException` has changed to not include the trailing comma. Most exceptions are affected by this change. (Contributed by Serhiy Storchaka in bpo-30399.)

- `repr` for `datetime.timedelta` has changed to include the keyword arguments in the output. (Contributed by Utkarsh Upadhyay in bpo-30302.)

- Because `shutil.rmtree()` is now implemented using the `os.scandir()` function, the user specified handler onerror is now called with the first argument `os.scandir` instead of `os.listdir` when listing the directory is failed.

- Support for nested sets and set operations in regular expressions as in Unicode Technical Standard #18 might be added in the future. This would change the syntax. To facilitate this future change a `FutureWarning` will be raised in ambiguous cases for the time being. That include sets starting with a literal `'['` or containing literal character sequences `'--'`, `'&&'`, `'~~'`, and `'||'`. To avoid a warning, escape them with a backslash. (Contributed by Serhiy Storchaka in bpo-30349.)

- The result of splitting a string on a `regular expression` that could match an empty string has been changed. For example splitting on `r'\s*'` will now split not only on whitespaces as it did previously, but also on empty strings before all non-whitespace characters and just before the end of the string. The previous behavior can be restored by changing the pattern to `r'\s+'`. A `FutureWarning` was emitted for such patterns since Python 3.5.

  For patterns that match both empty and non-empty strings, the result of searching for all matches may also be changed in other cases. For example in the string `'a\n\n'`, the pattern `r'(?m)^\s*?$'` will not only match empty strings at positions 2 and 3, but also the string `'\n'` at positions 2--3. To match only blank lines, the pattern should be rewritten as `r'(?m)^[^\S\n]*$'`.

  `re.sub()` now replaces empty matches adjacent to a previous non-empty match. For example `re.sub('x*', '-', 'abxd')` returns now `'-a-b--d-'` instead of `'-a-b-d-'` (the first minus between 'b' and 'd' replaces 'x', and the second minus replaces an empty string between 'x' and 'd').

  (Contributed by Serhiy Storchaka in bpo-25054 and bpo-32308.)

- Change `re.escape()` to only escape regex special characters instead of escaping all characters other than ASCII letters, numbers, and `'_'`. (Contributed by Serhiy Storchaka in bpo-29995.)

- `tracemalloc.Traceback` frames are now sorted from oldest to most recent to be more consistent with `traceback`. (Contributed by Jesse Bakker in bpo-32121.)

- On OSes that support `socket.SOCK_NONBLOCK` or `socket.SOCK_CLOEXEC` bit flags, the `socket.type` no longer has them applied. Therefore, checks like `if sock.type == socket.SOCK_STREAM` work as expected on all platforms. (Contributed by Yury Selivanov in bpo-32331.)

- On Windows the default for the close_fds argument of `subprocess.Popen` was changed from `False` to `True` when redirecting the standard handles. If you previously depended on handles being inherited when using `subprocess.Popen` with standard io redirection, you will have to pass `close_fds=False` to preserve the previous behaviour, or use `STARTUPINFO.lpAttributeList`.

- `importlib.machinery.PathFinder.invalidate_caches()` -- which implicitly affects `importlib.invalidate_caches()` -- now deletes entries in `sys.path_importer_cache` which are set to `None`. (Contributed by Brett Cannon in bpo-33169.)

- In `asyncio`, `loop.sock_recv()`, `loop.sock_sendall()`, `loop.sock_accept()`, `loop.getaddrinfo()`, `loop.getnameinfo()` have been changed to be proper coroutine methods to match their documentation. Previously, these methods returned `asyncio.Future` instances. (Contributed by Yury Selivanov in bpo-32327.)

- `asyncio.Server.sockets` now returns a copy of the internal list of server sockets, instead of returning it directly. (Contributed by Yury Selivanov in bpo-32662.)

- `Struct.format` is now a `str` instance instead of a `bytes` instance. (Contributed by Victor Stinner in bpo-21071.)

- `ast.literal_eval()` is now stricter. Addition and subtraction of arbitrary numbers are no longer allowed. (Contributed by Serhiy Storchaka in bpo-31778.)

- `Calendar.itermonthdates` will now consistently raise an exception when a date falls outside of the `0001-01-01` through `9999-12-31` range. To support applications that cannot tolerate such exceptions, the new `Calendar.itermonthdays3` and `Calendar.itermonthdays4` can be used. The new methods return tuples and are not restricted by the range supported by `datetime.date`. (Contributed by Alexander Belopolsky in bpo-28292.)

- `collections.ChainMap` now preserves the order of the underlying mappings. (Contributed by Raymond Hettinger in bpo-32792.)

- The `submit()` method of `concurrent.futures.ThreadPoolExecutor` and `concurrent.futures.ProcessPoolExecutor` now raises a `RuntimeError` if called during interpreter shutdown. (Contributed by Mark Nemec in bpo-33097.)

- The `configparser.ConfigParser` constructor now uses `read_dict()` to process the default values, making its behavior consistent with the rest of the parser. Non-string keys and values in the defaults dictionary are now being implicitly converted to strings. (Contributed by James Tocknell in bpo-23835.)

- Several undocumented internal imports were removed. One example is that `os.errno` is no longer available; use `import errno` directly instead. Note that such undocumented internal imports may be removed any time without notice, even in micro version releases.

### Changes in the C API

The function `PySlice_GetIndicesEx()` is considered unsafe for resizable sequences. If the slice indices are not instances of `int`, but objects that implement the `__index__()` method, the sequence can be resized after passing its length to `PySlice_GetIndicesEx()`. This can lead to returning indices out of the length of the sequence. For avoiding possible problems use new functions `PySlice_Unpack()` and `PySlice_AdjustIndices()`. (Contributed by Serhiy Storchaka in bpo-27867.)

### CPython bytecode changes

There are two new opcodes: `LOAD_METHOD` and `CALL_METHOD`. (Contributed by Yury Selivanov and INADA Naoki in bpo-26110.)

The `STORE_ANNOTATION` opcode has been removed. (Contributed by Mark Shannon in bpo-32550.)

### Windows-only Changes

The file used to override `sys.path` is now called `<python-executable>._pth` instead of `'sys.path'`. See Finding modules for more information. (Contributed by Steve Dower in bpo-28137.)

### Other CPython implementation changes

In preparation for potential future changes to the public CPython runtime initialization API (see PEP 432 for an initial, but somewhat outdated, draft), CPython's internal startup and configuration management logic has been significantly refactored. While these updates are intended to be entirely transparent to both embedding applications and users of the regular CPython CLI, they're being mentioned here as the refactoring changes the internal order of various operations during interpreter startup, and hence may uncover previously latent defects, either in embedding applications, or in CPython itself. (Initially contributed by Nick Coghlan and Eric Snow as part of bpo-22257, and further updated by Nick, Eric, and Victor Stinner in a number of other issues). Some known details affected:

- `PySys_AddWarnOptionUnicode()` is not currently usable by embedding applications due to the requirement to create a Unicode object prior to calling Py_Initialize. Use `PySys_AddWarnOption()` instead.
- warnings filters added by an embedding application with `PySys_AddWarnOption()` should now more consistently take precedence over the default filters set by the interpreter

Due to changes in the way the default warnings filters are configured, setting `Py_BytesWarningFlag` to a value greater than one is no longer sufficient to both emit `BytesWarning` messages and have them converted to exceptions. Instead, the flag must be set (to cause the warnings to be emitted in the first place), and an explicit `error::BytesWarning` warnings filter added to convert them to exceptions.

Due to a change in the way docstrings are handled by the compiler, the implicit `return None` in a function body consisting solely of a docstring is now marked as occurring on the same line as the docstring, not on the function's header line.

The current exception state has been moved from the frame object to the co-routine. This simplified the interpreter and fixed a couple of obscure bugs caused by having swap exception state when entering or exiting a generator. (Contributed by Mark Shannon in bpo-25612.)

## Notable changes in Python 3.7.1

Starting in 3.7.1, `Py_Initialize()` now consistently reads and respects all of the same environment settings as `Py_Main()` (in earlier Python versions, it respected an ill-defined subset of those environment variables, while in Python 3.7.0 it didn't read any of them due to bpo-34247). If this behavior is unwanted, set `Py_IgnoreEnvironmentFlag` to 1 before calling `Py_Initialize()`.

In 3.7.1 the C API for Context Variables was updated to use `PyObject` pointers. See also bpo-34762.

In 3.7.1 the `tokenize` module now implicitly emits a `NEWLINE` token when provided with input that does not have a trailing new line. This behavior now matches what the C tokenizer does internally. (Contributed by Ammar Askar in bpo-33899.)

## Notable changes in Python 3.7.2

In 3.7.2, `venv` on Windows no longer copies the original binaries, but creates redirector scripts named `python.exe` and `pythonw.exe` instead. This resolves a long standing issue where all virtual environments would have to be upgraded or recreated with each Python update. However, note that this release will still require recreation of virtual environments in order to get the new scripts.