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# 15.2. `io` — Core tools for working with streams

2.6 版新加入.

The `io` module provides the Python interfaces to stream handling. Under Python 2.x, this is proposed as an alternative to the built-in `file` object, but in Python 3.x it is the default interface to access files and streams.

備註

Since this module has been designed primarily for Python 3.x, you have to be aware that all uses of 「bytes」 in this document refer to the `str` type (of which `bytes` is an alias), and all uses of 「text」 refer to the `unicode` type. Furthermore, those two types are not interchangeable in the `io` APIs.

At the top of the I/O hierarchy is the abstract base class `IOBase`. It defines the basic interface to a stream. Note, however, that there is no separation between reading and writing to streams; implementations are allowed to raise an `IOError` if they do not support a given operation.

Extending `IOBase` is `RawIOBase` which deals simply with the reading and writing of raw bytes to a stream. `FileIO` subclasses `RawIOBase` to provide an interface to files in the machine’s file system.

`BufferedIOBase` deals with buffering on a raw byte stream (`RawIOBase`). Its subclasses, `BufferedWriter`, `BufferedReader`, and `BufferedRWPair` buffer streams that are readable, writable, and both readable and writable. `BufferedRandom` provides a buffered interface to random access streams. `BytesIO` is a simple stream of in-memory bytes.

Another `IOBase` subclass, `TextIOBase`, deals with streams whose bytes represent text, and handles encoding and decoding from and to `unicode` strings. `TextIOWrapper`, which extends it, is a buffered text interface to a buffered raw stream (`BufferedIOBase`). Finally, `StringIO` is an in-memory stream for unicode text.

Argument names are not part of the specification, and only the arguments of `open()` are intended to be used as keyword arguments.

## 15.2.1. Module Interface

`io.``DEFAULT_BUFFER_SIZE`

  An int containing the default buffer size used by the module’s buffered I/O classes. `open()` uses the file’s blksize (as obtained by `os.stat()`) if possible.

`io.``open`(file, mode='r', buffering=-1, encoding=None, errors=None, newline=None, closefd=True)

  Open file and return a corresponding stream. If the file cannot be opened, an `IOError` is raised.

  file is either a string giving the pathname (absolute or relative to the current working directory) of the file to be opened or an integer file descriptor of the file to be wrapped. (If a file descriptor is given, it is closed when the returned I/O object is closed, unless closefd is set to `False`.)

  mode is an optional string that specifies the mode in which the file is opened. It defaults to `'r'` which means open for reading in text mode. Other common values are `'w'` for writing (truncating the file if it already exists), and `'a'` for appending (which on some Unix systems, means that all writes append to the end of the file regardless of the current seek position). In text mode, if encoding is not specified the encoding used is platform dependent. (For reading and writing raw bytes use binary mode and leave encoding unspecified.) The available modes are:

  Character | Meaning
  `'r'` | open for reading (default)
  `'w'` | open for writing, truncating the file first
  `'a'` | open for writing, appending to the end of the file if it exists
  `'b'` | binary mode
  `'t'` | text mode (default)
  `'+'` | open a disk file for updating (reading and writing)
  `'U'` | universal newlines mode (for backwards compatibility; should not be used in new code)

  The default mode is `'rt'` (open for reading text). For binary random access, the mode `'w+b'` opens and truncates the file to 0 bytes, while `'r+b'` opens the file without truncation.

  Python distinguishes between files opened in binary and text modes, even when the underlying operating system doesn’t. Files opened in binary mode (including `'b'` in the mode argument) return contents as `bytes` objects without any decoding. In text mode (the default, or when `'t'` is included in the mode argument), the contents of the file are returned as `unicode` strings, the bytes having been first decoded using a platform-dependent encoding or using the specified encoding if given.

  buffering is an optional integer used to set the buffering policy. Pass 0 to switch buffering off (only allowed in binary mode), 1 to select line buffering (only usable in text mode), and an integer > 1 to indicate the size of a fixed-size chunk buffer. When no buffering argument is given, the default buffering policy works as follows:

  - Binary files are buffered in fixed-size chunks; the size of the buffer is chosen using a heuristic trying to determine the underlying device’s 「block size」 and falling back on `DEFAULT_BUFFER_SIZE`. On many systems, the buffer will typically be 4096 or 8192 bytes long.

  - 「Interactive」 text files (files for which `isatty()` returns True) use line buffering. Other text files use the policy described above for binary files.

  encoding is the name of the encoding used to decode or encode the file. This should only be used in text mode. The default encoding is platform dependent (whatever `locale.getpreferredencoding()` returns), but any encoding supported by Python can be used. See the `codecs` module for the list of supported encodings.

  errors is an optional string that specifies how encoding and decoding errors are to be handled—this cannot be used in binary mode. Pass `'strict'` to raise a `ValueError` exception if there is an encoding error (the default of `None` has the same effect), or pass `'ignore'` to ignore errors. (Note that ignoring encoding errors can lead to data loss.) `'replace'` causes a replacement marker (such as `'?'`) to be inserted where there is malformed data. When writing, `'xmlcharrefreplace'` (replace with the appropriate XML character reference) or `'backslashreplace'` (replace with backslashed escape sequences) can be used. Any other error handling name that has been registered with `codecs.register_error()` is also valid.

  newline controls how universal newlines works (it only applies to text mode). It can be `None`, `''`, `'\n'`, `'\r'`, and `'\r\n'`. It works as follows:

  - On input, if newline is `None`, universal newlines mode is enabled. Lines in the input can end in `'\n'`, `'\r'`, or `'\r\n'`, and these are translated into `'\n'` before being returned to the caller. If it is `''`, universal newlines mode is enabled, but line endings are returned to the caller untranslated. If it has any of the other legal values, input lines are only terminated by the given string, and the line ending is returned to the caller untranslated.

  - On output, if newline is `None`, any `'\n'` characters written are translated to the system default line separator, `os.linesep`. If newline is `''`, no translation takes place. If newline is any of the other legal values, any `'\n'` characters written are translated to the given string.

  If closefd is `False` and a file descriptor rather than a filename was given, the underlying file descriptor will be kept open when the file is closed. If a filename is given closefd has no effect and must be `True` (the default).

  The type of file object returned by the `open()` function depends on the mode. When `open()` is used to open a file in a text mode (`'w'`, `'r'`, `'wt'`, `'rt'`, etc.), it returns a subclass of `TextIOBase` (specifically `TextIOWrapper`). When used to open a file in a binary mode with buffering, the returned class is a subclass of `BufferedIOBase`. The exact class varies: in read binary mode, it returns a `BufferedReader`; in write binary and append binary modes, it returns a `BufferedWriter`, and in read/write mode, it returns a `BufferedRandom`. When buffering is disabled, the raw stream, a subclass of `RawIOBase`, `FileIO`, is returned.

  It is also possible to use an `unicode` or `bytes` string as a file for both reading and writing. For `unicode` strings `StringIO` can be used like a file opened in text mode, and for `bytes` a `BytesIO` can be used like a file opened in a binary mode.

exception `io.``BlockingIOError`

  Error raised when blocking would occur on a non-blocking stream. It inherits `IOError`.

  In addition to those of `IOError`, `BlockingIOError` has one attribute:

  `characters_written`

    An integer containing the number of characters written to the stream before it blocked.

exception `io.``UnsupportedOperation`

  An exception inheriting `IOError` and `ValueError` that is raised when an unsupported operation is called on a stream.

## 15.2.2. I/O Base Classes

class `io.``IOBase`

  The abstract base class for all I/O classes, acting on streams of bytes. There is no public constructor.

  This class provides empty abstract implementations for many methods that derived classes can override selectively; the default implementations represent a file that cannot be read, written or seeked.

  Even though `IOBase` does not declare `read()`, `readinto()`, or `write()` because their signatures will vary, implementations and clients should consider those methods part of the interface. Also, implementations may raise an `IOError` when operations they do not support are called.

  The basic type used for binary data read from or written to a file is `bytes` (also known as `str`). Method arguments may also be `bytearray` or `memoryview` of arrays of bytes. In some cases, such as `readinto()`, a writable object such as `bytearray` is required. Text I/O classes work with `unicode` data.

  2.7 版更變: Implementations should support `memoryview` arguments.

  Note that calling any method (even inquiries) on a closed stream is undefined. Implementations may raise `IOError` in this case.

  IOBase (and its subclasses) support the iterator protocol, meaning that an `IOBase` object can be iterated over yielding the lines in a stream. Lines are defined slightly differently depending on whether the stream is a binary stream (yielding `bytes`), or a text stream (yielding `unicode` strings). See `readline()` below.

  IOBase is also a context manager and therefore supports the `with` statement. In this example, file is closed after the `with` statement’s suite is finished—even if an exception occurs:

      with io.open('spam.txt', 'w') as file:
          file.write(u'Spam and eggs!')

  `IOBase` provides these data attributes and methods:

  `close`()

    Flush and close this stream. This method has no effect if the file is already closed. Once the file is closed, any operation on the file (e.g. reading or writing) will raise a `ValueError`.

    As a convenience, it is allowed to call this method more than once; only the first call, however, will have an effect.

  `closed`

    True if the stream is closed.

  `fileno`()

    Return the underlying file descriptor (an integer) of the stream if it exists. An `IOError` is raised if the IO object does not use a file descriptor.

  `flush`()

    Flush the write buffers of the stream if applicable. This does nothing for read-only and non-blocking streams.

  `isatty`()

    Return `True` if the stream is interactive (i.e., connected to a terminal/tty device).

  `readable`()

    Return `True` if the stream can be read from. If `False`, `read()` will raise `IOError`.

  `readline`(limit=-1)

    Read and return one line from the stream. If limit is specified, at most limit bytes will be read.

    The line terminator is always `b'\n'` for binary files; for text files, the newline argument to `open()` can be used to select the line terminator(s) recognized.

  `readlines`(hint=-1)

    Read and return a list of lines from the stream. hint can be specified to control the number of lines read: no more lines will be read if the total size (in bytes/characters) of all lines so far exceeds hint.

    Note that it’s already possible to iterate on file objects using `for line in file: ...` without calling `file.readlines()`.

  `seek`(offset, whence=SEEK_SET)

    Change the stream position to the given byte offset. offset is interpreted relative to the position indicated by whence. The default value for whence is `SEEK_SET`. Values for whence are:

    - `SEEK_SET` or `0` – start of the stream (the default); offset should be zero or positive

    - `SEEK_CUR` or `1` – current stream position; offset may be negative

    - `SEEK_END` or `2` – end of the stream; offset is usually negative

    Return the new absolute position.

    2.7 版新加入: The `SEEK_*` constants

  `seekable`()

    Return `True` if the stream supports random access. If `False`, `seek()`, `tell()` and `truncate()` will raise `IOError`.

  `tell`()

    Return the current stream position.

  `truncate`(size=None)

    Resize the stream to the given size in bytes (or the current position if size is not specified). The current stream position isn’t changed. This resizing can extend or reduce the current file size. In case of extension, the contents of the new file area depend on the platform (on most systems, additional bytes are zero-filled, on Windows they’re undetermined). The new file size is returned.

  `writable`()

    Return `True` if the stream supports writing. If `False`, `write()` and `truncate()` will raise `IOError`.

  `writelines`(lines)

    Write a list of lines to the stream. Line separators are not added, so it is usual for each of the lines provided to have a line separator at the end.

  `__del__`()

    Prepare for object destruction. `IOBase` provides a default implementation of this method that calls the instance’s `close()` method.

class `io.``RawIOBase`

  Base class for raw binary I/O. It inherits `IOBase`. There is no public constructor.

  Raw binary I/O typically provides low-level access to an underlying OS device or API, and does not try to encapsulate it in high-level primitives (this is left to Buffered I/O and Text I/O, described later in this page).

  In addition to the attributes and methods from `IOBase`, RawIOBase provides the following methods:

  `read`(n=-1)

    Read up to n bytes from the object and return them. As a convenience, if n is unspecified or -1, `readall()` is called. Otherwise, only one system call is ever made. Fewer than n bytes may be returned if the operating system call returns fewer than n bytes.

    If 0 bytes are returned, and n was not 0, this indicates end of file. If the object is in non-blocking mode and no bytes are available, `None` is returned.

  `readall`()

    Read and return all the bytes from the stream until EOF, using multiple calls to the stream if necessary.

  `readinto`(b)

    Read up to len(b) bytes into b, and return the number of bytes read. The object b should be a pre-allocated, writable array of bytes, either `bytearray` or `memoryview`. If the object is in non-blocking mode and no bytes are available, `None` is returned.

  `write`(b)

    Write b to the underlying raw stream, and return the number of bytes written. The object b should be an array of bytes, either `bytes`, `bytearray`, or `memoryview`. The return value can be less than `len(b)`, depending on specifics of the underlying raw stream, and especially if it is in non-blocking mode. `None` is returned if the raw stream is set not to block and no single byte could be readily written to it. The caller may release or mutate b after this method returns, so the implementation should only access b during the method call.

class `io.``BufferedIOBase`

  Base class for binary streams that support some kind of buffering. It inherits `IOBase`. There is no public constructor.

  The main difference with `RawIOBase` is that methods `read()`, `readinto()` and `write()` will try (respectively) to read as much input as requested or to consume all given output, at the expense of making perhaps more than one system call.

  In addition, those methods can raise `BlockingIOError` if the underlying raw stream is in non-blocking mode and cannot take or give enough data; unlike their `RawIOBase` counterparts, they will never return `None`.

  Besides, the `read()` method does not have a default implementation that defers to `readinto()`.

  A typical `BufferedIOBase` implementation should not inherit from a `RawIOBase` implementation, but wrap one, like `BufferedWriter` and `BufferedReader` do.

  `BufferedIOBase` provides or overrides these methods and attribute in addition to those from `IOBase`:

  `raw`

    The underlying raw stream (a `RawIOBase` instance) that `BufferedIOBase` deals with. This is not part of the `BufferedIOBase` API and may not exist on some implementations.

  `detach`()

    Separate the underlying raw stream from the buffer and return it.

    After the raw stream has been detached, the buffer is in an unusable state.

    Some buffers, like `BytesIO`, do not have the concept of a single raw stream to return from this method. They raise `UnsupportedOperation`.

    2.7 版新加入.

  `read`(n=-1)

    Read and return up to n bytes. If the argument is omitted, `None`, or negative, data is read and returned until EOF is reached. An empty bytes object is returned if the stream is already at EOF.

    If the argument is positive, and the underlying raw stream is not interactive, multiple raw reads may be issued to satisfy the byte count (unless EOF is reached first). But for interactive raw streams, at most one raw read will be issued, and a short result does not imply that EOF is imminent.

    A `BlockingIOError` is raised if the underlying raw stream is in non blocking-mode, and has no data available at the moment.

  `read1`(n=-1)

    Read and return up to n bytes, with at most one call to the underlying raw stream’s `read()` method. This can be useful if you are implementing your own buffering on top of a `BufferedIOBase` object.

  `readinto`(b)

    Read up to len(b) bytes into b, and return the number of bytes read. The object b should be a pre-allocated, writable array of bytes, either `bytearray` or `memoryview`.

    Like `read()`, multiple reads may be issued to the underlying raw stream, unless the latter is 『interactive』.

    A `BlockingIOError` is raised if the underlying raw stream is in non blocking-mode, and has no data available at the moment.

  `write`(b)

    Write b, and return the number of bytes written (always equal to `len(b)`, since if the write fails an `IOError` will be raised). The object b should be an array of bytes, either `bytes`, `bytearray`, or `memoryview`. Depending on the actual implementation, these bytes may be readily written to the underlying stream, or held in a buffer for performance and latency reasons.

    When in non-blocking mode, a `BlockingIOError` is raised if the data needed to be written to the raw stream but it couldn’t accept all the data without blocking.

    The caller may release or mutate b after this method returns, so the implementation should only access b during the method call.

## 15.2.3. Raw File I/O

class `io.``FileIO`(name, mode='r', closefd=True)

  `FileIO` represents an OS-level file containing bytes data. It implements the `RawIOBase` interface (and therefore the `IOBase` interface, too).

  The name can be one of two things:

  - a string representing the path to the file which will be opened;

  - an integer representing the number of an existing OS-level file descriptor to which the resulting `FileIO` object will give access.

  The mode can be `'r'`, `'w'` or `'a'` for reading (default), writing, or appending. The file will be created if it doesn’t exist when opened for writing or appending; it will be truncated when opened for writing. Add a `'+'` to the mode to allow simultaneous reading and writing.

  The `read()` (when called with a positive argument), `readinto()` and `write()` methods on this class will only make one system call.

  In addition to the attributes and methods from `IOBase` and `RawIOBase`, `FileIO` provides the following data attributes and methods:

  `mode`

    The mode as given in the constructor.

  `name`

    The file name. This is the file descriptor of the file when no name is given in the constructor.

## 15.2.4. Buffered Streams

Buffered I/O streams provide a higher-level interface to an I/O device than raw I/O does.

class `io.``BytesIO`([initial_bytes])

  A stream implementation using an in-memory bytes buffer. It inherits `BufferedIOBase`.

  The optional argument initial_bytes is a `bytes` object that contains initial data.

  `BytesIO` provides or overrides these methods in addition to those from `BufferedIOBase` and `IOBase`:

  `getvalue`()

    Return `bytes` containing the entire contents of the buffer.

  `read1`()

    In `BytesIO`, this is the same as `read()`.

class `io.``BufferedReader`(raw, buffer_size=DEFAULT_BUFFER_SIZE)

  A buffer providing higher-level access to a readable, sequential `RawIOBase` object. It inherits `BufferedIOBase`. When reading data from this object, a larger amount of data may be requested from the underlying raw stream, and kept in an internal buffer. The buffered data can then be returned directly on subsequent reads.

  The constructor creates a `BufferedReader` for the given readable raw stream and buffer_size. If buffer_size is omitted, `DEFAULT_BUFFER_SIZE` is used.

  `BufferedReader` provides or overrides these methods in addition to those from `BufferedIOBase` and `IOBase`:

  `peek`([n])

    Return bytes from the stream without advancing the position. At most one single read on the raw stream is done to satisfy the call. The number of bytes returned may be less or more than requested.

  `read`([n])

    Read and return n bytes, or if n is not given or negative, until EOF or if the read call would block in non-blocking mode.

  `read1`(n)

    Read and return up to n bytes with only one call on the raw stream. If at least one byte is buffered, only buffered bytes are returned. Otherwise, one raw stream read call is made.

class `io.``BufferedWriter`(raw, buffer_size=DEFAULT_BUFFER_SIZE)

  A buffer providing higher-level access to a writeable, sequential `RawIOBase` object. It inherits `BufferedIOBase`. When writing to this object, data is normally held into an internal buffer. The buffer will be written out to the underlying `RawIOBase` object under various conditions, including:

  - when the buffer gets too small for all pending data;

  - when `flush()` is called;

  - when a `seek()` is requested (for `BufferedRandom` objects);

  - when the `BufferedWriter` object is closed or destroyed.

  The constructor creates a `BufferedWriter` for the given writeable raw stream. If the buffer_size is not given, it defaults to `DEFAULT_BUFFER_SIZE`.

  A third argument, max_buffer_size, is supported, but unused and deprecated.

  `BufferedWriter` provides or overrides these methods in addition to those from `BufferedIOBase` and `IOBase`:

  `flush`()

    Force bytes held in the buffer into the raw stream. A `BlockingIOError` should be raised if the raw stream blocks.

  `write`(b)

    Write b, and return the number of bytes written. The object b should be an array of bytes, either `bytes`, `bytearray`, or `memoryview`. When in non-blocking mode, a `BlockingIOError` is raised if the buffer needs to be written out but the raw stream blocks.

class `io.``BufferedRandom`(raw, buffer_size=DEFAULT_BUFFER_SIZE)

  A buffered interface to random access streams. It inherits `BufferedReader` and `BufferedWriter`, and further supports `seek()` and `tell()` functionality.

  The constructor creates a reader and writer for a seekable raw stream, given in the first argument. If the buffer_size is omitted it defaults to `DEFAULT_BUFFER_SIZE`.

  A third argument, max_buffer_size, is supported, but unused and deprecated.

  `BufferedRandom` is capable of anything `BufferedReader` or `BufferedWriter` can do.

class `io.``BufferedRWPair`(reader, writer, buffer_size=DEFAULT_BUFFER_SIZE)

  A buffered I/O object combining two unidirectional `RawIOBase` objects – one readable, the other writeable – into a single bidirectional endpoint. It inherits `BufferedIOBase`.

  reader and writer are `RawIOBase` objects that are readable and writeable respectively. If the buffer_size is omitted it defaults to `DEFAULT_BUFFER_SIZE`.

  A fourth argument, max_buffer_size, is supported, but unused and deprecated.

  `BufferedRWPair` implements all of `BufferedIOBase`’s methods except for `detach()`, which raises `UnsupportedOperation`.

  警告

  `BufferedRWPair` does not attempt to synchronize accesses to its underlying raw streams. You should not pass it the same object as reader and writer; use `BufferedRandom` instead.

## 15.2.5. Text I/O

class `io.``TextIOBase`

  Base class for text streams. This class provides a unicode character and line based interface to stream I/O. There is no `readinto()` method because Python’s `unicode` strings are immutable. It inherits `IOBase`. There is no public constructor.

  `TextIOBase` provides or overrides these data attributes and methods in addition to those from `IOBase`:

  `encoding`

    The name of the encoding used to decode the stream’s bytes into strings, and to encode strings into bytes.

  `errors`

    The error setting of the decoder or encoder.

  `newlines`

    A string, a tuple of strings, or `None`, indicating the newlines translated so far. Depending on the implementation and the initial constructor flags, this may not be available.

  `buffer`

    The underlying binary buffer (a `BufferedIOBase` instance) that `TextIOBase` deals with. This is not part of the `TextIOBase` API and may not exist on some implementations.

  `detach`()

    Separate the underlying binary buffer from the `TextIOBase` and return it.

    After the underlying buffer has been detached, the `TextIOBase` is in an unusable state.

    Some `TextIOBase` implementations, like `StringIO`, may not have the concept of an underlying buffer and calling this method will raise `UnsupportedOperation`.

    2.7 版新加入.

  `read`(n=-1)

    Read and return at most n characters from the stream as a single `unicode`. If n is negative or `None`, reads until EOF.

  `readline`(limit=-1)

    Read until newline or EOF and return a single `unicode`. If the stream is already at EOF, an empty string is returned.

    If limit is specified, at most limit characters will be read.

  `seek`(offset, whence=SEEK_SET)

    Change the stream position to the given offset. Behaviour depends on the whence parameter. The default value for whence is `SEEK_SET`.

    - `SEEK_SET` or `0`: seek from the start of the stream (the default); offset must either be a number returned by `TextIOBase.tell()`, or zero. Any other offset value produces undefined behaviour.

    - `SEEK_CUR` or `1`: 「seek」 to the current position; offset must be zero, which is a no-operation (all other values are unsupported).

    - `SEEK_END` or `2`: seek to the end of the stream; offset must be zero (all other values are unsupported).

    Return the new absolute position as an opaque number.

    2.7 版新加入: The `SEEK_*` constants.

  `tell`()

    Return the current stream position as an opaque number. The number does not usually represent a number of bytes in the underlying binary storage.

  `write`(s)

    Write the `unicode` string s to the stream and return the number of characters written.

class `io.``TextIOWrapper`(buffer, encoding=None, errors=None, newline=None, line_buffering=False)

  A buffered text stream over a `BufferedIOBase` binary stream. It inherits `TextIOBase`.

  encoding gives the name of the encoding that the stream will be decoded or encoded with. It defaults to `locale.getpreferredencoding()`.

  errors is an optional string that specifies how encoding and decoding errors are to be handled. Pass `'strict'` to raise a `ValueError` exception if there is an encoding error (the default of `None` has the same effect), or pass `'ignore'` to ignore errors. (Note that ignoring encoding errors can lead to data loss.) `'replace'` causes a replacement marker (such as `'?'`) to be inserted where there is malformed data. When writing, `'xmlcharrefreplace'` (replace with the appropriate XML character reference) or `'backslashreplace'` (replace with backslashed escape sequences) can be used. Any other error handling name that has been registered with `codecs.register_error()` is also valid.

  newline controls how line endings are handled. It can be `None`, `''`, `'\n'`, `'\r'`, and `'\r\n'`. It works as follows:

  - On input, if newline is `None`, universal newlines mode is enabled. Lines in the input can end in `'\n'`, `'\r'`, or `'\r\n'`, and these are translated into `'\n'` before being returned to the caller. If it is `''`, universal newlines mode is enabled, but line endings are returned to the caller untranslated. If it has any of the other legal values, input lines are only terminated by the given string, and the line ending is returned to the caller untranslated.

  - On output, if newline is `None`, any `'\n'` characters written are translated to the system default line separator, `os.linesep`. If newline is `''`, no translation takes place. If newline is any of the other legal values, any `'\n'` characters written are translated to the given string.

  If line_buffering is `True`, `flush()` is implied when a call to write contains a newline character or a carriage return.

  `TextIOWrapper` provides one attribute in addition to those of `TextIOBase` and its parents:

  `line_buffering`

    Whether line buffering is enabled.

class `io.``StringIO`(initial_value=u'', newline=u'\n')

  An in-memory stream for unicode text. It inherits `TextIOWrapper`.

  The initial value of the buffer can be set by providing initial_value. If newline translation is enabled, newlines will be encoded as if by `write()`. The stream is positioned at the start of the buffer.

  The newline argument works like that of `TextIOWrapper`. The default is to consider only `\n` characters as ends of lines and to do no newline translation. If newline is set to `None`, newlines are written as `\n` on all platforms, but universal newline decoding is still performed when reading.

  `StringIO` provides this method in addition to those from `TextIOWrapper` and its parents:

  `getvalue`()

    Return a `unicode` containing the entire contents of the buffer at any time before the `StringIO` object’s `close()` method is called. Newlines are decoded as if by `read()`, although the stream position is not changed.

  Example usage:

      import io

      output = io.StringIO()
      output.write(u'First line.\n')
      output.write(u'Second line.\n')

      # Retrieve file contents -- this will be
      # u'First line.\nSecond line.\n'
      contents = output.getvalue()

      # Close object and discard memory buffer --
      # .getvalue() will now raise an exception.
      output.close()

class `io.``IncrementalNewlineDecoder`

  A helper codec that decodes newlines for universal newlines mode. It inherits `codecs.IncrementalDecoder`.

## 15.2.6. Advanced topics

Here we will discuss several advanced topics pertaining to the concrete I/O implementations described above.

### 15.2.6.1. Performance

#### 15.2.6.1.1. Binary I/O

By reading and writing only large chunks of data even when the user asks for a single byte, buffered I/O is designed to hide any inefficiency in calling and executing the operating system’s unbuffered I/O routines. The gain will vary very much depending on the OS and the kind of I/O which is performed (for example, on some contemporary OSes such as Linux, unbuffered disk I/O can be as fast as buffered I/O). The bottom line, however, is that buffered I/O will offer you predictable performance regardless of the platform and the backing device. Therefore, it is most always preferable to use buffered I/O rather than unbuffered I/O.

#### 15.2.6.1.2. Text I/O

Text I/O over a binary storage (such as a file) is significantly slower than binary I/O over the same storage, because it implies conversions from unicode to binary data using a character codec. This can become noticeable if you handle huge amounts of text data (for example very large log files). Also, `TextIOWrapper.tell()` and `TextIOWrapper.seek()` are both quite slow due to the reconstruction algorithm used.

`StringIO`, however, is a native in-memory unicode container and will exhibit similar speed to `BytesIO`.

### 15.2.6.2. Multi-threading

`FileIO` objects are thread-safe to the extent that the operating system calls (such as `read(2)` under Unix) they are wrapping are thread-safe too.

Binary buffered objects (instances of `BufferedReader`, `BufferedWriter`, `BufferedRandom` and `BufferedRWPair`) protect their internal structures using a lock; it is therefore safe to call them from multiple threads at once.

`TextIOWrapper` objects are not thread-safe.

### 15.2.6.3. Reentrancy

Binary buffered objects (instances of `BufferedReader`, `BufferedWriter`, `BufferedRandom` and `BufferedRWPair`) are not reentrant. While reentrant calls will not happen in normal situations, they can arise if you are doing I/O in a `signal` handler. If it is attempted to enter a buffered object again while already being accessed from the same thread, then a `RuntimeError` is raised.

The above implicitly extends to text files, since the `open()` function will wrap a buffered object inside a `TextIOWrapper`. This includes standard streams and therefore affects the built-in function `print()` as well.