# 5. O sistema de importação

O código Python em um módulo obtém acesso ao código em outro módulo pelo processo de importação dele. A instrução `import` é a maneira mais comum de chamar o mecanismo de importação, mas não é a única maneira. Funções como `importlib.import_module()` e a embutida `__import__()` também podem ser usadas para chamar o mecanismo de importação.

The `import` statement combines two operations; it searches for the named module, then it binds the results of that search to a name in the local scope. The search operation of the `import` statement is defined as a call to the `__import__()` function, with the appropriate arguments. The return value of `__import__()` is used to perform the name binding operation of the `import` statement. See the `import` statement for the exact details of that name binding operation.

Uma chamada direta para `__import__()` realiza apenas a pesquisa do módulo e, se encontrada, a operação de criação do módulo. Embora certos efeitos colaterais possam ocorrer, como a importação de pacotes pai e a atualização de vários caches (incluindo `sys.modules`), apenas a instrução `import` realiza uma operação de ligação de nome.

When calling `__import__()` as part of an import statement, the standard builtin `__import__()` is called. Other mechanisms for invoking the import system (such as `importlib.import_module()`) may choose to subvert `__import__()` and use its own solution to implement import semantics.

Quando um módulo é importado pela primeira vez, o Python procura pelo módulo e, se encontrado, cria um objeto de módulo 1, inicializando-o. Se o módulo nomeado não puder ser encontrado, uma `ModuleNotFoundError` será levantada. O Python implementa várias estratégias para procurar o módulo nomeado quando o mecanismo de importação é chamado. Essas estratégias podem ser modificadas e estendidas usando vários ganchos descritos nas seções abaixo.

Alterado na versão 3.3: O sistema de importação foi atualizado para implementar completamente a segunda fase da PEP 302. Não há mais um mecanismo de importação implícito – o sistema completo de importação é exposto através de `sys.meta_path`. Além disso, o suporte nativo a pacote de espaço de nomes foi implementado (consulte PEP 420).

## 5.1. `importlib`

O módulo `importlib` fornece uma API rica para interagir com o sistema de importação. Por exemplo `importlib.import_module()` fornece uma API mais simples e recomendada do que a embutida `__import__()` para chamar o mecanismo de importação. Consulte a documentação da biblioteca `importlib` para obter detalhes adicionais.

## 5.2. Pacotes

O Python possui apenas um tipo de objeto de módulo e todos os módulos são desse tipo, independentemente de o módulo estar implementado em Python, C ou qualquer outra coisa. Para ajudar a organizar os módulos e fornecer uma hierarquia de nomes, o Python tem o conceito de pacotes.

Você pode pensar em pacotes como os diretórios em um sistema de arquivos e os módulos como arquivos nos diretórios, mas não tome essa analogia muito literalmente, já que pacotes e módulos não precisam se originar do sistema de arquivos. Para os fins desta documentação, usaremos essa analogia conveniente de diretórios e arquivos. Como os diretórios do sistema de arquivos, os pacotes são organizados hierarquicamente e os próprios pacotes podem conter subpacotes e módulos regulares.

É importante ter em mente que todos os pacotes são módulos, mas nem todos os módulos são pacotes. Ou, dito de outra forma, os pacotes são apenas um tipo especial de módulo. Especificamente, qualquer módulo que contenha um atributo `__path__` é considerado um pacote.

Todos os módulos têm um nome. Os nomes dos subpacotes são separados do nome do pacote pai por pontos, semelhante à sintaxe de acesso aos atributos padrão do Python. Assim, você pode ter um módulo chamado `sys` e um pacote chamado `email`, que por sua vez possui um subpacote chamado `email.mime` e um módulo dentro desse subpacote chamado `email.mime.text`.

### 5.2.1. Pacotes regulares

O Python define dois tipos de pacotes, pacotes regulares e pacotes de espaço de nomes. Pacotes regulares são pacotes tradicionais, como existiam no Python 3.2 e versões anteriores. Um pacote regular é normalmente implementado como um diretório que contém um arquivo `__init__.py`. Quando um pacote regular é importado, esse arquivo `__init__.py` é executado implicitamente, e os objetos que ele define são vinculados aos nomes no espaço de nomes do pacote. O arquivo `__init__.py` pode conter o mesmo código Python que qualquer outro módulo pode conter, e o Python adicionará alguns atributos adicionais ao módulo quando ele for importado.

Por exemplo, o layout do sistema de arquivos a seguir define um pacote `parent` de nível superior com três subpacotes:

    parent/
        __init__.py
        one/
            __init__.py
        two/
            __init__.py
        three/
            __init__.py

A importação de `parent.one` vai executar implicitamente `parent/__init__.py` e `parent/one/__init__.py`. Importações subsequentes de `parent.two` ou `parent.three` vão executar `parent/two/__init__.py` e `parent/three/__init__.py`, respectivamente.

### 5.2.2. Pacotes de espaço de nomes

Um pacote de espaço de nomes é um composto de várias porções, em que cada parte contribui com um subpacote para o pacote pai. Partes podem residir em locais diferentes no sistema de arquivos. Partes também podem ser encontradas em arquivos zip, na rede ou em qualquer outro lugar que o Python pesquisar durante a importação. Os pacotes de espaço de nomes podem ou não corresponder diretamente aos objetos no sistema de arquivos; eles podem ser módulos virtuais que não têm representação concreta.

Os pacotes de espaço de nomes não usam uma lista comum para o atributo `__path__`. Em vez disso, eles usam um tipo iterável personalizado que executará automaticamente uma nova pesquisa por partes do pacote na próxima tentativa de importação dentro desse pacote, se o caminho do pacote pai (ou `sys.path` para um pacote de nível superior) for alterado.

Com pacotes de espaço de nomes, não há arquivo `pai/__init__.py`. De fato, pode haver vários diretórios `pai` encontrados durante a pesquisa de importação, onde cada um é fornecido por uma parte diferente. Portanto, `pai/um` pode não estar fisicamente localizado próximo a `pai/dois`. Nesse caso, o Python criará um pacote de espaço de nomes para o pacote `pai` de nível superior sempre que ele ou um de seus subpacotes for importado.

Veja também PEP 420 para a especificação de pacotes de espaço de nomes.

## 5.3. Searching

To begin the search, Python needs the fully qualified name of the module (or package, but for the purposes of this discussion, the difference is immaterial) being imported. This name may come from various arguments to the `import` statement, or from the parameters to the `importlib.import_module()` or `__import__()` functions.

This name will be used in various phases of the import search, and it may be the dotted path to a submodule, e.g. `foo.bar.baz`. In this case, Python first tries to import `foo`, then `foo.bar`, and finally `foo.bar.baz`. If any of the intermediate imports fail, a `ModuleNotFoundError` is raised.

### 5.3.1. The module cache

The first place checked during import search is `sys.modules`. This mapping serves as a cache of all modules that have been previously imported, including the intermediate paths. So if `foo.bar.baz` was previously imported, `sys.modules` will contain entries for `foo`, `foo.bar`, and `foo.bar.baz`. Each key will have as its value the corresponding module object.

During import, the module name is looked up in `sys.modules` and if present, the associated value is the module satisfying the import, and the process completes. However, if the value is `None`, then a `ModuleNotFoundError` is raised. If the module name is missing, Python will continue searching for the module.

`sys.modules` is writable. Deleting a key may not destroy the associated module (as other modules may hold references to it), but it will invalidate the cache entry for the named module, causing Python to search anew for the named module upon its next import. The key can also be assigned to `None`, forcing the next import of the module to result in a `ModuleNotFoundError`.

Beware though, as if you keep a reference to the module object, invalidate its cache entry in `sys.modules`, and then re-import the named module, the two module objects will not be the same. By contrast, `importlib.reload()` will reuse the same module object, and simply reinitialise the module contents by rerunning the module’s code.

### 5.3.2. Finders and loaders

If the named module is not found in `sys.modules`, then Python’s import protocol is invoked to find and load the module. This protocol consists of two conceptual objects, finders and loaders. A finder’s job is to determine whether it can find the named module using whatever strategy it knows about. Objects that implement both of these interfaces are referred to as importers - they return themselves when they find that they can load the requested module.

Python includes a number of default finders and importers. The first one knows how to locate built-in modules, and the second knows how to locate frozen modules. A third default finder searches an import path for modules. The import path is a list of locations that may name file system paths or zip files. It can also be extended to search for any locatable resource, such as those identified by URLs.

The import machinery is extensible, so new finders can be added to extend the range and scope of module searching.

Finders do not actually load modules. If they can find the named module, they return a module spec, an encapsulation of the module’s import-related information, which the import machinery then uses when loading the module.

The following sections describe the protocol for finders and loaders in more detail, including how you can create and register new ones to extend the import machinery.

Alterado na versão 3.4: In previous versions of Python, finders returned loaders directly, whereas now they return module specs which contain loaders. Loaders are still used during import but have fewer responsibilities.

### 5.3.3. Import hooks

The import machinery is designed to be extensible; the primary mechanism for this are the import hooks. There are two types of import hooks: meta hooks and import path hooks.

Meta hooks are called at the start of import processing, before any other import processing has occurred, other than `sys.modules` cache look up. This allows meta hooks to override `sys.path` processing, frozen modules, or even built-in modules. Meta hooks are registered by adding new finder objects to `sys.meta_path`, as described below.

Import path hooks are called as part of `sys.path` (or `package.__path__`) processing, at the point where their associated path item is encountered. Import path hooks are registered by adding new callables to `sys.path_hooks` as described below.

### 5.3.4. The meta path

When the named module is not found in `sys.modules`, Python next searches `sys.meta_path`, which contains a list of meta path finder objects. These finders are queried in order to see if they know how to handle the named module. Meta path finders must implement a method called `find_spec()` which takes three arguments: a name, an import path, and (optionally) a target module. The meta path finder can use any strategy it wants to determine whether it can handle the named module or not.

If the meta path finder knows how to handle the named module, it returns a spec object. If it cannot handle the named module, it returns `None`. If `sys.meta_path` processing reaches the end of its list without returning a spec, then a `ModuleNotFoundError` is raised. Any other exceptions raised are simply propagated up, aborting the import process.

The `find_spec()` method of meta path finders is called with two or three arguments. The first is the fully qualified name of the module being imported, for example `foo.bar.baz`. The second argument is the path entries to use for the module search. For top-level modules, the second argument is `None`, but for submodules or subpackages, the second argument is the value of the parent package’s `__path__` attribute. If the appropriate `__path__` attribute cannot be accessed, a `ModuleNotFoundError` is raised. The third argument is an existing module object that will be the target of loading later. The import system passes in a target module only during reload.

The meta path may be traversed multiple times for a single import request. For example, assuming none of the modules involved has already been cached, importing `foo.bar.baz` will first perform a top level import, calling `mpf.find_spec("foo", None, None)` on each meta path finder (`mpf`). After `foo` has been imported, `foo.bar` will be imported by traversing the meta path a second time, calling `mpf.find_spec("foo.bar", foo.__path__, None)`. Once `foo.bar` has been imported, the final traversal will call `mpf.find_spec("foo.bar.baz", foo.bar.__path__, None)`.

Some meta path finders only support top level imports. These importers will always return `None` when anything other than `None` is passed as the second argument.

Python’s default `sys.meta_path` has three meta path finders, one that knows how to import built-in modules, one that knows how to import frozen modules, and one that knows how to import modules from an import path (i.e. the path based finder).

Alterado na versão 3.4: The `find_spec()` method of meta path finders replaced `find_module()`, which is now deprecated. While it will continue to work without change, the import machinery will try it only if the finder does not implement `find_spec()`.

## 5.4. Loading

If and when a module spec is found, the import machinery will use it (and the loader it contains) when loading the module. Here is an approximation of what happens during the loading portion of import:

    module = None
    if spec.loader is not None and hasattr(spec.loader, 'create_module'):
        # It is assumed 'exec_module' will also be defined on the loader.
        module = spec.loader.create_module(spec)
    if module is None:
        module = ModuleType(spec.name)
    # The import-related module attributes get set here:
    _init_module_attrs(spec, module)

    if spec.loader is None:
        if spec.submodule_search_locations is not None:
            # namespace package
            sys.modules[spec.name] = module
        else:
            # unsupported
            raise ImportError
    elif not hasattr(spec.loader, 'exec_module'):
        module = spec.loader.load_module(spec.name)
        # Set __loader__ and __package__ if missing.
    else:
        sys.modules[spec.name] = module
        try:
            spec.loader.exec_module(module)
        except BaseException:
            try:
                del sys.modules[spec.name]
            except KeyError:
                pass
            raise
    return sys.modules[spec.name]

Note the following details:

> - If there is an existing module object with the given name in `sys.modules`, import will have already returned it.

> - The module will exist in `sys.modules` before the loader executes the module code. This is crucial because the module code may (directly or indirectly) import itself; adding it to `sys.modules` beforehand prevents unbounded recursion in the worst case and multiple loading in the best.

> - If loading fails, the failing module – and only the failing module – gets removed from `sys.modules`. Any module already in the `sys.modules` cache, and any module that was successfully loaded as a side-effect, must remain in the cache. This contrasts with reloading where even the failing module is left in `sys.modules`.

> - After the module is created but before execution, the import machinery sets the import-related module attributes (“_init_module_attrs” in the pseudo-code example above), as summarized in a later section.

> - Module execution is the key moment of loading in which the module’s namespace gets populated. Execution is entirely delegated to the loader, which gets to decide what gets populated and how.

> - The module created during loading and passed to exec_module() may not be the one returned at the end of import 2.

Alterado na versão 3.4: The import system has taken over the boilerplate responsibilities of loaders. These were previously performed by the `importlib.abc.Loader.load_module()` method.

### 5.4.1. Loaders

Module loaders provide the critical function of loading: module execution. The import machinery calls the `importlib.abc.Loader.exec_module()` method with a single argument, the module object to execute. Any value returned from `exec_module()` is ignored.

Loaders must satisfy the following requirements:

> - If the module is a Python module (as opposed to a built-in module or a dynamically loaded extension), the loader should execute the module’s code in the module’s global name space (`module.__dict__`).

> - If the loader cannot execute the module, it should raise an `ImportError`, although any other exception raised during `exec_module()` will be propagated.

In many cases, the finder and loader can be the same object; in such cases the `find_spec()` method would just return a spec with the loader set to `self`.

Module loaders may opt in to creating the module object during loading by implementing a `create_module()` method. It takes one argument, the module spec, and returns the new module object to use during loading. `create_module()` does not need to set any attributes on the module object. If the method returns `None`, the import machinery will create the new module itself.

Novo na versão 3.4: The `create_module()` method of loaders.

Alterado na versão 3.4: The `load_module()` method was replaced by `exec_module()` and the import machinery assumed all the boilerplate responsibilities of loading.

For compatibility with existing loaders, the import machinery will use the `load_module()` method of loaders if it exists and the loader does not also implement `exec_module()`. However, `load_module()` has been deprecated and loaders should implement `exec_module()` instead.

The `load_module()` method must implement all the boilerplate loading functionality described above in addition to executing the module. All the same constraints apply, with some additional clarification:

> - If there is an existing module object with the given name in `sys.modules`, the loader must use that existing module. (Otherwise, `importlib.reload()` will not work correctly.) If the named module does not exist in `sys.modules`, the loader must create a new module object and add it to `sys.modules`.

> - The module must exist in `sys.modules` before the loader executes the module code, to prevent unbounded recursion or multiple loading.

> - If loading fails, the loader must remove any modules it has inserted into `sys.modules`, but it must remove only the failing module(s), and only if the loader itself has loaded the module(s) explicitly.

Alterado na versão 3.5: A `DeprecationWarning` is raised when `exec_module()` is defined but `create_module()` is not.

Alterado na versão 3.6: An `ImportError` is raised when `exec_module()` is defined but `create_module()` is not.

### 5.4.2. Submódulos

When a submodule is loaded using any mechanism (e.g. `importlib` APIs, the `import` or `import-from` statements, or built-in `__import__()`) a binding is placed in the parent module’s namespace to the submodule object. For example, if package `spam` has a submodule `foo`, after importing `spam.foo`, `spam` will have an attribute `foo` which is bound to the submodule. Let’s say you have the following directory structure:

    spam/
        __init__.py
        foo.py
        bar.py

and `spam/__init__.py` has the following lines in it:

    from .foo import Foo
    from .bar import Bar

then executing the following puts a name binding to `foo` and `bar` in the `spam` module:

    >>> import spam
    >>> spam.foo
    <module 'spam.foo' from '/tmp/imports/spam/foo.py'>
    >>> spam.bar
    <module 'spam.bar' from '/tmp/imports/spam/bar.py'>

Given Python’s familiar name binding rules this might seem surprising, but it’s actually a fundamental feature of the import system. The invariant holding is that if you have `sys.modules['spam']` and `sys.modules['spam.foo']` (as you would after the above import), the latter must appear as the `foo` attribute of the former.

### 5.4.3. Module spec

The import machinery uses a variety of information about each module during import, especially before loading. Most of the information is common to all modules. The purpose of a module’s spec is to encapsulate this import-related information on a per-module basis.

Using a spec during import allows state to be transferred between import system components, e.g. between the finder that creates the module spec and the loader that executes it. Most importantly, it allows the import machinery to perform the boilerplate operations of loading, whereas without a module spec the loader had that responsibility.

The module’s spec is exposed as the `__spec__` attribute on a module object. See `ModuleSpec` for details on the contents of the module spec.

Novo na versão 3.4.

### 5.4.4. Import-related module attributes

The import machinery fills in these attributes on each module object during loading, based on the module’s spec, before the loader executes the module.

`__name__`

  The `__name__` attribute must be set to the fully-qualified name of the module. This name is used to uniquely identify the module in the import system.

`__loader__`

  The `__loader__` attribute must be set to the loader object that the import machinery used when loading the module. This is mostly for introspection, but can be used for additional loader-specific functionality, for example getting data associated with a loader.

`__package__`

  The module’s `__package__` attribute must be set. Its value must be a string, but it can be the same value as its `__name__`. When the module is a package, its `__package__` value should be set to its `__name__`. When the module is not a package, `__package__` should be set to the empty string for top-level modules, or for submodules, to the parent package’s name. See PEP 366 for further details.

  This attribute is used instead of `__name__` to calculate explicit relative imports for main modules, as defined in PEP 366. It is expected to have the same value as `__spec__.parent`.

  Alterado na versão 3.6: The value of `__package__` is expected to be the same as `__spec__.parent`.

`__spec__`

  The `__spec__` attribute must be set to the module spec that was used when importing the module. Setting `__spec__` appropriately applies equally to modules initialized during interpreter startup. The one exception is `__main__`, where `__spec__` is set to None in some cases.

  When `__package__` is not defined, `__spec__.parent` is used as a fallback.

  Novo na versão 3.4.

  Alterado na versão 3.6: `__spec__.parent` is used as a fallback when `__package__` is not defined.

`__path__`

  If the module is a package (either regular or namespace), the module object’s `__path__` attribute must be set. The value must be iterable, but may be empty if `__path__` has no further significance. If `__path__` is not empty, it must produce strings when iterated over. More details on the semantics of `__path__` are given below.

  Non-package modules should not have a `__path__` attribute.

`__file__`

`__cached__`

  `__file__` is optional. If set, this attribute’s value must be a string. The import system may opt to leave `__file__` unset if it has no semantic meaning (e.g. a module loaded from a database).

  If `__file__` is set, it may also be appropriate to set the `__cached__` attribute which is the path to any compiled version of the code (e.g. byte-compiled file). The file does not need to exist to set this attribute; the path can simply point to where the compiled file would exist (see PEP 3147).

  It is also appropriate to set `__cached__` when `__file__` is not set. However, that scenario is quite atypical. Ultimately, the loader is what makes use of `__file__` and/or `__cached__`. So if a loader can load from a cached module but otherwise does not load from a file, that atypical scenario may be appropriate.

### 5.4.5. module.__path__

By definition, if a module has a `__path__` attribute, it is a package.

A package’s `__path__` attribute is used during imports of its subpackages. Within the import machinery, it functions much the same as `sys.path`, i.e. providing a list of locations to search for modules during import. However, `__path__` is typically much more constrained than `sys.path`.

`__path__` must be an iterable of strings, but it may be empty. The same rules used for `sys.path` also apply to a package’s `__path__`, and `sys.path_hooks` (described below) are consulted when traversing a package’s `__path__`.

A package’s `__init__.py` file may set or alter the package’s `__path__` attribute, and this was typically the way namespace packages were implemented prior to PEP 420. With the adoption of PEP 420, namespace packages no longer need to supply `__init__.py` files containing only `__path__` manipulation code; the import machinery automatically sets `__path__` correctly for the namespace package.

### 5.4.6. Module reprs

By default, all modules have a usable repr, however depending on the attributes set above, and in the module’s spec, you can more explicitly control the repr of module objects.

If the module has a spec (`__spec__`), the import machinery will try to generate a repr from it. If that fails or there is no spec, the import system will craft a default repr using whatever information is available on the module. It will try to use the `module.__name__`, `module.__file__`, and `module.__loader__` as input into the repr, with defaults for whatever information is missing.

Here are the exact rules used:

> - If the module has a `__spec__` attribute, the information in the spec is used to generate the repr. The “name”, “loader”, “origin”, and “has_location” attributes are consulted.

> - If the module has a `__file__` attribute, this is used as part of the module’s repr.

> - If the module has no `__file__` but does have a `__loader__` that is not `None`, then the loader’s repr is used as part of the module’s repr.

> - Otherwise, just use the module’s `__name__` in the repr.

Alterado na versão 3.4: Use of `loader.module_repr()` has been deprecated and the module spec is now used by the import machinery to generate a module repr.

For backward compatibility with Python 3.3, the module repr will be generated by calling the loader’s `module_repr()` method, if defined, before trying either approach described above. However, the method is deprecated.

## 5.5. The Path Based Finder

As mentioned previously, Python comes with several default meta path finders. One of these, called the path based finder (`PathFinder`), searches an import path, which contains a list of path entries. Each path entry names a location to search for modules.

The path based finder itself doesn’t know how to import anything. Instead, it traverses the individual path entries, associating each of them with a path entry finder that knows how to handle that particular kind of path.

The default set of path entry finders implement all the semantics for finding modules on the file system, handling special file types such as Python source code (`.py` files), Python byte code (`.pyc` files) and shared libraries (e.g. `.so` files). When supported by the `zipimport` module in the standard library, the default path entry finders also handle loading all of these file types (other than shared libraries) from zipfiles.

Path entries need not be limited to file system locations. They can refer to URLs, database queries, or any other location that can be specified as a string.

The path based finder provides additional hooks and protocols so that you can extend and customize the types of searchable path entries. For example, if you wanted to support path entries as network URLs, you could write a hook that implements HTTP semantics to find modules on the web. This hook (a callable) would return a path entry finder supporting the protocol described below, which was then used to get a loader for the module from the web.

A word of warning: this section and the previous both use the term finder, distinguishing between them by using the terms meta path finder and path entry finder. These two types of finders are very similar, support similar protocols, and function in similar ways during the import process, but it’s important to keep in mind that they are subtly different. In particular, meta path finders operate at the beginning of the import process, as keyed off the `sys.meta_path` traversal.

By contrast, path entry finders are in a sense an implementation detail of the path based finder, and in fact, if the path based finder were to be removed from `sys.meta_path`, none of the path entry finder semantics would be invoked.

### 5.5.1. Path entry finders

The path based finder is responsible for finding and loading Python modules and packages whose location is specified with a string path entry. Most path entries name locations in the file system, but they need not be limited to this.

As a meta path finder, the path based finder implements the `find_spec()` protocol previously described, however it exposes additional hooks that can be used to customize how modules are found and loaded from the import path.

Three variables are used by the path based finder, `sys.path`, `sys.path_hooks` and `sys.path_importer_cache`. The `__path__` attributes on package objects are also used. These provide additional ways that the import machinery can be customized.

`sys.path` contains a list of strings providing search locations for modules and packages. It is initialized from the `PYTHONPATH` environment variable and various other installation- and implementation-specific defaults. Entries in `sys.path` can name directories on the file system, zip files, and potentially other “locations” (see the `site` module) that should be searched for modules, such as URLs, or database queries. Only strings and bytes should be present on `sys.path`; all other data types are ignored. The encoding of bytes entries is determined by the individual path entry finders.

The path based finder is a meta path finder, so the import machinery begins the import path search by calling the path based finder’s `find_spec()` method as described previously. When the `path` argument to `find_spec()` is given, it will be a list of string paths to traverse - typically a package’s `__path__` attribute for an import within that package. If the `path` argument is `None`, this indicates a top level import and `sys.path` is used.

The path based finder iterates over every entry in the search path, and for each of these, looks for an appropriate path entry finder (`PathEntryFinder`) for the path entry. Because this can be an expensive operation (e.g. there may be stat() call overheads for this search), the path based finder maintains a cache mapping path entries to path entry finders. This cache is maintained in `sys.path_importer_cache` (despite the name, this cache actually stores finder objects rather than being limited to importer objects). In this way, the expensive search for a particular path entry location’s path entry finder need only be done once. User code is free to remove cache entries from `sys.path_importer_cache` forcing the path based finder to perform the path entry search again 3.

If the path entry is not present in the cache, the path based finder iterates over every callable in `sys.path_hooks`. Each of the path entry hooks in this list is called with a single argument, the path entry to be searched. This callable may either return a path entry finder that can handle the path entry, or it may raise `ImportError`. An `ImportError` is used by the path based finder to signal that the hook cannot find a path entry finder for that path entry. The exception is ignored and import path iteration continues. The hook should expect either a string or bytes object; the encoding of bytes objects is up to the hook (e.g. it may be a file system encoding, UTF-8, or something else), and if the hook cannot decode the argument, it should raise `ImportError`.

If `sys.path_hooks` iteration ends with no path entry finder being returned, then the path based finder’s `find_spec()` method will store `None` in `sys.path_importer_cache` (to indicate that there is no finder for this path entry) and return `None`, indicating that this meta path finder could not find the module.

If a path entry finder is returned by one of the path entry hook callables on `sys.path_hooks`, then the following protocol is used to ask the finder for a module spec, which is then used when loading the module.

The current working directory – denoted by an empty string – is handled slightly differently from other entries on `sys.path`. First, if the current working directory is found to not exist, no value is stored in `sys.path_importer_cache`. Second, the value for the current working directory is looked up fresh for each module lookup. Third, the path used for `sys.path_importer_cache` and returned by `importlib.machinery.PathFinder.find_spec()` will be the actual current working directory and not the empty string.

### 5.5.2. Path entry finder protocol

In order to support imports of modules and initialized packages and also to contribute portions to namespace packages, path entry finders must implement the `find_spec()` method.

`find_spec()` takes two argument, the fully qualified name of the module being imported, and the (optional) target module. `find_spec()` returns a fully populated spec for the module. This spec will always have “loader” set (with one exception).

To indicate to the import machinery that the spec represents a namespace portion. the path entry finder sets “loader” on the spec to `None` and “submodule_search_locations” to a list containing the portion.

Alterado na versão 3.4: `find_spec()` replaced `find_loader()` and `find_module()`, both of which are now deprecated, but will be used if `find_spec()` is not defined.

Older path entry finders may implement one of these two deprecated methods instead of `find_spec()`. The methods are still respected for the sake of backward compatibility. However, if `find_spec()` is implemented on the path entry finder, the legacy methods are ignored.

`find_loader()` takes one argument, the fully qualified name of the module being imported. `find_loader()` returns a 2-tuple where the first item is the loader and the second item is a namespace portion. When the first item (i.e. the loader) is `None`, this means that while the path entry finder does not have a loader for the named module, it knows that the path entry contributes to a namespace portion for the named module. This will almost always be the case where Python is asked to import a namespace package that has no physical presence on the file system. When a path entry finder returns `None` for the loader, the second item of the 2-tuple return value must be a sequence, although it can be empty.

If `find_loader()` returns a non-`None` loader value, the portion is ignored and the loader is returned from the path based finder, terminating the search through the path entries.

For backwards compatibility with other implementations of the import protocol, many path entry finders also support the same, traditional `find_module()` method that meta path finders support. However path entry finder `find_module()` methods are never called with a `path` argument (they are expected to record the appropriate path information from the initial call to the path hook).

The `find_module()` method on path entry finders is deprecated, as it does not allow the path entry finder to contribute portions to namespace packages. If both `find_loader()` and `find_module()` exist on a path entry finder, the import system will always call `find_loader()` in preference to `find_module()`.

## 5.6. Replacing the standard import system

The most reliable mechanism for replacing the entire import system is to delete the default contents of `sys.meta_path`, replacing them entirely with a custom meta path hook.

If it is acceptable to only alter the behaviour of import statements without affecting other APIs that access the import system, then replacing the builtin `__import__()` function may be sufficient. This technique may also be employed at the module level to only alter the behaviour of import statements within that module.

To selectively prevent import of some modules from a hook early on the meta path (rather than disabling the standard import system entirely), it is sufficient to raise `ModuleNotFoundError` directly from `find_spec()` instead of returning `None`. The latter indicates that the meta path search should continue, while raising an exception terminates it immediately.

## 5.7. Special considerations for __main__

The `__main__` module is a special case relative to Python’s import system. As noted elsewhere, the `__main__` module is directly initialized at interpreter startup, much like `sys` and `builtins`. However, unlike those two, it doesn’t strictly qualify as a built-in module. This is because the manner in which `__main__` is initialized depends on the flags and other options with which the interpreter is invoked.

### 5.7.1. __main__.__spec__

Depending on how `__main__` is initialized, `__main__.__spec__` gets set appropriately or to `None`.

When Python is started with the `-m` option, `__spec__` is set to the module spec of the corresponding module or package. `__spec__` is also populated when the `__main__` module is loaded as part of executing a directory, zipfile or other `sys.path` entry.

In the remaining cases `__main__.__spec__` is set to `None`, as the code used to populate the `__main__` does not correspond directly with an importable module:

- interactive prompt

- `-c` option

- running from stdin

- running directly from a source or bytecode file

Note that `__main__.__spec__` is always `None` in the last case, even if the file could technically be imported directly as a module instead. Use the `-m` switch if valid module metadata is desired in `__main__`.

Note also that even when `__main__` corresponds with an importable module and `__main__.__spec__` is set accordingly, they’re still considered distinct modules. This is due to the fact that blocks guarded by `if __name__ == "__main__":` checks only execute when the module is used to populate the `__main__` namespace, and not during normal import.

## 5.8. Open issues

XXX It would be really nice to have a diagram.

XXX * (import_machinery.rst) how about a section devoted just to the attributes of modules and packages, perhaps expanding upon or supplanting the related entries in the data model reference page?

XXX runpy, pkgutil, et al in the library manual should all get “See Also” links at the top pointing to the new import system section.

XXX Add more explanation regarding the different ways in which `__main__` is initialized?

XXX Add more info on `__main__` quirks/pitfalls (i.e. copy from PEP 395).

## 5.9. Referências

The import machinery has evolved considerably since Python’s early days. The original specification for packages is still available to read, although some details have changed since the writing of that document.

The original specification for `sys.meta_path` was PEP 302, with subsequent extension in PEP 420.

PEP 420 introduced namespace packages for Python 3.3. PEP 420 also introduced the `find_loader()` protocol as an alternative to `find_module()`.

PEP 366 describes the addition of the `__package__` attribute for explicit relative imports in main modules.

PEP 328 introduced absolute and explicit relative imports and initially proposed `__name__` for semantics PEP 366 would eventually specify for `__package__`.

PEP 338 defines executing modules as scripts.

PEP 451 adds the encapsulation of per-module import state in spec objects. It also off-loads most of the boilerplate responsibilities of loaders back onto the import machinery. These changes allow the deprecation of several APIs in the import system and also addition of new methods to finders and loaders.

Notas de Rodapé

1

  See `types.ModuleType`.

2

  The importlib implementation avoids using the return value directly. Instead, it gets the module object by looking the module name up in `sys.modules`. The indirect effect of this is that an imported module may replace itself in `sys.modules`. This is implementation-specific behavior that is not guaranteed to work in other Python implementations.

3

  In legacy code, it is possible to find instances of `imp.NullImporter` in the `sys.path_importer_cache`. It is recommended that code be changed to use `None` instead. See Porting Python code for more details.