30.6. importlib – The implementation of import — Python 3.3.5 documentation

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# 30.6. `importlib` – The implementation of `import`

New in version 3.1.

## 30.6.1. Introduction

The purpose of the `importlib` package is two-fold. One is to provide the implementation of the `import` statement (and thus, by extension, the `__import__()` function) in Python source code. This provides an implementation of `import` which is portable to any Python interpreter. This also provides an implementation which is easier to comprehend than one implemented in a programming language other than Python.

Two, the components to implement `import` are exposed in this package, making it easier for users to create their own custom objects (known generically as an importer) to participate in the import process.

See also

The import statement
  The language reference for the `import` statement.
Packages specification
  Original specification of packages. Some semantics have changed since the writing of this document (e.g. redirecting based on `None` in `sys.modules`).
The `__import__()` function
  The `import` statement is syntactic sugar for this function.
PEP 235
  Import on Case-Insensitive Platforms
PEP 263
  Defining Python Source Code Encodings
PEP 302
  New Import Hooks
PEP 328
  Imports: Multi-Line and Absolute/Relative
PEP 366
  Main module explicit relative imports
PEP 451
  A ModuleSpec Type for the Import System
PEP 3120
  Using UTF-8 as the Default Source Encoding
PEP 3147
  PYC Repository Directories

## 30.6.2. Functions

`importlib.` `__import__`(name, globals=None, locals=None, fromlist=(), level=0)

  An implementation of the built-in `__import__()` function.

`importlib.` `import_module`(name, package=None)

  Import a module. The name argument specifies what module to import in absolute or relative terms (e.g. either `pkg.mod` or `..mod`). If the name is specified in relative terms, then the package argument must be set to the name of the package which is to act as the anchor for resolving the package name (e.g. `import_module('..mod', 'pkg.subpkg')` will import `pkg.mod`).

  The `import_module()` function acts as a simplifying wrapper around `importlib.__import__()`. This means all semantics of the function are derived from `importlib.__import__()`, including requiring the package from which an import is occurring to have been previously imported (i.e., package must already be imported). The most important difference is that `import_module()` returns the specified package or module (e.g. `pkg.mod`), while `__import__()` returns the top-level package or module (e.g. `pkg`).

  Changed in version 3.3: Parent packages are automatically imported.

`importlib.` `find_loader`(name, path=None)

  Find the loader for a module, optionally within the specified path. If the module is in `sys.modules`, then `sys.modules[name].__loader__` is returned (unless the loader would be `None`, in which case `ValueError` is raised). Otherwise a search using `sys.meta_path` is done. `None` is returned if no loader is found.

  A dotted name does not have its parent’s implicitly imported as that requires loading them and that may not be desired. To properly import a submodule you will need to import all parent packages of the submodule and use the correct argument to path.

`importlib.` `invalidate_caches`()

  Invalidate the internal caches of finders stored at `sys.meta_path`. If a finder implements `invalidate_caches()` then it will be called to perform the invalidation. This function should be called if any modules are created/installed while your program is running to guarantee all finders will notice the new module’s existence.

  New in version 3.3.

## 30.6.3. `importlib.abc` – Abstract base classes related to import

The `importlib.abc` module contains all of the core abstract base classes used by `import`. Some subclasses of the core abstract base classes are also provided to help in implementing the core ABCs.

ABC hierarchy:

    object
     +-- Finder (deprecated)
     |    +-- MetaPathFinder
     |    +-- PathEntryFinder
     +-- Loader
          +-- ResourceLoader --------+
          +-- InspectLoader          |
               +-- ExecutionLoader --+
                                     +-- FileLoader
                                     +-- SourceLoader
                                          +-- PyLoader (deprecated)
                                          +-- PyPycLoader (deprecated)

class `importlib.abc.` `Finder`

  An abstract base class representing a finder.

  Deprecated since version 3.3: Use `MetaPathFinder` or `PathEntryFinder` instead.

  `find_module`(fullname, path=None)

    An abstact method for finding a loader for the specified module. Originally specified in PEP 302, this method was meant for use in `sys.meta_path` and in the path-based import subsystem.

class `importlib.abc.` `MetaPathFinder`

  An abstract base class representing a meta path finder. For compatibility, this is a subclass of `Finder`.

  New in version 3.3.

  `find_module`(fullname, path)

    An abstract method for finding a loader for the specified module. If this is a top-level import, path will be `None`. Otherwise, this is a search for a subpackage or module and path will be the value of `__path__` from the parent package. If a loader cannot be found, `None` is returned.

  `invalidate_caches`()

    An optional method which, when called, should invalidate any internal cache used by the finder. Used by `importlib.invalidate_caches()` when invalidating the caches of all finders on `sys.meta_path`.

class `importlib.abc.` `PathEntryFinder`

  An abstract base class representing a path entry finder. Though it bears some similarities to `MetaPathFinder`, `PathEntryFinder` is meant for use only within the path-based import subsystem provided by `PathFinder`. This ABC is a subclass of `Finder` for compatibility.

  New in version 3.3.

  `find_loader`(fullname)

    An abstract method for finding a loader for the specified module. Returns a 2-tuple of `(loader, portion)` where `portion` is a sequence of file system locations contributing to part of a namespace package. The loader may be `None` while specifying `portion` to signify the contribution of the file system locations to a namespace package. An empty list can be used for `portion` to signify the loader is not part of a package. If `loader` is `None` and `portion` is the empty list then no loader or location for a namespace package were found (i.e. failure to find anything for the module).

  `find_module`(fullname)

    A concrete implementation of `Finder.find_module()` which is equivalent to `self.find_loader(fullname)[0]`.

  `invalidate_caches`()

    An optional method which, when called, should invalidate any internal cache used by the finder. Used by `PathFinder.invalidate_caches()` when invalidating the caches of all cached finders.

class `importlib.abc.` `Loader`

  An abstract base class for a loader. See PEP 302 for the exact definition for a loader.

  `load_module`(fullname)

    An abstract method for loading a module. If the module cannot be loaded, `ImportError` is raised, otherwise the loaded module is returned.

    If the requested module already exists in `sys.modules`, that module should be used and reloaded. Otherwise the loader should create a new module and insert it into `sys.modules` before any loading begins, to prevent recursion from the import. If the loader inserted a module and the load fails, it must be removed by the loader from `sys.modules`; modules already in `sys.modules` before the loader began execution should be left alone. The `importlib.util.module_for_loader()` decorator handles all of these details.

    The loader should set several attributes on the module. (Note that some of these attributes can change when a module is reloaded.)

    - `__name__`

        The name of the module.

    - `__file__`

        The path to where the module data is stored (not set for built-in modules).

    - `__cached__`

        The path to where a compiled version of the module is/should be stored (not set when the attribute would be inappropriate).

    - `__path__`

        A list of strings specifying the search path within a package. This attribute is not set on modules.

    - `__package__`

        The parent package for the module/package. If the module is top-level then it has a value of the empty string. The `importlib.util.set_package()` decorator can handle the details for `__package__`.

    - `__loader__`

        The loader used to load the module. (This is not set by the built-in import machinery, but it should be set whenever a loader is used.)

  `module_repr`(module)

    An abstract method which when implemented calculates and returns the given module’s repr, as a string.

class `importlib.abc.` `ResourceLoader`

  An abstract base class for a loader which implements the optional PEP 302 protocol for loading arbitrary resources from the storage back-end.

  `get_data`(path)

    An abstract method to return the bytes for the data located at path. Loaders that have a file-like storage back-end that allows storing arbitrary data can implement this abstract method to give direct access to the data stored. `IOError` is to be raised if the path cannot be found. The path is expected to be constructed using a module’s `__file__` attribute or an item from a package’s `__path__`.

class `importlib.abc.` `InspectLoader`

  An abstract base class for a loader which implements the optional PEP 302 protocol for loaders that inspect modules.

  `get_code`(fullname)

    An abstract method to return the `code` object for a module. `None` is returned if the module does not have a code object (e.g. built-in module). `ImportError` is raised if loader cannot find the requested module.

  `get_source`(fullname)

    An abstract method to return the source of a module. It is returned as a text string using universal newlines, translating all recognized line separators into `'\n'` characters. Returns `None` if no source is available (e.g. a built-in module). Raises `ImportError` if the loader cannot find the module specified.

  `is_package`(fullname)

    An abstract method to return a true value if the module is a package, a false value otherwise. `ImportError` is raised if the loader cannot find the module.

class `importlib.abc.` `ExecutionLoader`

  An abstract base class which inherits from `InspectLoader` that, when implemented, helps a module to be executed as a script. The ABC represents an optional PEP 302 protocol.

  `get_filename`(fullname)

    An abstract method that is to return the value of `__file__` for the specified module. If no path is available, `ImportError` is raised.

    If source code is available, then the method should return the path to the source file, regardless of whether a bytecode was used to load the module.

class `importlib.abc.` `FileLoader`(fullname, path)

  An abstract base class which inherits from `ResourceLoader` and `ExecutionLoader`, providing concrete implementations of `ResourceLoader.get_data()` and `ExecutionLoader.get_filename()`.

  The fullname argument is a fully resolved name of the module the loader is to handle. The path argument is the path to the file for the module.

  New in version 3.3.

  `name`

    The name of the module the loader can handle.

  `path`

    Path to the file of the module.

  `load_module`(fullname)

    Calls super’s `load_module()`.

  `get_filename`(fullname)

    Returns `path`.

  `get_data`(path)

    Returns the open, binary file for path.

class `importlib.abc.` `SourceLoader`

  An abstract base class for implementing source (and optionally bytecode) file loading. The class inherits from both `ResourceLoader` and `ExecutionLoader`, requiring the implementation of:

  - `ResourceLoader.get_data()`

  - `ExecutionLoader.get_filename()`

      Should only return the path to the source file; sourceless loading is not supported.

  The abstract methods defined by this class are to add optional bytecode file support. Not implementing these optional methods causes the loader to only work with source code. Implementing the methods allows the loader to work with source and bytecode files; it does not allow for sourceless loading where only bytecode is provided. Bytecode files are an optimization to speed up loading by removing the parsing step of Python’s compiler, and so no bytecode-specific API is exposed.

  `path_stats`(path)

    Optional abstract method which returns a `dict` containing metadata about the specifed path. Supported dictionary keys are:

    - `'mtime'` (mandatory): an integer or floating-point number representing the modification time of the source code;
    - `'size'` (optional): the size in bytes of the source code.

    Any other keys in the dictionary are ignored, to allow for future extensions.

    New in version 3.3.

  `path_mtime`(path)

    Optional abstract method which returns the modification time for the specified path.

    Deprecated since version 3.3: This method is deprecated in favour of `path_stats()`. You don’t have to implement it, but it is still available for compatibility purposes.

  `set_data`(path, data)

    Optional abstract method which writes the specified bytes to a file path. Any intermediate directories which do not exist are to be created automatically.

    When writing to the path fails because the path is read-only (`errno.EACCES`/`PermissionError`), do not propagate the exception.

  `get_code`(fullname)

    Concrete implementation of `InspectLoader.get_code()`.

  `load_module`(fullname)

    Concrete implementation of `Loader.load_module()`.

  `get_source`(fullname)

    Concrete implementation of `InspectLoader.get_source()`.

  `is_package`(fullname)

    Concrete implementation of `InspectLoader.is_package()`. A module is determined to be a package if its file path (as provided by `ExecutionLoader.get_filename()`) is a file named `__init__` when the file extension is removed and the module name itself does not end in `__init__`.

class `importlib.abc.` `PyLoader`

  An abstract base class inheriting from `ExecutionLoader` and `ResourceLoader` designed to ease the loading of Python source modules (bytecode is not handled; see `SourceLoader` for a source/bytecode ABC). A subclass implementing this ABC will only need to worry about exposing how the source code is stored; all other details for loading Python source code will be handled by the concrete implementations of key methods.

  Deprecated since version 3.2: This class has been deprecated in favor of `SourceLoader` and is slated for removal in Python 3.4. See below for how to create a subclass that is compatible with Python 3.1 onwards.

  If compatibility with Python 3.1 is required, then use the following idiom to implement a subclass that will work with Python 3.1 onwards (make sure to implement `ExecutionLoader.get_filename()`):

      try:
          from importlib.abc import SourceLoader
      except ImportError:
          from importlib.abc import PyLoader as SourceLoader

      class CustomLoader(SourceLoader):
          def get_filename(self, fullname):
              """Return the path to the source file."""
              # Implement ...

          def source_path(self, fullname):
              """Implement source_path in terms of get_filename."""
              try:
                  return self.get_filename(fullname)
              except ImportError:
                  return None

          def is_package(self, fullname):
              """Implement is_package by looking for an __init__ file
              name as returned by get_filename."""
              filename = os.path.basename(self.get_filename(fullname))
              return os.path.splitext(filename)[0] == '__init__'

  `source_path`(fullname)

    An abstract method that returns the path to the source code for a module. Should return `None` if there is no source code. Raises `ImportError` if the loader knows it cannot handle the module.

  `get_filename`(fullname)

    A concrete implementation of `importlib.abc.ExecutionLoader.get_filename()` that relies on `source_path()`. If `source_path()` returns `None`, then `ImportError` is raised.

  `load_module`(fullname)

    A concrete implementation of `importlib.abc.Loader.load_module()` that loads Python source code. All needed information comes from the abstract methods required by this ABC. The only pertinent assumption made by this method is that when loading a package `__path__` is set to `[os.path.dirname(__file__)]`.

  `get_code`(fullname)

    A concrete implementation of `importlib.abc.InspectLoader.get_code()` that creates code objects from Python source code, by requesting the source code (using `source_path()` and `get_data()`) and compiling it with the built-in `compile()` function.

  `get_source`(fullname)

    A concrete implementation of `importlib.abc.InspectLoader.get_source()`. Uses `importlib.abc.ResourceLoader.get_data()` and `source_path()` to get the source code. It tries to guess the source encoding using `tokenize.detect_encoding()`.

class `importlib.abc.` `PyPycLoader`

  An abstract base class inheriting from `PyLoader`. This ABC is meant to help in creating loaders that support both Python source and bytecode.

  Deprecated since version 3.2: This class has been deprecated in favor of `SourceLoader` and to properly support PEP 3147. If compatibility is required with Python 3.1, implement both `SourceLoader` and `PyLoader`; instructions on how to do so are included in the documentation for `PyLoader`. Do note that this solution will not support sourceless/bytecode-only loading; only source and bytecode loading.

  Changed in version 3.3: Updated to parse (but not use) the new source size field in bytecode files when reading and to write out the field properly when writing.

  `source_mtime`(fullname)

    An abstract method which returns the modification time for the source code of the specified module. The modification time should be an integer. If there is no source code, return `None`. If the module cannot be found then `ImportError` is raised.

  `bytecode_path`(fullname)

    An abstract method which returns the path to the bytecode for the specified module, if it exists. It returns `None` if no bytecode exists (yet). Raises `ImportError` if the loader knows it cannot handle the module.

  `get_filename`(fullname)

    A concrete implementation of `ExecutionLoader.get_filename()` that relies on `PyLoader.source_path()` and `bytecode_path()`. If `source_path()` returns a path, then that value is returned. Else if `bytecode_path()` returns a path, that path will be returned. If a path is not available from both methods, `ImportError` is raised.

  `write_bytecode`(fullname, bytecode)

    An abstract method which has the loader write bytecode for future use. If the bytecode is written, return `True`. Return `False` if the bytecode could not be written. This method should not be called if `sys.dont_write_bytecode` is true. The bytecode argument should be a bytes string or bytes array.

## 30.6.4. `importlib.machinery` – Importers and path hooks

This module contains the various objects that help `import` find and load modules.

`importlib.machinery.` `SOURCE_SUFFIXES`

  A list of strings representing the recognized file suffixes for source modules.

  New in version 3.3.

`importlib.machinery.` `DEBUG_BYTECODE_SUFFIXES`

  A list of strings representing the file suffixes for non-optimized bytecode modules.

  New in version 3.3.

`importlib.machinery.` `OPTIMIZED_BYTECODE_SUFFIXES`

  A list of strings representing the file suffixes for optimized bytecode modules.

  New in version 3.3.

`importlib.machinery.` `BYTECODE_SUFFIXES`

  A list of strings representing the recognized file suffixes for bytecode modules. Set to either `DEBUG_BYTECODE_SUFFIXES` or `OPTIMIZED_BYTECODE_SUFFIXES` based on whether `__debug__` is true.

  New in version 3.3.

`importlib.machinery.` `EXTENSION_SUFFIXES`

  A list of strings representing the recognized file suffixes for extension modules.

  New in version 3.3.

`importlib.machinery.` `all_suffixes`()

  Returns a combined list of strings representing all file suffixes for modules recognized by the standard import machinery. This is a helper for code which simply needs to know if a filesystem path potentially refers to a module without needing any details on the kind of module (for example, `inspect.getmodulename()`)

  New in version 3.3.

class `importlib.machinery.` `BuiltinImporter`

  An importer for built-in modules. All known built-in modules are listed in `sys.builtin_module_names`. This class implements the `importlib.abc.MetaPathFinder` and `importlib.abc.InspectLoader` ABCs.

  Only class methods are defined by this class to alleviate the need for instantiation.

class `importlib.machinery.` `FrozenImporter`

  An importer for frozen modules. This class implements the `importlib.abc.MetaPathFinder` and `importlib.abc.InspectLoader` ABCs.

  Only class methods are defined by this class to alleviate the need for instantiation.

class `importlib.machinery.` `WindowsRegistryFinder`

  Finder for modules declared in the Windows registry. This class implements the `importlib.abc.Finder` ABC.

  Only class methods are defined by this class to alleviate the need for instantiation.

  New in version 3.3.

class `importlib.machinery.` `PathFinder`

  A Finder for `sys.path` and package `__path__` attributes. This class implements the `importlib.abc.MetaPathFinder` ABC.

  Only class methods are defined by this class to alleviate the need for instantiation.

  classmethod `find_module`(fullname, path=None)

    Class method that attempts to find a loader for the module specified by fullname on `sys.path` or, if defined, on path. For each path entry that is searched, `sys.path_importer_cache` is checked. If a non-false object is found then it is used as the path entry finder to look for the module being searched for. If no entry is found in `sys.path_importer_cache`, then `sys.path_hooks` is searched for a finder for the path entry and, if found, is stored in `sys.path_importer_cache` along with being queried about the module. If no finder is ever found then `None` is both stored in the cache and returned.

  classmethod `invalidate_caches`()

    Calls `importlib.abc.PathEntryFinder.invalidate_caches()` on all finders stored in `sys.path_importer_cache`.

class `importlib.machinery.` `FileFinder`(path, *loader_details)

  A concrete implementation of `importlib.abc.PathEntryFinder` which caches results from the file system.

  The path argument is the directory for which the finder is in charge of searching.

  The loader_details argument is a variable number of 2-item tuples each containing a loader and a sequence of file suffixes the loader recognizes. The loaders are expected to be callables which accept two arguments of the module’s name and the path to the file found.

  The finder will cache the directory contents as necessary, making stat calls for each module search to verify the cache is not outdated. Because cache staleness relies upon the granularity of the operating system’s state information of the file system, there is a potential race condition of searching for a module, creating a new file, and then searching for the module the new file represents. If the operations happen fast enough to fit within the granularity of stat calls, then the module search will fail. To prevent this from happening, when you create a module dynamically, make sure to call `importlib.invalidate_caches()`.

  New in version 3.3.

  `path`

    The path the finder will search in.

  `find_loader`(fullname)

    Attempt to find the loader to handle fullname within `path`.

  `invalidate_caches`()

    Clear out the internal cache.

  classmethod `path_hook`(*loader_details)

    A class method which returns a closure for use on `sys.path_hooks`. An instance of `FileFinder` is returned by the closure using the path argument given to the closure directly and loader_details indirectly.

    If the argument to the closure is not an existing directory, `ImportError` is raised.

class `importlib.machinery.` `SourceFileLoader`(fullname, path)

  A concrete implementation of `importlib.abc.SourceLoader` by subclassing `importlib.abc.FileLoader` and providing some concrete implementations of other methods.

  New in version 3.3.

  `name`

    The name of the module that this loader will handle.

  `path`

    The path to the source file.

  `is_package`(fullname)

    Return true if `path` appears to be for a package.

  `path_stats`(path)

    Concrete implementation of `importlib.abc.SourceLoader.path_stats()`.

  `set_data`(path, data)

    Concrete implementation of `importlib.abc.SourceLoader.set_data()`.

class `importlib.machinery.` `SourcelessFileLoader`(fullname, path)

  A concrete implementation of `importlib.abc.FileLoader` which can import bytecode files (i.e. no source code files exist).

  Please note that direct use of bytecode files (and thus not source code files) inhibits your modules from being usable by all Python implementations or new versions of Python which change the bytecode format.

  New in version 3.3.

  `name`

    The name of the module the loader will handle.

  `path`

    The path to the bytecode file.

  `is_package`(fullname)

    Determines if the module is a package based on `path`.

  `get_code`(fullname)

    Returns the code object for `name` created from `path`.

  `get_source`(fullname)

    Returns `None` as bytecode files have no source when this loader is used.

class `importlib.machinery.` `ExtensionFileLoader`(fullname, path)

  A concrete implementation of `importlib.abc.InspectLoader` for extension modules.

  The fullname argument specifies the name of the module the loader is to support. The path argument is the path to the extension module’s file.

  New in version 3.3.

  `name`

    Name of the module the loader supports.

  `path`

    Path to the extension module.

  `load_module`(fullname)

    Loads the extension module if and only if fullname is the same as `name` or is `None`.

  `is_package`(fullname)

    Returns `True` if the file path points to a package’s `__init__` module based on `EXTENSION_SUFFIXES`.

  `get_code`(fullname)

    Returns `None` as extension modules lack a code object.

  `get_source`(fullname)

    Returns `None` as extension modules do not have source code.

## 30.6.5. `importlib.util` – Utility code for importers

This module contains the various objects that help in the construction of an importer.

`importlib.util.` `resolve_name`(name, package)

  Resolve a relative module name to an absolute one.

  If name has no leading dots, then name is simply returned. This allows for usage such as `importlib.util.resolve_name('sys', __package__)` without doing a check to see if the package argument is needed.

  `ValueError` is raised if name is a relative module name but package is a false value (e.g. `None` or the empty string). `ValueError` is also raised a relative name would escape its containing package (e.g. requesting `..bacon` from within the `spam` package).

  New in version 3.3.

`@``importlib.util.` `module_for_loader`

  A decorator for a loader method, to handle selecting the proper module object to load with. The decorated method is expected to have a call signature taking two positional arguments (e.g. `load_module(self, module)`) for which the second argument will be the module object to be used by the loader. Note that the decorator will not work on static methods because of the assumption of two arguments.

  The decorated method will take in the name of the module to be loaded as expected for a loader. If the module is not found in `sys.modules` then a new one is constructed with its `__name__` attribute set to name, `__loader__` set to self, and `__package__` set if `importlib.abc.InspectLoader.is_package()` is defined for self and does not raise `ImportError` for name. If a new module is not needed then the module found in `sys.modules` will be passed into the method.

  If an exception is raised by the decorated method and a module was added to `sys.modules` it will be removed to prevent a partially initialized module from being in left in `sys.modules`. If the module was already in `sys.modules` then it is left alone.

  Use of this decorator handles all the details of which module object a loader should initialize as specified by PEP 302 as best as possible.

  Changed in version 3.3: `__loader__` and `__package__` are automatically set (when possible).

`@``importlib.util.` `set_loader`

  > A decorator for a loader method, to set the `__loader__` attribute on loaded modules. If the attribute is already set the decorator does nothing. It is assumed that the first positional argument to the wrapped method (i.e. `self`) is what `__loader__` should be set to.

  Note

  It is recommended that `module_for_loader()` be used over this decorator as it subsumes this functionality.

`@``importlib.util.` `set_package`

  > A decorator for a loader to set the `__package__` attribute on the module returned by the loader. If `__package__` is set and has a value other than `None` it will not be changed. Note that the module returned by the loader is what has the attribute set on and not the module found in `sys.modules`.

  > Reliance on this decorator is discouraged when it is possible to set `__package__` before importing. By setting it beforehand the code for the module is executed with the attribute set and thus can be used by global level code during initialization.

  Note

  It is recommended that `module_for_loader()` be used over this decorator as it subsumes this functionality.

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