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2022-07-25 Python

Python docs - The Import System

The Import system from the Python Docs#

Python code in one module gains access to code in another module by importing it

The import statement is the most common way but you can also:

  • importlib.import_module()
  • __import__()

The import statement:

  1. it searches for the named module
  2. it binds the results of that search to a name in the local scope

A direct call to __import__() performs only the module search and, if found, the module creation operation.

If the named module cannot be found, a ModuleNotFoundError is raised

importlib - is an interface for interacting with the import system

Packages#

Packages:

  • organise modules
  • provide a naming hierachy

You can think of packages as the directories on a file system and modules as files within directories

It’s important to keep in mind that all packages are modules, but not all modules are packages.

Any module that contains a __path__ attribute is considered a package

All modules have a name. Subpackage names are separated from their parent package name by a dot, akin to Python’s standard attribute access syntax. Eg. email.mime.text

Regular packages#
  • A regular package is typically implemented as a directory containing an __init__.py file.
  • When a regular package is imported, this __init__.py file is implicitly executed, and the objects it defines are bound to names in the package’s namespace. (A good place to setup logging)

  • The __init__.py file can contain the same Python code that any other module can contain

    parent/ init.py one/ init.py two/ init.py three/ init.py

That is a top level parent with 3 subpackages. Importing parent.one implicitly executes parent/__init__.py and parent/one/__init__.py

Namespace packages#
  • Namespace packages may or may not correspond directly to objects on the file system; they may be virtual modules that have no concrete representation.
  • With namespace packages, there is no parent/__init__.py file. In fact, there may be multiple parent directories found during import search, where each one is provided by a different portion.

Searching#

  • Python needs the fully qualified name - dotted name showing the “path” from a module’s global scope to a class, function or method.
  • 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.
  • During import, the module name is looked up in sys.modules
  • 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.

Keywords#

  • __path__ - if a module has a __path__ attribute, it is a package. Non-package modules should not have a __path__ attribute.
  • __name__ - fully qualified name of the module - uniquely identifies the module
  • __loader__ - the loader object used to laod the module
  • __package__ - 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
  • __spec__ - the moduel specification used to import the module
  • __file__ - is optional. If set, this attribute’s value must be a string. Import system may leave it unset.

Source#