Modules

• Modules can be imported using the import statement. When a module is imported:

  • System cache is checked first (sys.modules)
  • Otherwise, locate it from finders (e.g: sys.meta_path: gets list of finders). Module is retrieved (loaded) from loaders (returned by finders). empty module typed object is created. A reference to the module is added to the system cache (sys.modules) and the module is compiled. When the module is executed, it sets up the module's namespace

• Module Finders:

  In [2]: sys.meta_path
  Out[2]:
  [_frozen_importlib.BuiltinImporter, #--> finds built-ins, such as math
   _frozen_importlib.FrozenImporter,  #--> finds frozen modules
   _frozen_importlib_external.PathFinder] #--> file-based modules
  

• PathFinder: Finds file-based modules based on sys.path and package __path__

• Module Properties: __spec__, __name__, __package__ (empty for modules), __file__ (found by PathFinder)

• Modules may reside:

  • in the built-ins
  • in files on disk
  • they can even be pre-compiled, frozen or even inside zip archives
  • anywhere else that can be accessed by a finder and a loader
  • For file based modules (PathFinder), they must exist in path specified in sys.path or in a path specified by <package>.__path__

Packages

• Packages are modules. They can contain modules, packages.

• If a module is a package, then it must have a value set for `path

• Packages represent a hierarchy of modules/packages

• How to define package?: Directory name becomes package name & __init__.py is where the code for package will go.

• If we don't have the __init__.py file, then Python will create an implicit namespace package

• Package will have __path__ & __file__ property defined:

  • __path__ : file system directory path (absolute)
  • __file__: file system path to __init__.py (absolute)

• Module will also have above properties:

  • __file__ : location of module code in file system
  • __package__: package the module code is located in (is empty string if module is located in application root)
  • __path__: only set if module is also a package. Location of the package (directory) in the file system

• Nested packages:

  app/
      module.py
      pack1/
          __init__.py
          module1.py
          module2.py
          pack1_1/
              __init__.py
              module1_1a.py
              module1_1b.py
  

  • module inside pack1: pack1.module1
  • package inside pack1: pack1.pack1_1
  • module inside pack1.pack1_1: pack1.pack1_1.module1_1a
  • module.__file__:.../app/module.py
  • module:__path__: Not set
  • module.__package__: ""
  • pack1.__file__:.../app/pack1/__init__.py
  • pack1.__path__: .../app/pack1/
  • pack1.__package__: pack1
  • Python will load all the modules referenced in the import statement and the modules in the files of the imported modules and store it in sys.modules

• __all__: List of public objects of a module, as interpreted by import *. It overrides the default of hiding everything that begins with an underscore.

• Example:

  # date.py
  ​
  __all__ = ['is_date', 'constant', 'date_helper_1']
  ​
  constant = 3.14
  ​
  def is_date(arg):
      pass
  ​
  def date_helper_1():
      pass
  ​
  def date_helper_2():
      pass
  ​
  def constant():
      pass
  

  • Only 'is_date', 'constant', 'date_helper_1' can be used. Other funcs are hidden.