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Modules and Packages

• The very basic: Every source file is a module. The import module will load AND execute a module.
• Modules are a namespace for the definitions inside, and as expected, a layer over a Python dictionary (the globals of that module). Special variables: __file__ (name of source file), __name__ (name of the module) and __doc__. -> The main module: __name__ == __main__.
• import foo in bar.py: Executes foo and adds a reference to foo in bar.__dict__
  • from foo import func : foo still executes, but only func is added to bar’s dictionary
  • from foo import *: All names that don’t start with underscore get added to bar
• Each module is loaded only once. Use sys.modules to get a list of loaded modules.
• Module reloading (with importlib): This is not advised, since existing instances of classes will still use old code, specific names imported from foo import name don’t get updated, and can break typechecks/ code with super()
• Module import basics: Relative imports of submodules don’t work - imports are always absolute
  • Use “.” prefix
• Packages: __package__ (name of the enclosing package), __path__ (search path for subcomponents)
• Main use of __init__.py: stitching together multiple source files into a “unified” top-level import
• Controlling exports: Submodules should define __all__ to control from module import *. This is useful in __init__!
• __main__.py designates an entrypoint for a package! Makes the module executable python -m module
• Inject dynamic import magic with __import__ or importlib. Can do things like __import__(f"{__package__}.formats.{format}") (this imports a file from .formats if present)