modules.md

Modules

Modules

• module is a Python file (e.g. my_mod.py) on sys.path:
  • directory from which the input script was run, or the current directory if the interpreter is being run interactively
  • list of directories contained in the PYTHONPATH environment variable, if it is set
  • installation-dependent list of directories configured at the time Python is installed
• module types
  • written in Python itself
  • built-in module (e.g.itertools)
  • written in C and loaded dynamically at run-time (e.g. re)
• dir() - list of names in the current local scope
• dir(object) - list of names for the given object (module, class, ..)
• my_mod.__file__ - module file (not available for built-in modules)

Packages

• folder with Python files (modules)
• __init__.py - invoked when importing the package or a module from the package
• when invoking a subpackage, parent packages are initialized first

Import

• module is initialized the first time it is imported (like static blocks in Java)
• dynamic imports - imports can be anywhere in the code

  if visual_mode:
      import draw_visual as draw
  else:
      import draw_textual as draw

• handling import errors - except ImportError:

import MODULE [as ALIAS]

• imports the module/package namespace (private symbol table)

  import foo.bar
  foo.bar.x  # OK, foo.bar was imported
  dir()  # .., foo

from MODULE import OBJECT [as ALIAS]

• directly imports the given object name into the current namespace (like static import in Java)

  from foo.bar import x
  x  # OK, foo.bar.x was imported
  dir()  # .., x

from MODULE/PACKAGE import *

• module - directly imports all the names from the given module except for names starting with _
  • similar to packages, what's imported can be changed by defining __all__ variable
• package - by default does nothing
  • define __all__ in __init__.py and define what * should import
• risk of overwriting already defined names