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Python Module

Module
A file containing Python definitions and statements

The file name is the module name with .py suffixed.

The module is name is available in the module as a string with the global __name__ variable.

A module can contain statements, which are intended to initialize the module. These are executed only the first time a module is imported, or when a module is executed as a script.

A module has its own private namespace. This namespace is the global namespace within the module.

You can import modules like

import <module-name>
# Let's you import names directly into the caller's namespace.
# Doesn't import the module name itself.
from <module-name> import <name-in-module> 
# Imports everything except the names starting with _
from <module-name> import *
# Import module name as a different name
import <module-name> as different-name
# Import names within a module as a different name
from <module-name> import <name-in-module> as different-name

Running Module as Scripts

You can run a module:

$ python foo.py <args>

When running as a script the __name__ global variable is set to "__main__"

You can use this fact to have that only runs during scripts, and not imports.

if __name__ == "__main__":
    print("HELLO")

Module Search Path

Python searches three places when trying to import a module:

  1. Directory containing the script, or current directory if no file is specified.
  2. $PYTHONPATH (a list of directory names, with the same syntax as the shell variable PATH).
  3. The installation-dependent default

The search path is accessible in python via sys.path variable.

dir()

The built-int function dir() returns all names defined for the module.

Packages

Packages
allows a way to structure module namespaces
cool-package/
├── __init__.py
├── sub-pkg-1/
│   ├── __init__.py
│   └── foo.py
└── sub-pkg-2/
    ├── __init__.py
    └── bar.py

The __init__.py files are required to make Python treat directories containing the file as packages. It can be empty or contain initialization code.

You can then import packages using dot notation.

import cool-package.sub-pkg-1.foo

# Then we can use it via the full name
cool-package.sub-pkg-1.foo.do("HI")

alternatively

from cool-package.sub-pkg-1 import foo

foo.do("HI")

or even

from cool-package.sub-pkg-1.foo import do

do("HI")

Importing * From a Package

TODO