Python: How to Write a Module

You can define functions, save it in a file, then later on, load the file and use these functions. For example, save the following line in a file and name it xy.py.

# -*- coding: utf-8 -*-
# python

def f1(n):
    return n+1

To load the file, use import import xy, then to call the function, use xy.f1. Example:

# -*- coding: utf-8 -*-
# python

import xy           # import the module

print xy.f1(5)      # calling a function

Python Module/Package/Namespace Details

• a “module” is a single file of Python code.
• a “package” is a directory of Python modules. (but can mean a single file module too.)

If you have multiple files, they make up a package. You put all files inside directory, possibly with nested sub directories. Like this:

fangame
 __init__.py
 soundx
  __init__.py
  rabbit_punch.py
 vid
  __init__.py
  tiger.py
 thinky
  __init__.py
  supertramp.py
  subby
   __init__.py
   a.py
   b.py

For example, in the above, you can load a module by:

import fangame.soundx.rabbit_punch
import fangame.thinky.subby.a
# ...

Dir with __init__.py means that directory is part of the package. Python will look at the dir structure and file names as module's name when import is called.

For example, if dir soundx does not have a __init__.py file, then import fangame.soundx.rabbit_punch won't work, because Python won't consider dir soundx as part of the package.

Similarly, fangame must have a __init__.py. And, if you want import fangame to automatically load soundx/rabbit_punch, then, in fangame/__init__.py you must have import soundx.rabbit_punch.

The file __init__.py will be executed when that particular module in that directory is loaded. If you don't need any such init file, just create a empty file.

Here's a summary:

• When you do import name (or variant syntax of import), Python searches for modules at current dir and sys.path. 〔►see Python: List Modules, Search Path, Loaded Modules〕
• Module files are just regular Python code. There's no code for module/package/namespace declaration.
• A package is directory of modules.
• A package's directory must have __init__.py file, but it can be empty. This file is loaded when modules in that dir is imported.
• File names and directory structure are automatically taken to be the module's name and form namespace. For example, a dir named a/b/c.py is a module that can be imported with import a.b.c

Note: it is suggested that module names be short and lower case only.

Note: the Python language does not have direct technical concept of “module” nor “package”. For example, there's not keyword “module” nor “package”. These words are only used to describe the concept of libraries. Technically, Python language only has a sense of namespace, and is exhibited by {import, __init__.py, __name__, __all__, …}.

Syntax 「from x.y import z」

Alternative syntax for loading module is from x.y import z.

Typically, this is used to import the name z, of single function z of module file at x/y.py. But the actual semantics is a bit complex.

from x.y import z will do ONE of two things:

• ① Import the name z of a single function/variable z of a module at path x/y.py, if that module does define the name z.
• ② Import the module names as prefix name z of a module at path x/y/z.py, if the file x/y.py does not contain the name z. For example, if x/y/z.py contains a function named f, then after from x.y import z, you can call f by z.f().

If the module x/y.py doesn't have the name z, and there's no module at x/y/z.py, then a ImportError is raised.

6. Built-in Exceptions — Python v2.7.6 documentation #exceptions.ImportError

Syntax 「from x.y import *」

Another syntax for loading module is from x.y import *.

from x.y import * will do this:

If the file at x/y/__init__.py defines a variable named __all__ (which should contain a list of strings, each string is a submodule name), then, all those sub-modules are imported. The names imported are the strings of __all__.

For example, if __all__ = ["a","b"], then the module at x/y/a.py is imported as name a and the module at x/y/b.py is imported as name b.

If __all__ is not defined, then from x.y import * just imports the name x.y (for module x/y.py) and all names from module x/y.py

• 5. Built-in Types — Python v2.7.6 documentation #modules
• 6. Modules — Python v2.7.6 documentation #modules
• 6. Modules — Python v2.7.6 documentation #packages

2014-04-09 Thanks to Kaito Kumashiro for correction. Thanks to Demian Brecht for suggestion.