Python: How to Write a Module

By Xah Lee. Date: . Last updated: .

You can define functions, save it in a file, then later on, load the file and use these functions.

Save the following in a file and name it

# python 3

def f1(n):
    return n+1

To load the file, use import yy, then to call the function, use yy.f1

# python 3

# import the module
import yy

# calling a function
print yy.f1(5)

See also:

Python Module/Package/Namespace

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


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

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

Dir with 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 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 And, if you want import fangame to automatically load soundx/rabbit_punch, then, in fangame/ you must have import soundx.rabbit_punch.

The file 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:

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,, __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/ But the actual semantics is a bit complex.

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

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

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/ 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/ is imported as name a and the module at x/y/ is imported as name b.

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

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

Python: Paths Topic

  1. File Path
  2. Print Version String from Script
  3. Executable Path, Script Path
  4. List Modules, Search Path, Loaded Modules
  5. How to Write a Module

If you have a question, put $5 at patreon and message me.


  1. Python 3 Basics
  2. Python 2 Basics
  3. Python 2 and 3 Difference
  4. Print Version
  5. Builtin Help
  6. Quote String
  7. String Methods
  8. Format String
  9. Operators
  10. Complex Numbers
  11. True, False
  12. if then else
  13. Loop
  14. List Basics
  15. Loop Thru List
  16. Map f to List
  17. Copy Nested List
  18. List Comprehension
  19. List Methods
  20. Sort
  21. Dictionary
  22. Loop Thru Dict
  23. Dict Methods
  24. Tuple
  25. Sets
  26. Function
  27. Closure
  28. 2 Closure
  29. Decorator
  30. Class
  31. Object, ID, Type
  32. List Modules
  33. Write a Module
  34. Unicode 🐍


  1. Regex Basics
  2. Regex Reference

Text Processing

  1. Read/Write File
  2. Traverse Directory
  3. File Path
  4. Process Unicode
  5. Convert File Encoding
  6. Find Replace in dir
  7. Find Replace by Regex
  8. Count Word Frequency


  1. Send Email
  2. GET Web Page
  3. Web Crawler


  1. JSON
  2. Find Script Path
  3. Get Env Var
  4. System Call
  5. Decompress Gzip
  6. Append String in Loop
  7. Timing f timeit
  8. Keyword Arg Default Value Unstable
  9. Check Page Load Size
  10. Thumbnail Generation