1. Python’s Modules
Noah Black

2. Contents What are modules? Importing modules
Accessing module components
Executing modules as scripts
Module search path
Compiled Python files
Standard modules
The dir() function
Packages
Importing * from a package
Intra-package references
Packages in multiple directories

3. What are modules?
Definitions of functions and variables are not saved when interpreter is exited
Modules allow definitions to be saved for later access
Modules also allow statements to be run as executable scripts
Modules are files with .py extension

4. Importing modules
Modules are imported by using built-in import command, without the .py extension
>>> import example
To make access easier, individual definitions within a module may be imported
>>> from function import func1, func2
When modules are imported, all statements and definitions will be executed

5. Accessing module components
To use functions defined in module, type module name followed by dot
>>> example.func(3)
If function in module was imported individually with from, the module name and the dot may be excluded
>>> func(3)

6. Executing modules as scripts
Python scripts with .py extension maybe executed in command line with
python example.py <arguments>
Imported modules and executed modules can be distinguished by accessing global variable __name__: the value of __name__ is “__main__” only when modules are executed.

7. Modules as scripts, cont’d
Control statements can be used as follows:
if __name__ == "__main__":
All code contained in if statement will not run if module is imported

8. Module search path
Interpreter looks for all imported modules in certain designated places, in the following order, until the module is found:
Current directory
The list of directories in PYTHONPATH environment variable
Installation-dependent default path

9. Module search path, cont’d
Environment variable PYTHONPATH is a list of directories
Current directory, PYTHONPATH, and installation-dependent default path are all stored in sys.path variable

10. Compiled Python files
Compiled version of example.py is stored in example.pyc
If no .pyc file exists when a module is imported, it is created
Upon future imports, the already compiled version is used to save time, unless original .py file has a new modification time

11. Standard modules There is a standard library of Python modules
These modules contain built-in operations that are not actually a part of Python at its core
Some modules are only imported if a certain operating system is being used
One important library module is sys

12. The sys module
Two variable contained in sys, ps1 and ps2, hold the values of the strings in the primary and secondary prompts. These strings can be changed to your preference.
>>> import sys
>>> sys.ps1 = 'plurt> '
plurt> print ‘plurt'
plurt
plurt>

13. The dir() function
The dir() function returns a list of names (variables and functions) that are defined by the module passed to it.
>>> dir(sqrt)
[‘num', ‘result']
When dir is executed with no arguments, it returns a list of all currently defined names, except those that are built-in

14. Packages
Packages are organized collections of modules, modules within modules
Modules are stored in directories, with each directory containing a __init__.py file that tells the interpreter that it is a package directory
Each subdirectory of root package directory is considered a subpackage

15. Packages, cont’d
The package or any of its subpackages may then be imported.
import currency.conversion.euro
Any of euro’s definitions or subpackages can now be accessed, but only using dots:
euro.convertfromdollar(10)

16. Importing * from a package
Sometimes you may want to import everything inside a package or subpackage. In this case you may use this command:
from example.subexample import *
This loads all module names contained in variable __all__, which is itself contained in the package directory’s __init__.py file.

17. Importing * from a package, cont’d
If __all__ variable is not defined, then by default, importing * from a package will import all defined names of the package, and the package itself. Any submodules will not be imported unless they are explicitly imported by __init__.py or by

18. Intra-package references
Any module in a package may import any other modules in the same directory by name without using any dots
For importing any modules in a sibling folder, a module may use an absolute import

19. Packages in multiple directories
The location in which a package searches for its __init__.py may be changed, by modifying the variable __path__, which stores the value of the directory the __init__.py is in

20. Sources http://docs.python.org/library/
Information on Python’s standard library
Tutorial on Python’s modules