Python’s Modules

Noah Black

• 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

Contents

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

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

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)

Executing modules as scripts

• Python scripts with .py extension maybe executed in command line withpython 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.

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

Module search path

• Interpreter looks for all imported modules in certain designated places, in the following order, until the module is found:1. Current directory2. The list of directories in PYTHONPATH

environment variable3. Installation-dependent default path

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

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

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

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>

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

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

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)

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.

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

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

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

Sources

• http://docs.python.org/library/– Information on Python’s standard library

• http://docs.python.org/tutorial/modules.html– Tutorial on Python’s modules