Python’s Modules

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 >>> importexample • To make access easier, individual definitions within a module may be imported >>> fromfunctionimport 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. >>> importsys >>> 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. importcurrency.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: fromexample.subexampleimport* • 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 • http://docs.python.org/tutorial/modules.html • Tutorial on Python’s modules