1. Modules

2. Modules Modules are the highest level program organization unit, usually correspond to source files and serve as libraries of tools. Each file is a module and modules import other modules to use the name they define. import import: lets a client fetch a module as a whole from from: allows to fetch particular names from module reload reload: provides a way to reload code without stopping Python

3. Why Use Modules? Modules provide an easy way to organize components into a system, by serving as packages of names Code reuse: code in module is persistent: can be reloaded and run as many times as needed System namespace partitioning: Everything “lives” in a module: code and objects are always implicitly enclosed by a module Implementing shared services or data

4. Structure a Program General architecture General architecture: Multiple text files with Python statements One top-level file with zero or more modules (imports) Top-level file contains the main control flow import A file imports a module to gain access to the tools it defines

5. Structure a Program Attributes Attributes are tools defined by a module (functions). in b.py: def spam(t): print t, in c.py: import b object.attribute b.spam(“hi”) # object.attribute

6. How Imports Work module search path: Find the module using module search path: Home directory of top-level file PYTHONPATH directories Standard directory libraries The contents of any *.pth (path configuration) files. It checks: source file(*.py), byte-code file(*.pyc), a directory for package imports, C extension module, in-memory image (frozen executables), java class, zip file(zipimport module)

7. How Imports Work Compile it to byte-code (if needed) *.pyc is saved only for imported modules Run its code to build the objects it defines Future imports skip all three steps and reuse already-loaded module

8. Modules Usage Clients can use the module file we just wrote by running “import” statement >>> import math >>> import random >>> import module1 # assume this is the file name we saved Clients can use the module file by running from statement >>>from fib import fib, fibTopN, list without module The copied names are used directly without going through module

9. Modules examples We create a file name module1: def print_out(aa): print aa*3 x=1 y=2

10. Modules Example now we write another file named module2 import module1 module1.print_out(“Hello World!! ”) # Use module1’s function print module1.x, module1.y # Reference module1’s variable x=10 y=20 print x, y The result of execute this program is: Hello World!! Hello World!! Hello World!! 1 2 10 20

11. Modules Example You may import as many modules as you like, for example: import module1 import module2 print module1.x print module2.x The result of execute this program is: 1 10