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Modules and stuff

Any Python program can be imported as a module.

A simple module:

print("Hello, world!")

Then you can tell your interpreter where to look for the module by executing the following (using the Windows directory):

>>> import sys
>>> sys.path.append('C:/python')

Now you can import your module:

>>> import hello
Hello, world!

Defining a function in a module

def hello():
    print("Hello, world!")


>>> import hello2

The module is then executed, which means that the function hello is defined in the scope of the module, so you can access the function like this:

>>> hello2.hello()
Hello, world!

Adding Test Code in a Module

def hello():
    print("Hello, world!")

# A test:

If you import it as a module, to use the hello function in another program, the test code is executed, as in the first hello module in this chapter.

>>> import hello3
Hello, world!
>>> hello3.hello()
Hello, world!

This is not what you want. The key to avoiding this behavior is checking whether the module is run as a program on its own or imported into another program. To do that, you need the variable name.

>>> __name__
>>> hello3.__name__

Correct version:


def hello():
    print("Hello, world!")

def test():

if __name__ == '__main__': test()

If you run this as a program, the hello function is executed; if you import it, it behaves like a normal module.

>>> import hello4
>>> hello4.hello()
Hello, world!

Make your module available

You have to put your modules to the right place:

>>> import sys, pprint
>>> pprint.pprint(sys.path)

Place your hello script to one of the site-packages directory and try to import module again.

Telling the Interpreter Where to Look

Use environment variable PYTHONPATH.



To structure your modules, you can group them into packages. A package is basically just another type of module. The interesting thing about them is that they can contain other modules.

While a module is stored in a file (with the file name extension .py), a package is a directory. To make Python treat it as a package, it must contain a file named

Simple example:

File/Directory Description
∼/python/ Directory in PYTHONPATH
∼/python/drawing/ Package directory (drawing package)
∼/python/drawing/ Package code (drawing module)
∼/python/drawing/ colors module
∼/python/drawing/ shapes module

Now we can do following:

import drawing             # (1) Imports the drawing package
import drawing.colors      # (2) Imports the colors module
from drawing import shapes # (3) Imports the shapes module

Using dir()

To find out what a module contains, you can use the dir function , which lists all the attributes of an object (and therefore all functions, classes, variables, and so on, of a module). If you print out dir(copy), you get a long list of names.

>>> import copy
>>> [n for n in dir(copy) if not n.startswith('_')]
['Error', 'PyStringMap', 'copy', 'deepcopy', 'dispatch_table', 'error', 'name', 't', 'weakref']

Help and documentation

>>> help(copy.copy)
Help on function copy in module copy:

    Shallow copy operation on arbitrary Python objects.

    See the module's __doc__ string for more info.

This tells you that copy takes a single argument x and that it is a "shallow copy operation."

range(stop) -> range object
range(start, stop[, step]) -> range object

Return an object that produces a sequence of integers from start (inclusive)
to stop (exclusive) by step.  range(i, j) produces i, i+1, i+2, ..., j-1.
start defaults to 0, and stop is omitted!  range(4) produces 0, 1, 2, 3.
These are exactly the valid indices for a list of 4 elements.
When step is given, it specifies the increment (or decrement).

Source code

If you want to find source code of the script on filesystem, try following:

>>> print(copy.__file__)

The standard library


The sys module gives you access to variables and functions that are closely linked to the Python interpreter.

Function/Variable Description
argv The command-line arguments, including the script name
exit([arg]) Exits the current program, optionally with a given return value or error message
modules A dictionary mapping module names to loaded modules
path A list of directory names where modules can be found
platform A platform identifier such as sunos5 or win32
stdin Standard input stream—a file-like object
stdout Standard output stream—a file-like object
stderr Standard error stream—a file-like object


Function/Variable Description
environ Mapping with environment variables
system(command) Executes an operating system command in a subshell
sep Separator used in paths
linesep Line separator ('\n', '\r', or '\r\n')
urandom(n) Returns n bytes of cryptographically strong random data

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