If the function is from a source file available on the filesystem, then inspect.getsource(foo) might be of help:

If foo is defined as:

def foo(arg1,arg2):         
    #do something with args 
    a = arg1 + arg2         
    return a  

Then:

import inspect
lines = inspect.getsource(foo)
print(lines)

Returns:

def foo(arg1,arg2):         
    #do something with args 
    a = arg1 + arg2         
    return a                

But I believe that if the function is compiled from a string, stream or imported from a compiled file, then you cannot retrieve its source code.

Since Python is open source you can read the source code.

To find out what file a particular module or function is implemented in you can usually print the __file__ attribute. Alternatively, you may use the inspect module, see the section Retrieving Source Code in the documentation of inspect.

For built-in classes and methods this is not so straightforward since inspect.getfile and inspect.getsource will return a type error stating that the object is built-in. However, many of the built-in types can be found in the Objects sub-directory of the Python source trunk. For example, see here for the implementation of the enumerate class or here for the implementation of the list type.

The source code for the reference implementation of Python is available here at their GitHub mirror. However, it's worth noting that large parts of the Python language are implemented in C, including the core engine and many of the standard library libraries. Really understanding how everything is implemented under the hood requires a fair amount of C fluency.

1. If you enable the source code repositories in "Software sources" application (software-properties-gtk), you can download the latest python source that is related to the package:

   apt-get source python
   

   You can do that for any ubuntu/debian package.

2. You can always google your way to the source code: http://www.python.org/download/releases/