Introduction
This function is a function that returns a function. You pass it a function, and it returns a function that takes a collection as a parameter and applies your function to it. The difference between this and map is that this returns a function that you can use again which takes a collection as a parameter.

For Example
If I had a collection c, and a function p that prints its argument, I can create a visitor function using the following:
printVisitor=createVisitor(p)

Then any time I want to print a collection I just type:
printVisitor(c)

A Better Example
If you were writing an IRC server you would probably have many functions that operated on a list of users, such as ban,makeop, etc. You could create those functions in a manor similar to the following:

banUsers=createVisitor(ban)
makeOperators=createVisitor(makeop)

Then in a seperate file you could use these functions as if they were regular functions.

you would type banUsers(users) to ban, or makeOperators(users) to promote those users.

Very fast method for extracting all of the unique items from a list while preserving their original ordering.

Reads a file line by line using Python in 3 lines

Reads well formed RSS entries using standard python libraries

Computes the difference between two dates in seconds

Decompress a .tar.bz2 archive in Pytho

Quick way to get the users home directory in Unix/Linux and Windows

Builds and prints a list of all links in a specified webpage

create a random string of 16 chars

Copy a file to a new location, optionally marking the file as a temporary file (to be deleted on application exit).

Note there are issues for temporary files - not all the parent directories that might be created are guaranteed to be deleted - only the immediate parent directory will be deleted on exit if created. Other higher-level directories will not be deleted, even if the file is marked as temporary.