When I started learning Python, I used it's IDLE. That was a bit annoying, comparing with Java IDEs, such as NetBeans, or Eclipse (which I used them regularly that time). However, it was a good way to master coding with Python.
Python interpreter could be used to test smaller codes, but it is not a feasible way to write fast code for huge projects and systems. So Eclipse is probably the best open source IDE in the market for creating and managing Python programs.
Pydev is a plugin that enables users to use Eclipse for Python and Jython development -- making Eclipse a first class Python IDE. It comes with many goodies such as code completion, syntax highlighting, syntax analysis, refactor, debug and many others.
Pydev Extensions is a plugin that works with Pydev, providing many other features such as code-analysis, code completion with auto-import, a much better implementation of go to definition, remote debugging, 'quick-fixes', mark-occurrences, quick outline, browse globals, among others.