There are as many Python editors and interactive systems as hydrological models. Since cmf is nothing more than an extension to Python, you can just use what you like. However, if you are new to Python, you can start with the software tools used by the cmf developers:

Interactive use

IPython

http://ipython.org/ - Works just great together with matplotlib for plotting results of cmf. Provides fantastic helper functions and code completion with the tab key.

Spyder

http://packages.python.org/spyder/ - Integrates IPython into a bigger environment which is by coincidence quite similar to some proprietary software for scientific modelling.

Both programs are integrated for Windows in scientific Python distributions, like WinPython, python(x,y) and Anaconda. WinPython is smaller and available for 32bit and 64bit, while python(x,y) has more modules and non-python tools integrated, but it is only available for 32bit Python. I recommend the usage of a scientific Python distribution on Windows.

Editors

Eclipse with PyDev

http://eclipse.org and http://pydev.org - The Diva among the Python Editors (personal opinion). Takes years to start, fails sometimes completely, but if it is there it is simply the best - with code completion like in Visual Studio. Since it is eclipse based, everything can be done with the proper plugin. Not the nicest for installation.

PyCharm

https://www.jetbrains.com/pycharm/ - The only software for money on this website but it is really good. Academics, like students and teachers get a free licence.

Notepad++

http://notepad-plus-plus.org/ - Great, light, fast editor for Windows, offers code highlighting for many languages.

gedit, kate, emacs, vim

If you are familiar Linux, I'm sure you have already your favourite light editor with syntax highlighting, otherwise use what comes with your distribution.