> Python is Not a Configuration File Format

But a subset of python syntax is.

if so, tell that to twistedmatrix guys, that .tac files they use in twisted - was wrong choice. choice which do not require additional parsing, which is specific for python, which is really simplier than xml (parsing xml is always expensive). or... maybe twisted is too small and too young framework as for You? ;)

if You are reading some configuration in python application - what would be output produced from this configuration? most probably - python variables... so at end You will convert your configuration into ... python representation.
btw. what You have in mind by writing "your application is now not able to configure itself". what this has connected with configuration file format? do You know other method of reading configuration from .py file than importing it?

most configuration files start little, so there's really no reason to kick in the power of a real programming language. but sooner or later, you need to add conditional configuration ("if windows:... elif linux:..."), and then it might make sense to split your configuration over several files which include/import a common one, and the list of features grows longer.

long story short - at some point, you will need a powerful language (maybe not turing complete, but surely more powerful than key=value), so why invent a new DSL when you can just use python?

I would not suggest JSON as configuration file format, because it is lacking support for comments. Just sayin :)

My rule of thumb for configuration files:

If I cannot figure it out how to do something with .ini files, I should not be doing that anyway, and should be looking for a simpler solution that indeed works with .ini files :-D.

Agreed. However, I think there is one thing to be said in favor of using Python as configuration, or rather, one prominent reason that many apps choose to use it: it's dead simple to use. Compare "import settings" to the whole ConfigParser song and dance you're required to use for that format, and for an early stage project using Python starts to look really appealing.

Are there any good modules that can make this easer? Probably. Are they well known? Maybe not (I don't know of any, nor does the Hitchhiker's Guide). I think if we can solve this hurdle, or find and promote a package that already does, we can begin to effectively combat this bad practice; until then, and with Django and Twisted setting bad examples (in my opinion, and I believe in yours), it's unlikely to change.

As with any binary argument, I think the answer is "both".

The actual danger is expecting to be able to use "import" to retrieve configuration settings. It's a convenience that leads to tears. I suspect most Django core developers would agree (e.g. "import settings" is a source of circref problems and an inability to put more than one Django configuration into the same Python process).

Instead, it seems saner to put true configuration in a non-importable format (e.g. ini). Then have your code read that file, and operate *against the result* to do conditionalized things, e.g.:

def config(the_ini_file):
adict = read_ini_file(the_ini_file)
if adict.get('debug'):
...
...

In this way you can get the benefit of simplicity of pure-ini-syntax when you don't need conditionals, but you have a place to stick Python code to do conditionals and looping where necessary. You can also always create a "settings.py" that reads the ini at module scope and injects the result into module globals, so if you really *must* "import settings" you can do it.. you just can't go the other way around and turn a system that expects settings to be importable into one that does not.