Using Design by Contract in Python

Question

I am looking to start using DBC on a large number of Python-based projects at work and am wondering what experiences others have had with it. So far my research turned up the following:

• http://www.python.org/dev/peps/pep-0316/ - PEP 316 that is supposed to standardize design by contract for Python which has been deferred. This PEP suggests using docstrings.
• http://www.wayforward.net/pycontract/ - Contracts for Python. This seems to be a complete, but unmaintained framework using docstrings.
• http://www.nongnu.org/pydbc/ - PyDBC which implements contracts using metaclasses. Also unmaintained for a few years.

My questions are: have you used DBC with Python for mature production code? How well did it work/was it worth the effort? Which tools would you recommend?

Answer 1

The PEP you found hasn't yet been accepted, so there isn't a standard or accepted way of doing this (yet -- you could always implement the PEP yourself!). However, there are a few different approaches, as you have found.

Probably the most light-weight is just to simply use Python decorators. There's a set of decorators for pre-/post-conditions in the Python Decorator Library that are quite straight-forward to use. Here's an example from that page:

  >>> def in_ge20(inval):   ...    assert inval >= 20, 'Input value < 20'   ...   >>> def out_lt30(retval, inval):   ...    assert retval < 30, 'Return value >= 30'   ...   >>> @precondition(in_ge20)   ... @postcondition(out_lt30)   ... def inc(value):   ...   return value + 1   ...   >>> inc(5)   Traceback (most recent call last):     ...   AssertionError: Input value < 20 

Now, you mention class invariants. These are a bit more difficult, but the way I would go about it is to define a callable to check the invariant, then have something like the post-condition decorator check that invariant at the end of every method call. As a first cut you could probably just use the postcondition decorator as-is.