When to use assert

Python's assert statement is a very useful feature that unfortunately often gets misused. assert takes an expression and an optional error message, evaluates the expression, and if it gives a true value, does nothing. If the expression evaluates to a false value, it raises an AssertionError exception with optional error message. For example:

py> x = 23
py> assert x > 0, "x is zero or negative"
py> assert x%2 == 0, "x is an odd number"
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AssertionError: x is an odd number

Many people use assertions as a quick and easy way to raise an exception if an argument is given the wrong value. But this is wrong, badly wrong, for two reasons. ( Read more... )

Lies in code

Quote of the week:

At Resolver we've found it useful to short-circuit any doubt and just refer to comments in code as 'lies'.

-- Michael Foord paraphrases Christian Muirhead on python-dev.