pitfall for your amusement

[Russell E. Owen]

In my own learning of Python I found a fair number of warnings about the 
problem you mentioned, so I feel it is pretty well documented. On the 
other hand, it was a familiar problem from Smalltalk and I may just have 
gotten lucky in my reading.

If you are collecting pitfalls, here's my least favorite: "never use a 
mutable object as a default value". A classic example:

def badfunc(alist=[]):
   ...

The default value of alist will not stay [] (an empty list) but instead 
is affected by whatever you pass in for "alist". Very tricky and 
unpleasant.

Here's my usual solution:
def okfunc(alist=None):
   alist = alist or []
   ...

-- Russell