Dictionaries

The function has_key is useful to find out if a particular key exists in a dictionary, before trying to use it.

>>> primitive['square']
Traceback (most recent call last):
  File "", line 1, in ?
KeyError: square

>>> if primitive.has_key('square'):
...     print primitive['square']
... else:
...     print 'no such key'
...
no such key