Learning Python

Exception Gotchas

There isn’t much to trip over here, but here are a few general pointers on exception use.

Exceptions Match by Identity, Not Equality

As we’ve seen, when an exception is raised (by you or by Python itself), Python searches for the most recently entered try statement with a matching except clause, where matching means the same string object, the same class object, or a superclass of the raised class object. It’s important to notice that matching is performed by identity, not equality. For instance, suppose we define two string objects we want to raise as exceptions:

>>> ex1 = "spam"
>>> ex2 = "spam"
>>>
>>> ex1 == ex2, ex1 is ex2
(1, 0)

Applying the == test returns true (1) because they have equal values, but is returns false (0) since they are two distinct string objects in memory. Now, an except clause that names the same string object will always match:

>>> try:
...    raise ex1
... except ex1:
...    print 'got it'
...
got it

But one that lists an equal but not identical object will fail:

>>> try:
...    raise ex1
... except ex2:
...    print 'Got it'
...
Traceback (innermost last):
  File "<stdin>", line 2, in ?
spam

Here, the exception isn’t caught, so Python climbs to the top level of the process and prints a stack trace and the exception automatically (the string "spam"). For class exceptions, the behavior is similar, but Python generalizes the notion of exception matching to include superclass relationships.

Catching Too Much?

Because Python lets you pick and choose which ...