Python FAQ: “How fast are exceptions?”

Question

I was just looking at the Python FAQ because it was mentioned in another question. Having never really looked at it in detail before, I came across this question: “How fast are exceptions?”:

A try/except block is extremely efficient. Actually catching an exception is expensive. In versions of Python prior to 2.0 it was common to use this idiom:

try:
    value = mydict[key]
except KeyError:
    mydict[key] = getvalue(key)
    value = mydict[key]

I was a little bit surprised about the “catching an exception is expensive” part. Is this referring only to those except cases where you actually save the exception in a variable, or generally all excepts (including the one in the example above)?

I’ve always thought that using such idioms as shown would be very pythonic, especially as in Python “it is Easier to Ask Forgiveness than it is to get Permission”. Also many answers on SO generally follow this idea.

Is the performance for catching Exceptions really that bad? Should one rather follow LBYL (“Look before you leap”) in such cases?

(Note that I’m not directly talking about the example from the FAQ; there are many other examples where you just look out for an exception instead of checking the types before.)

Answer 1

Catching exceptions is expensive, but exceptions should be exceptional (read, not happen very often). If exceptions are rare, try/catch is faster than LBYL.

The following example times a dictionary key lookup using exceptions and LBYL when the key exists and when it doesn't exist:

import timeit

s = []

s.append('''\
try:
    x = D['key']
except KeyError:
    x = None
''')

s.append('''\
x = D['key'] if 'key' in D else None
''')

s.append('''\
try:
    x = D['xxx']
except KeyError:
    x = None
''')

s.append('''\
x = D['xxx'] if 'xxx' in D else None
''')

for i,c in enumerate(s,1):
    t = timeit.Timer(c,"D={'key':'value'}")
    print('Run',i,'=',min(t.repeat()))

Output

Run 1 = 0.05600167960596991       # try/catch, key exists
Run 2 = 0.08530091918578364       # LBYL, key exists (slower)
Run 3 = 0.3486251291120652        # try/catch, key doesn't exist (MUCH slower)
Run 4 = 0.050621117060586585      # LBYL, key doesn't exist

When the usual case is no exception, try/catch is "extremely efficient" when compared to LBYL.