Python FAQ: “How fast are exceptions?”

by

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.

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