I don't think the try-except is 'more performant'. At least from the below benchmark test it doesn't seem to be so.
>>> from timeit import timeit
>>> timeit(setup='x=dict([(i,i*2) for i in range(10)])',stmt=
"""
if 20 in x:
pass""")
0.07420943164572691
>>> timeit(setup='x=dict([(i,i*2) for i in range(10)])',
stmt="""
try:
x[20]
except KeyError:
pass""")
1.1514457843105674
I am on my phone and cannot test this, but the try:except: construct is optimised for the non-exceptional path. The latter is probably faster for x[0] than x[20].
