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Python FTW: Under the Hood

Slowing Down dict Lookups

Explore how Python's dictionary lookup functions work, particularly the difference between specialized string-key lookups and generic lookups. Understand the causes of slowed dictionary access when non-string keys are introduced, and learn how this behavior impacts your code's performance.

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Let’s show you how to slow down dict lookup at will.

Python 3.5
some_dict = {str(i): 1 for i in range(1000000)}
another_dict = {str(i): 1 for i in range(1000000)}
>>> timeit.timeit("some_dict['5']", number=1000, globals=globals())
6.216699989636254e-05
>>> some_dict[1] = 1
>>> timeit.timeit("some_dict['5']", number=1000, globals=globals())
9.339500002170098e-05
>>> timeit.timeit("another_dict['5']", number=1000, globals=globals())
6.14590001077886e-05
>>> another_dict[1]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
KeyError: 1
>>> timeit.timeit("another_dict['5']", number=1000, globals=globals())
8.72460000209685e-05

Try it out in the terminal below:

Terminal 1
Terminal
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Why are the same lookups becoming slower?

Explanation

  • CPython has a generic dictionary lookup function that handles all types of
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