Tuples Are Slow

Python tuples (immutable sequences) can be much slower to work with than their mutable cousins, lists. This surprised me; I would have thought they would have been faster all ’round, as they seem “simpler”. That doesn’t seem to be the case.

Methodology

I ran some simple timing experiments comparing list and tuple operations in Python 2.5.1, from inside the PythonWin environment (build 210). I used the timeit module for all tests, running the default 1,000,000 trials.

Creation

If you’re creating a sequence from an explicit set of values, tuples are much faster to create:

>>> timeit.Timer('[1,2,3,4,5,6,7,8,9]').timeit()
0.6413645024165362
>>> timeit.Timer('(1,2,3,4,5,6,7,8,9)').timeit()
0.050510743951576842

If you’re creating a sequence algorithmically, e.g. from a list comprehension, tuples are slower to create:

>>> timeit.Timer('[i*2 for i in range(10)]').timeit()
4.1430525274962866
>>> timeit.Timer('tuple(i*2 for i in range(10))').timeit()
11.855401369060489

It takes nearly three times as long to create a short tuple from a list comprehension as it does to create a list.

Slices

Tuples are quicker to slice than lists:

>>> timeit.Timer('t[:5]', 't = [i*2 for i in range(10)]').timeit()
0.54482657497751319
>>> timeit.Timer('t[:5]', 't = tuple(i*2 for i in range(10))').timeit()
0.25133068111281887

It takes about 1/2 the time to slice a tuple as it does to slice a list.

Deletion

However, since tuples are immutable, it’s very expensive to remove items from them:

>>> timeit.Timer('t[:].remove(6)', 't = [i*2 for i in range(10)]').timeit()
1.007475770140104
>>> timeit.Timer('tuple(e for e in t if e != 6)', 't = tuple(i*2 for i in range(10))').timeit()
7.2336211566225757

Since tuples have no equivalent to the list.remove() method, a new tuple must be constructed that omits the undesired element. For short tuples, this process can take over 7 times longer than the equivalent list.remove() call. (Note that the actual difference is even greater, as my test code copies the list before invoking list.remove().)

Summary

In inner loops, the choice between lists and tuples can make a big difference in performance. It’s advisable to consider exactly how you plan to use your sequence objects, and choose accordingly.