List vs generator comprehension speed with join fu

Question

This question already has an answer here:

• List comprehension vs generator expression's weird timeit results? 3 answers
• List comprehension without [ ] in Python 7 answers

So I got these examples from the official documentation. https://docs.python.org/2/library/timeit.html

What exactly makes the first example (generator expression) slower than the second (list comprehension)?

>>> timeit.timeit('"-".join(str(n) for n in range(100))', number=10000)
0.8187260627746582
>>> timeit.timeit('"-".join([str(n) for n in range(100)])', number=10000)
0.7288308143615723

Answer 1

The str.join method converts its iterable parameter to a list if it's not a list or tuple already. This lets the joining logic iterate over the items multiple times (it makes one pass to calculate the size of the result string, then a second pass to actually copy the data).

You can see this in the CPython source code:

PyObject *
PyUnicode_Join(PyObject *separator, PyObject *seq)
{
    /* lots of variable declarations at the start of the function omitted */

    fseq = PySequence_Fast(seq, "can only join an iterable");

    /* ... */
}

The PySequence_Fast function in the C API does just what I described. It converts an arbitrary iterable into a list (essentially by calling list on it), unless it already is a list or tuple.

The conversion of the generator expression to a list means that the usual benefits of generators (a smaller memory footprint and the potential for short-circuiting) don't apply to str.join, and so the (small) additional overhead that the generator has makes its performance worse.