Why is updating a list faster when using a list comprehension as opposed to a generator expression?

Question

According to this answer lists perform better than generators in a number of cases, for example when used together with str.join (since the algorithm needs to pass over the data twice).

In the following example using a list comprehension seems to yield better performance than using a corresponding generator expression though intuitively the list comprehension comes with an overhead of allocating and copying to additional memory which the generator sidesteps.

In [1]: l = list(range(2_000_000))

In [2]: %timeit l[:] = [i*3 for i in range(len(l))]
190 ms ± 4.65 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)

In [3]: %timeit l[:] = (i*3 for i in range(len(l)))
261 ms ± 7.14 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)

In [4]: %timeit l[::2] = [i*3 for i in range(len(l)//2)]
97.1 ms ± 2.07 ms per loop (mean ± std. dev. of 7 runs, 10 loops each)

In [5]: %timeit l[::2] = (i*3 for i in range(len(l)//2))
129 ms ± 2.21 ms per loop (mean ± std. dev. of 7 runs, 10 loops each)

In [6]: %timeit l[:len(l)//2] = [i*3 for i in range(len(l)//2)]
92.6 ms ± 2.34 ms per loop (mean ± std. dev. of 7 runs, 10 loops each)

In [7]: %timeit l[:len(l)//2] = (i*3 for i in range(len(l)//2))
118 ms ± 2.17 ms per loop (mean ± std. dev. of 7 runs, 10 loops each)

Why does a list comprehension yield better performance in these cases?

Answer 1

This answer concerns CPython implementation only. Using a list comprehension is faster, since the generator is first converted into a list anyway. This is done because the length of the sequence should be determined before proceeding to replace data, and a generator can't tell you its length.

For list slice assignment, this operation is handled by the amusingly named list_ass_slice. There is a special-case handling for assigning a list or tuple, here - they can use PySequence_Fast ops.

This is the v3.7.4 implementation of PySequence_Fast, where you can clearly see a type-check for list or tuples:

PyObject *
PySequence_Fast(PyObject *v, const char *m)
{
    PyObject *it;

    if (v == NULL) {
        return null_error();
    }

    if (PyList_CheckExact(v) || PyTuple_CheckExact(v)) {
        Py_INCREF(v);
        return v;
    }

    it = PyObject_GetIter(v);
    if (it == NULL) {
        if (PyErr_ExceptionMatches(PyExc_TypeError))
            PyErr_SetString(PyExc_TypeError, m);
        return NULL;
    }

    v = PySequence_List(it);
    Py_DECREF(it);

    return v;
}

A generator expression will fail this type check and continue to the fallback code, where it is converted into a list object, so that the length can be predetermined.

In the general case, a predetermined length is desirable in order to allow efficient allocation of list storage, and also to provide useful error messages with extended slice assignment:

>>> vals = (x for x in 'abc')
>>> L = [1,2,3]
>>> L[::2] = vals  # attempt assigning 3 values into 2 positions
---------------------------------------------------------------------------
                                          Traceback (most recent call last)
...
ValueError: attempt to assign sequence of size 3 to extended slice of size 2
>>> L  # data unchanged
[1, 2, 3]
>>> list(vals)  # generator was fully consumed
[]