python - Is there a need for range(len(a))?

Question

One frequently finds expressions of this type in python questions on SO. Either for just accessing all items of the iterable

for i in range(len(a)):
    print(a[i])

Which is just a clumbersome way of writing:

for e in a:
    print(e)

Or for assigning to elements of the iterable:

for i in range(len(a)):
    a[i] = a[i] * 2

Which should be the same as:

for i, e in enumerate(a):
     a[i] = e * 2
# Or if it isn't too expensive to create a new iterable
a = [e * 2 for e in a]

Or for filtering over the indices:

for i in range(len(a)):
    if i % 2 == 1: continue
    print(a[i])

Which could be expressed like this:

for e in a [::2]:
    print(e)

Or when you just need the length of the list, and not its content:

for _ in range(len(a)):
    doSomethingUnrelatedToA()

Which could be:

for _ in a:
    doSomethingUnrelatedToA()

In python we have enumerate, slicing, filter, sorted, etc... As python for constructs are intended to iterate over iterables and not only ranges of integers, are there real-world use-cases where you need in range(len(a))?

Answer 1

If you need to work with indices of a sequence, then yes - you use it... eg for the equivalent of numpy.argsort...:

>>> a = [6, 3, 1, 2, 5, 4]
>>> sorted(range(len(a)), key=a.__getitem__)
[2, 3, 1, 5, 4, 0]