Python Basics: for i, element in enumerate(seq)..why/how does this work?

Question

Hello I've found an intersting snippet:

seq = ["one", "two", "three"] #edited
for i, element in enumerate(seq):
    seq[i] = '%d: %s' % (i, seq[i])

>>> seq
['0: one', '1: two', '2: three']

I wonder how python is doing that.... for me element should be undefined...but obviously it isn't..what does python do here?

Thanks a lot!

Answer 1

enumerate essentially turns each element of the input list into a list of tuples with the first element as the index and the element as the second. enumerate(['one', 'two', 'three']) therefore turns into [(0, 'one'), (1, 'two'), (2, 'three')]

The bit just after the for pretty much assigns i to the first element and element to the second for each tuple in the enumeration. So for example in the first iteration, i == 0 and element == 'one', and you just go through the other tuples to get the values for the other iterations.