The built-in range() function is defined by an upper limit: the lower limit and step values are optional. The count() function, on the other hand, has a start and optional step, but no upper limit.
This function can be thought of as the primitive basis for a function such as enumerate(). We can define the enumerate() function in terms of zip() and count() functions, as follows:
enumerate = lambda x, start=0: zip(count(start), x)
The enumerate() function behaves as if it's a zip() function that uses the count() function to generate the values associated with some iterator.
Consequently, the following two commands are equivalent to each other:
>>> list(zip(count(), iter('word')))[(0, 'w'), (1, 'o'), (2, 'r'), (3, 'd')] ...
