Python enumerate downwards or with a custom step

Question

How to make Python's enumerate function to enumerate from bigger numbers to lesser (descending order, decrement, count down)? Or in general, how to use different step increment/decrement in enumerate?

For example, such function, applied to list ['a', 'b', 'c'], with start value 10 and step -2, would produce iterator [(10, 'a'), (8, 'b'), (6, 'c')].

Answer 1

I haven't found more elegant, idiomatic, and concise way, than to write a simple generator:

def enumerate2(xs, start=0, step=1):     for x in xs:         yield (start, x)         start += step 

Examples:

>>> list(enumerate2([1,2,3], 5, -1)) [(5, 1), (4, 2), (3, 3)] >>> list(enumerate2([1,2,3], 5, -2)) [(5, 1), (3, 2), (1, 3)] 

If you don't understand the above code, read What does the "yield" keyword do in Python? and Difference between Python's Generators and Iterators.

Answer 2

One option is to zip your iterable to a range:

for index, item in zip(range(10, 0, -2), ['a', 'b', 'c']):     ... 

This does have the limitation that you need to know how far the range should go (the minimum it should cover - as in my example, excess will be truncated by zip).

If you don't know, you could roll your own "infinite range" and use that:

>>> def inf_range(start, step):     """Generator function to provide a never-ending range."""     while True:         yield start         start += step   >>> list(zip(inf_range(10, -2), ['a', 'b', 'c'])) [(10, 'a'), (8, 'b'), (6, 'c')]