Cleanest way to obtain the numeric prefix of a string

Question

What is the cleanest way to obtain the numeric prefix of a string in Python?

By \"clean\" I mean simple, short, readable. I couldn\'t care less about performance, a

Answer 1

input = '123abc456def'
output = re.findall(r'^\d+', input)

Will return ['123'] too.

Answer 2

One way, but not very efficient since it works through the whole string without break would be:

input_string = '123abc456def'
[input_string[:c] for c in range(len(input_string)) if input_string[:c].isdigit()][-1]

This appends each substring with increasing size if it is a digit and then appends it. So the last element is the one you look for. Because it is the longest startstring that is still a digit.

Answer 3

You can use itertools.takewhile which will iterate over your string (the iterable argument) until it encounters the first item which returns False (by passing to predictor function):

>>> from itertools import takewhile
>>> input = '123abc456def'
>>> ''.join(takewhile(str.isdigit, input))
'123'