Replacing characters of a string by a given string

Question

Given this string 'www__ww_www_'

I need to replace all the '_' characters with characters from the following string '1234'. The result should be 'www12ww3www4'.

TEXT = 'aio__oo_ecc_'
INSERT = '1234'

insert = list(INSERT)
ret = ''

for char in TEXT:
    if char == '_':
        ret += insert[0]
        insert.pop(0)
    else:
        ret += char

print (ret)
>> aio12oo3ecc4

What is the right way to do this? Because this seems like the most inefficient way.

Answer 1

You can loop over the TEXT using a list comprehension that uses a ternary to select from an INSERT iterator or from the current element in TEXT:

>>> TEXT = 'aio__oo_ecc_'
>>> INSERT = '1234'
>>> it = iter(INSERT)
>>> "".join([next(it) if x == "_" else x for x in TEXT])
'aio12oo3ecc4'

The benefits include avoiding Shlemiel the Painter's Algorithm with ret += char. Also, pop(0) requires the whole list to be shifted forward, so it's linear (better would be reversing INSERT and using pop()).

In response to some of the comments here, list comprehensions tend to be faster than generators when the whole iterable will be consumed on the spot.