String replace method...though it was easy

Question

Took this one from a challenge:

>>> a = "abcefd"
>>> print(a.replace('cd','12'))

Output: 'abcefd' # why is that???????

I was expecting --> 'ab1ef2' .

Please explain a.replace('cd','12') is 'cd' given as an interval from c to d included, so 'cefd' , since it is not recognized as a substring???
Or shall I consider 'cd' as separate 'c' and 'd' ?

Usually if you look for tutorials about replace() method on strings, the typical example are always given with substrings of the original string, never seen something like that above.

Answer 1

It replace a substring 'cd' with string '12' but because in your source string ('abcdefd') its not present the substring 'cd', no replacement happen