How to replace an empty string with 0, but leave it alone if not empty

Question

Python's behavior seems inconsistent when replacing values in the case shown below (using python 3.6.5)

    >>> emptyString = '    '
    >>> emptyString.strip().replace('','0') #produces expected results
    '0'
    >>> notEmptyString = ' 50 '
    >>> notEmptyString.strip().replace('','0') #expected '50'
    '05000'
    >>> shortString = notEmptyString.strip()
    >>> shortString  #results as expected
    '50'
    >>> shortString.replace('','0') #unexpected results - expected '50'
    '05000'

This is what I'd like to see:

• if string has a value, just strip() the leading and trailing spaces.
• if string is empty (i.e. "") or string is just blank characters (i.e. " ") then strip it to be "" and replace "" with '0'

Example #1... string = " 10 ".... then just strip leading and trailing spaces
Example #2... string = ' ' .... then convert to '0'

I can get the results I want by other means, but I wondered if anybody understands why python produces these results.

Answer 1

If s is a string, then:

s.replace(old, new)

returns a copy of s with every occurrence of the string old replaced with new, so for example:

In [7]: "abracadabra".replace("a","4")
Out[7]: '4br4c4d4br4'

As a special case, if old is the empty string, it inserts new at the start and end of the string and between every pairs of characters:

In [8]: "12345678".replace("","_")
Out[8]: '_1_2_3_4_5_6_7_8_'

The rationale is that there's a "empty string" before the first character, between each pair of characters, and after the last character, and that's what's being replaced.

So, replace isn't doing what you were thinking.

To do what you what, you can use one of the solutions already proposed, or something like this if you're feeling clever:

s.strip() or "0"