Python中的string.find索引

Question

I'm trying to understand why the following python code incorrectly returns the string "dining":

def remove(somestring, sub):
"""Return somestring with sub removed."""
    location = somestring.find(sub)
    length = len(sub)
    part_before = somestring[:location]
    part_after = somestring[location + length:]
    return part_before + part_after

print remove('ding', 'do')

I realize the way to make the code run correctly is to add an if statement so that if the location variable returns a -1 it will simply return the original string (in this case "ding"). The code, for example, should be:

def remove(somestring, sub):
"""Return somestring with sub removed."""
    location = somestring.find(sub)
    if location == -1:
        return somestring
    length = len(sub)
    part_before = somestring[:location]
    part_after = somestring[location + length:]
    return part_before + part_after
print remove('ding', 'do')

Without using the if statement to fix the function, the part_before variable will return the string "din". I would love to know why this happens. Reading the python documentation on string.find (which is ultimately how part_before is formulated) I see that the location variable would become a -1 because "do" is NOT found. But if the part_before variable holds all letters before the -1 index, shouldn't it be blank and not "din"? What am I missing here?

For reference, Python documentation for string.find states:

string.find(s, sub[, start[, end]])

Return the lowest index in s where the substring sub is found such that sub is wholly contained in s[start:end]. Return -1 on failure. Defaults for start and end and interpretation of negative values is the same as for slices.

Answer 1

string = 'ding'
string[:-1]
>>> 'din'

Using a negative number as an index in python returns the n th element from the right-hand side. Accordingly, a slice with :-1 return all but the last element of the string.

Answer 2

If you have a string 'ding' and you are searching for 'do' , str.find() will return -1. 'ding'[:-1] is equal to 'din' and 'ding'[-1 + len(sub):] equals 'ding'[1:] which is equal to 'ing' . Putting the two together results in 'dining'. To get the right answer, try something like this:

def remove(string, sub):
    index = string.find(sub)
    if index == -1:
        return string
    else:
        return string[:index] + string[index + len(sub):]

The reason that string[:-1] is not equal to the whole string is that in slicing, the first number (in this case blank so equal to None , or for our purposes equivalent to 0) is inclusive, but the second number (-1) is exclusive. string[-1] is the last character, so that character is not included.