Question

I have always thought that using -1 in a condition is alway the same as the writing False (boolean value). But from my code, I get different results:

Using True and False:

def count(sub, s):
    count = 0
    index = 0
    while True:
        if string.find(s, sub, index) != False:
            count += 1
            index = string.find(s, sub, index) + 1
        else:
            return count


print count('nana', 'banana')

Result: Takes to long for interpreter to respond.

---

Using 1 and -1:

def count(sub, s):
    count = 0
    index = 0
    while 1:
        if string.find(s, sub, index) != -1:
            count += 1
            index = string.find(s, sub, index) + 1
        else:
            return count


print count('nana', 'banana')

Result: 1

Why does using -1 and 1 give me the correct result whereas using the bool values True and False do not?

Answer 1

string.find doesn’t return a boolean so string.find('banana', 'nana', index) will NEVER return 0 (False) regardless of the value of index.

>>> import string
>>> help(string.find)
Help on function find in module string:

find(s, *args)
    find(s, sub [, start [, end]]) -> int

    Return the lowest index in s where substring sub is found,
    such that sub is contained within s[start,end].  Optional
    arguments start and end are interpreted as in slice notation.

    Return -1 on failure.
>>>

Your example simply repeats:

index = string.find('banana', 'nana', 0) + 1 # index = 3
index = string.find('banana', 'nana', 3) + 1 # index = 0

The -1 version works because it correctly interprets the return value of string.find!