Python doesn't recognize -1 as False?

Question

I recently wrote a Python program that used:

if s.find('something'): 

or

if s.find('something')==True:

but nothing worked correctly until I used:

if s.find('something') >0

That's the only way I could get an accurate True decision in my program.

It seems -1 result wasn't recognized as False. Successful finds weren't recognized as True either. So looking for -1 (False) or a positive number >0 (True) was the only thing that worked for me.

What was I doing wrong? This can't be the way Python works, can it?

Answer 1

You are interpreting the return value of find incorrectly. The documentation states:

str.find(sub[, start[, end]])

Return the lowest index in the string where substring sub is found, such that sub is contained in the slice s[start:end] . Optional arguments start and end are interpreted as in slice notation. Return -1 if sub is not found.

Of all numbers, Python considers only 0 to be false; -1 is true . As find returns the index of the beginning of the match, and 0 is a valid index to a string, -1 is returned when no match was found.

Thus your code should rather* be:

if s.find('something') != -1: 

or

if s.find('something') >= 0:

Though they're not the most Pythonic either, if you just want to see if s contains 'something' - see below.

---

Now, about your second example: the bool constant False compares equal to 0 :

>>> 0 == False
True

and bool constant True only equal to number 1 :

>>> 1 == True
True
>>> 2 == True
False

Thus the expression s.find('something') == True would be totally equivalent with s.find('something') == 1 ; and the whole expression would evaluate to True only if the s would be of form xsomethingxxxx , with a match for something starting at the 2nd character (index 1).

---

The documentation for str.find continues with the following note:

Note

The find() method should be used only if you need to know the position of sub . To check if sub is a substring or not, use the in operator:

 >>> 'Py' in 'Python' True 

Thus the form that is the most Pythonic for the problem in your question is

if 'something' in s:

---

As suggested by @myaut, if you need the position of 'something' within s and expect it to contain the value, you should use str.index instead, which will throw an exception if the substring was not found within the string.

 try:
     position = s.index('something')
     print("'something' found at", position)
 except ValueError:
     print(s, "does not contain the substring 'something'")

Answer 2

Python thinks 0 is False , not -1 .

As you can see in the Python documentation , Python tries hard to have many different kinds of False values:

• False
• None
• 0 of any numeric type: 0 , 0L , 0.0 , 0j .
• any empty sequence: '' , () , [] .
• any empty mapping: {} .

Your intuitive notion that -1 should be False in Python makes some sense, but that is just not how Python was designed. Zeros and empties are falsey in Python.

Answer 3

If you only want to check wether a string is contained in another, use:

if 'something' in s:
    print 'something in %s' % s
else:
    print 'bad luck'

Answer 4

-1 as an error code came from C, but that leads to spaghetti code of checking return values.

Python has more reliable way to report an error: raise an Exception . Ie str.index does that:

>>> 'abcde'.find('cd')
2
>>> 'abcde'.index('cd')
2
>>> 'abcde'.find('zx')
-1
>>> 'abcde'.index('zx')
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ValueError: substring not found

And of course to check if string contains substring you should use in operator:

>>> 'ab' in 'abcde'
True
>>> 'zx' in 'abcde'
False

Speaking of if statements you should note that comparing to boolean is a bad idea (check for example PEP-8 ).

Every type in Python defines its own rules which values are considered "True": Truth value testing

Answer 5

The quickest way to verify what is the bool value of it;

bool(-1)
True

bool(0)
False

bool('')
False

and so on,

Answer 6

You are not doing something wrong.

In python bool inherit from int . False is 0 and True is 1 . You can try int(True) and int(False) . If you try bool(0) you will get False , but if you write bool(-1) or bool(1) you always will get True . If you use find you will get -1 so you need to use >= 0 . You can find 'something' inside 'something else' .

You can write if 'something' in s: . I think that find is deprecated.