Comparing strings with == gives wrong ? result

Question

I'm trying to compare two strings e.g. if self.Ends == '101100' or '001101' . self.Ends is in a class to compare two 3D co-ordinates. it may hold 000100. There are eight if elif comparisons.

The program always returns an 'a' even self.Ends holds another 01 string.

if self.Ends == '100101' or '101100':
     P_line.Line = 'a'   

elif self.Ends == '000100' or '100000':
     P_line.Line = 'b'

`'101100'` is a non-zero-length string and as such is always true. So is `'100000'`. Therefore both your `if` statements are always true. — kindall, Feb 04 '14 at 22:59
Ordinarily I'm a big fan of language constructs that read like English, but this one trips up far too many people. — Mark Ransom, Feb 04 '14 at 23:10

jramirez · Answer 1 · 2014-02-04T23:16:28.173

5

You need to compare the string to self.Ends on both sides of the or. Otherwise it will always be true. ie. if 1: is the same as if 'a': They will both evaluate to True

  if self.Ends == '100101' or self.Ends == '101100':
         P_line.Line = 'a'   

   elif self.Ends == '000100' or self.Ends == '100000':
         P_line.Line = 'b'

edited Feb 04 '14 at 23:16

answered Feb 04 '14 at 22:58

jramirez

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5

Can also use `if self.Ends in ('100101', '101100'):` – sberry Feb 04 '14 at 23:03
Yes, that is a more pythonic way for sure. – jramirez Feb 04 '14 at 23:15

score 1 · Answer 2 · answered Feb 04 '14 at 23:08

@jramirez answered the why, but this is an alternative approach to begin with.

This might be better than making all of those if, elif, else

>>> mapper = {'a': ('100101', '101100'), 'b': ('000100', '100000')}
>>> def get_line(bits, default='default'):
...     for k, v in mapper.items():
...         if bits in v:
...             return k
...     return default
...
>>> get_line('100101')
'a'
>>> get_line('000100')
'b'
>>> get_line('1', 'z')
'z'

So your code would look like

P_line.Line = get_line(self.Ends)

or with an optional default

P_line.Line = get_line(self.Ends, 'z')

Comparing strings with == gives wrong ? result

2 Answers2