Empty String Boolean Logic

Question

I just stumbled across this and I couldn't find a sufficient answer:

x = ""

Why then is:

x == True
False

x == False
False

x != True
True

x != False
True

Am I supposed to conclude that x is neither True nor False?

What?! You are to conclude that `x` isn't *equal to* either `True` or `False`. Why did you think it would be? Have you been confused somehow by https://docs.python.org/2/library/stdtypes.html#truth-value-testing? It will still evaluate false-y in a boolean context: `if x:`, `bool(x)`, etc.. — jonrsharpe, Mar 15 '16 at 14:00
Dr. Tautology, I think what you meant to test was if `bool(x) == False`. Username checks out, though. — pholtz, Mar 15 '16 at 14:08
Now see what happens with your tests if you set `x=0`. And then do the same thing with `x=1`. — PM 2Ring, Mar 15 '16 at 14:29
@PM2Ring Ask one bad question on stackoverflow and it ruins your day.FML — Dr.Tautology, Mar 15 '16 at 15:07
Hey, it's not _that_ bad. Sure, it's got a net zero score, but you won a few rep points from it, and you got some good answers. But yes, this is fairly basic info that isn't that hard to find in the official documentation. — PM 2Ring, Mar 15 '16 at 23:33

score 6 · Answer 1 · edited May 23 '17 at 10:27

6

to check if x is True of False:

bool("")
> False

bool("x")
> True

for details on the semantics of is and == see this question

edited May 23 '17 at 10:27

Community

1
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answered Mar 15 '16 at 14:01

scytale

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score 4 · Answer 2 · answered Mar 15 '16 at 14:18

Am I supposed to conclude that x is neither True nor False?

That's right. x is neither True nor False, it is "". The differences start with the type:

>>> print(type(""), type("x"), type(True), type(False))
builtins.str, builtins.str, builtins.bool, builtins.bool

Python is a highly object oriented language. Hence, strings are objects. The nice thing with python is that they can have a boolean representation for if x: print("yes"), e. g.. For strings this representation is len(x)!=0.

score 2 · Answer 3 · answered Mar 15 '16 at 14:01

2

In python '==' tests for equality. The empty string is not equal to True, so the result of your comparison is False.

You can determine the 'truthiness' of the empty string by passing it to the bool function:

>>> x = ''
>>> bool(x)
False

answered Mar 15 '16 at 14:01

snakecharmerb

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ode2k · Answer 4 · 2016-03-15T14:31:42.250

In a Boolean context, null / empty strings are false (Falsy). If you use

testString = ""

if not testString:
    print("NULL String")
else:
    print(testString)

As snakecharmerb said, if you pass the string to the bool() function it will return True or False based

>>> testString = ""
>>> bool(testString)
    False

>>> testString = "Not an empty string"
>>> bool(testString)
    True

See this doc on Truth Value Testing to learn more about this:

Python 2:

https://docs.python.org/2/library/stdtypes.html#truth-value-testing

Python 3:

https://docs.python.org/3/library/stdtypes.html#truth-value-testing

Empty String Boolean Logic

4 Answers4