python - compare string with boolean

Question

I came across a strange behaviour of python comparing a string with True/False.

I thought that python would print in the following:

if "Test" == True:
    print("Hello1")

but it does not. So I wrote some Test cases and I do not understand some of them.

if "Test" == True:
    print("Hello1")

if "Test" == False:
    print("Hello2")

#This I understand
if bool("Test") == True:
    print("Hello3")

#This I understand too    
if bool("") == False:
    print("Hello4")

if "Test":
    print("Hello5")

Output

>> Hello3
>> Hello4
>> Hello5

So I do not understand:

If Hello1 is not printed why is not Hello2 either?
Why does Hello5 get printed, is the cast to bool("Test") made implicit?

M.T · Accepted Answer · 2017-06-01 11:20:19Z

7

In the first two comparisons, you are checking whether the string "Test" has the same value as the object True or False. This is a value comparison.

If they have a different type, the comparison will return False. You can see this also when comparing lists, numbers etc.: [1]==1 (false), (1,)==[1] (false).

In the third and fourth comparisons, you are still doing a value comparison, but since both sides are of the same type (boolean), it will compare the values.

Hello5 is printed because it is not the null string "". You can see this by trying "Test" != None, which returns True.

While it is a comparison to None when it comes to most classes(None is Python's null value), Python's standard data types are compared to their "null" value, which are:

The empty string "" for strings,
[] for lists (similary () for tuples, {} for dictionaries),
0 for ints and floats,

just like a boolean comparison. Therefore it is not wrong to think of if expression as an implicit cast to if bool(expression).

What is going on under the hood is the evaluation of the __non-zero__(python2.x) or __bool__(python3.x) method of the class.

edited Jun 1, 2017 at 11:20

answered Dec 11, 2015 at 10:34

M.T

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3 Comments

JDurstberger Over a year ago

If you improve your answer to contain the object comparison too it is actually better then the on of user762353. An anwser should always stand on its own.

eestrada Over a year ago

@M.T The first four comparison are not comparing objects, but values. To compare objects (or in other words, identity) would be "Test" is True not "Test" == True. I think this is what you mean, but your verbiage is ambiguous. Please clarify your answer for future users.

eestrada Over a year ago

@M.T Also, another correction: None is not (strictly speaking or otherwise) a class; it is a singleton. In other words, it in an instance of class NoneType that has only one allowed instance: None. You should remove the statement that it is a class from your answer as it is false information. Other than the things I mentioned, this is a very good answer.

JDurstberger · Accepted Answer · 2015-12-11 08:53:01Z

2

In the case of Hello1, Hello2 and Hello5 there is an object comparison and not boolean comparions.

That means that
the string-object "Test" is not the same as object True ("Hello1")
the string object "Test" is not the same as object False("Hello2")
but the string object "Test" is not None ("Hello5")

edited Dec 11, 2015 at 8:53

JDurstberger

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answered Dec 11, 2015 at 8:28

user762353

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