Why does 1 == True but 2 != True in Python?
Question:
Possible Duplicate:
Is False == 0 and True == 1 in Python an implementation detail or is it guaranteed by the language?
A brief transcript from my interactive console:
Python 2.7.2 (default, Jun 29 2011, 11:10:00)
[GCC 4.6.1] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> True
True
>>> 0 == True
False
>>> 1 == True
True
>>> 2 == True
False
Why on earth is this the case?
Edit: For the sake of contrast, consider the is operator.
>>> 0 is False
False
>>> 1 is True
False
>>> 0 is 0
True
>>> True is True
True
That makes a lot of sense because though 1 and True both mean the same thing as the condition of an if statement, they really aren’t the same thing.
Edit again: More fun consequences of 1 == True:
>>> d = {}
>>> d[True] = "hello"
>>> d[1]
"hello"
Answers:
It’s standard binary conventions: 1 is true, 0 is false.
It’s like 1 means “on” in machine language, and 0 means “off”.
Because of this the bool type is just a subtype of int in Python, with 1 == True and 0 == False.
However any non-zero numeric value will be evaluated as True in a conditional statement:
>>> if 2: print "ok"
ok
>>> 2 == True
False
It’s a boolean thing. 1 == true, 0 == false.
It’s binary. 0 is always False and 1 is always True. In python though any non-zero value, including negative values are always True.
>>> if -1:
... print 'a'
...
a
>>> if 2:
... print 'a'
...
a
>>> if 0:
... print 'a'
...
>>>
Because instances of bool are also instances of int in python. True happens to be equals to integer 1.
Take a look at this example:
[GCC 4.2.1 (Based on Apple Inc. build 5658) (LLVM build 2335.15.00)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> isinstance(True, int)
True
>>> int(True)
1
>>>
because in binary 1 is true and 0 is false.
it also happens in PHP.
I’m not sure about other languages, but I think it’s going to be the same.
Because Boolean in Python is a subtype of integers. From the documentation:
Boolean values are the two constant objects False and True. They are used to represent truth values (although other values can also be considered false or true). In numeric contexts (for example when used as the argument to an arithmetic operator), they behave like the integers 0 and 1, respectively. The built-in function bool() can be used to cast any value to a Boolean, if the value can be interpreted as a truth value (see section Truth Value Testing above).
Equality testing is different than boolean testing:
>>> bool(1)
True
>>> bool(2)
True
>>> bool(-1)
True
>>> bool(0)
False
Once upon a time (in Python < 2.3), there was no boolean type, no False or True, only 0 and 1. Then PEP 285 came along and thus they were added to the language. They were defined so that True == 1 and False == 0, probably for reasons of backwards compatibility.
Possible Duplicate:
Is False == 0 and True == 1 in Python an implementation detail or is it guaranteed by the language?
A brief transcript from my interactive console:
Python 2.7.2 (default, Jun 29 2011, 11:10:00)
[GCC 4.6.1] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> True
True
>>> 0 == True
False
>>> 1 == True
True
>>> 2 == True
False
Why on earth is this the case?
Edit: For the sake of contrast, consider the is operator.
>>> 0 is False
False
>>> 1 is True
False
>>> 0 is 0
True
>>> True is True
True
That makes a lot of sense because though 1 and True both mean the same thing as the condition of an if statement, they really aren’t the same thing.
Edit again: More fun consequences of 1 == True:
>>> d = {}
>>> d[True] = "hello"
>>> d[1]
"hello"
It’s standard binary conventions: 1 is true, 0 is false.
It’s like 1 means “on” in machine language, and 0 means “off”.
Because of this the bool type is just a subtype of int in Python, with 1 == True and 0 == False.
However any non-zero numeric value will be evaluated as True in a conditional statement:
>>> if 2: print "ok"
ok
>>> 2 == True
False
It’s a boolean thing. 1 == true, 0 == false.
It’s binary. 0 is always False and 1 is always True. In python though any non-zero value, including negative values are always True.
>>> if -1:
... print 'a'
...
a
>>> if 2:
... print 'a'
...
a
>>> if 0:
... print 'a'
...
>>>
Because instances of bool are also instances of int in python. True happens to be equals to integer 1.
Take a look at this example:
[GCC 4.2.1 (Based on Apple Inc. build 5658) (LLVM build 2335.15.00)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> isinstance(True, int)
True
>>> int(True)
1
>>>
because in binary 1 is true and 0 is false.
it also happens in PHP.
I’m not sure about other languages, but I think it’s going to be the same.
Because Boolean in Python is a subtype of integers. From the documentation:
Boolean values are the two constant objects False and True. They are used to represent truth values (although other values can also be considered false or true). In numeric contexts (for example when used as the argument to an arithmetic operator), they behave like the integers 0 and 1, respectively. The built-in function bool() can be used to cast any value to a Boolean, if the value can be interpreted as a truth value (see section Truth Value Testing above).
Equality testing is different than boolean testing:
>>> bool(1)
True
>>> bool(2)
True
>>> bool(-1)
True
>>> bool(0)
False
Once upon a time (in Python < 2.3), there was no boolean type, no False or True, only 0 and 1. Then PEP 285 came along and thus they were added to the language. They were defined so that True == 1 and False == 0, probably for reasons of backwards compatibility.