9 PEP 285: A Boolean Type
A Boolean type was added to Python 2.3. Two new constants were added to the __builtin__ module, True and False. (True and False
constants were added to the built-ins in Python 2.2.1, but the 2.2.1 versions are simply
set to integer values of 1 and 0 and aren't a different type.)
The type object for this new type is named bool; the constructor
for it takes any Python value and converts it to True or False.
>>> bool(1)
True
>>> bool(0)
False
>>> bool([])
False
>>> bool( (1,) )
True
Most of the standard library modules and built-in functions have been changed to return
Booleans.
>>> obj = []
>>> hasattr(obj, 'append')
True
>>> isinstance(obj, list)
True
>>> isinstance(obj, tuple)
False
Python's Booleans were added with the primary goal of making code clearer. For example,
if you're reading a function and encounter the statement return 1, you might
wonder whether the 1 represents a Boolean truth value, an index, or a
coefficient that multiplies some other quantity. If the statement is return True,
however, the meaning of the return value is quite clear.
Python's Booleans were not added for the sake of strict type-checking. A very
strict language such as Pascal would also prevent you performing arithmetic with Booleans,
and would require that the expression in an if statement always
evaluate to a Boolean result. Python is not this strict and never will be, as PEP 285
explicitly says. This means you can still use any expression in an if
statement, even ones that evaluate to a list or tuple or some random object. The Boolean
type is a subclass of the int class so that arithmetic using a
Boolean still works.
>>> True + 1
2
>>> False + 1
1
>>> False * 75
0
>>> True * 75
75
To sum up True and False in a
sentence: they're alternative ways to spell the integer values 1 and 0, with the single
difference that str() and repr()
return the strings 'True' and 'False' instead of '1'
and '0'.
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