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I stumbled upon this line of code in SciPy's source, in the stats module:
return 1.0*(x==x)
Is this return something other than 1.0? In other words, is there any value of x such that x == x holds False?
722
Python interprets multiple (in)equalities the way you would expect in Math: In Math a = b = c mean all a = b , b = c and a = c . So True is False == False means True == False and False == False and True == False , which is False . For boolean constants, is is equivalent to == .
In Boolean logic "False" is a negative truth-value representing nothing.
It's used to represent the truth value of an expression. For example, the expression 1 <= 2 is True , while the expression 0 == 1 is False .
You can check if a value is either truthy or falsy with the built-in bool() function. According to the Python Documentation, this function: Returns a Boolean value, i.e. one of True or False .
According to the IEEE 754 standard not-a-number (NaN) must always compare false, no matter what it is compared to.
Python 2.7.2+ (default, Oct 4 2011, 20:06:09)
[GCC 4.6.1] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> x=float("NaN")
>>> x==x
False
A user-defined type can override the equality operator to do whatever you want:
Python 3.2.2 (default, Feb 10 2012, 09:23:17)
[GCC 4.4.5 20110214 (Red Hat 4.4.5-6)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> class A:
... def __eq__(self, other):
... return False
...
>>> x=A()
>>> x==x
False
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