In the python-ideas mailing list
http://mail.python.org/pipermail/pyt...ch/thread.html there
was a discussion about the fact that python has opeartors 'and', 'or' and
'not' (keywords of the language) but 'bool' is a type. Naturally we have
(not not x) == bool(x) # always True
If I take into account the fact that 'True' and 'False' are singletons
(guaranteed ?) :
(not not x) is bool(x) # should be always True.
.... return (not not x) is bool(x)>>def ok(x):
....
True>>ok(0)
True>>ok(False)
ok(11) is True
True
Interesting.... Let's make another function:
.... return (not not x) is bool(x) is True>>def foo(x):
....
False>>foo(False)
False>>foo(0.0)
True>>foo(0.1)
To see if the problem comes from using the a is b is True form, let's try
.... return (not not x) is bool(x) == True>>def ok3(x):
....
True>>ok3(44)
False>>ok3(0)
After much head scrating and experimenting with dis.dis() I have found that
chaining comparisons (with is or ==) a == b == c in Pythonis never a good
idea. It is interpreted as
( a == b ) and ( b == c)
Such a magic is fine when we write:
if 0.0 <= x < 1.0:
but it renders the outcome of if a == b == c: somewhat confusing. In the
reference documentation the sentences """Comparisons can be chained
arbitrarily, e.g., x < y <= z is equivalent to x < y and y <= z, except
that y is evaluated only once (but in both cases z is not evaluated at all
when x < y is found to be false). """ should be followed by a warning about
chaining '==' or similar operators.
Regards
PS. Sorry for bad english.
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