Tell me the truth

[francois.petitjean at bureauveritas.com]

In the python-ideas mailing list
http://mail.python.org/pipermail/python-ideas/2007-March/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.

>>> def ok(x):
...     return (not not x) is bool(x)
...
>>> ok(0)
True
>>> ok(False)
True

ok(11) is True
True
Interesting.... Let's make another function:
>>> def foo(x):
...     return (not not x) is bool(x)  is True
...
>>> foo(False)
False
>>> foo(0.0)
False
>>> foo(0.1)
True

To see if the problem comes from using the a is b is True form, let's try
>>> def ok3(x):
...     return (not not x) is bool(x) == True
...
>>> ok3(44)
True
>>> ok3(0)
False

After much head scrating and experimenting with dis.dis() I have found that
chaining comparisons (with is or ==)  a == b == c in Python is 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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