Tell me the truth

francois.petitjean at bureauveritas.com francois.petitjean at bureauveritas.com
Thu Mar 8 20:41:55 CET 2007


Duncan Booth wrote :
> francois.petitjean at bureauveritas.com wrote:

>> 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.

> I don't understand why it is confusing. What other meaning would you
> expect?
Consider:
>>> Python = cool = True
>>> Python is cool
True
>>> Python is cool is True
True
# heh  it seems possible to tackle is True to a boolean expression (not
always)
>>> it = False
>>> Python is cool is not it
True
# I knew that. slef evident...
>>> Python = cool = False
>>> Python is cool is not it
False
# Waht? desillution...
# Ough! You must reread the reference documentation on chaining the
comparison operators and understand the semantics (the two comparisons are
'anded' and 'and' shortcircuits in Python).
>>> Python is cool
True
# Yeah!

Regards


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