[Python-bugs-list] [ python-Bugs-513866 ] Float/long comparison anomaly

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Bugs item #513866, was opened at 2002-02-06 10:33
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Category: Python Interpreter Core
Group: Not a Bug
Status: Closed
Resolution: Wont Fix
Priority: 5
Submitted By: Andrew Koenig (arkoenig)
Assigned to: Tim Peters (tim_one)
Summary: Float/long comparison anomaly

Initial Comment:
Comparing a float and a long appears to convert the 
long to float and then compare the two floats.  This 
strategy is a problem because the conversion might 
lose precision.  As a result, == is not an equivalence 
relation and < is not an order relation.  For example, 
it is possible to create three numbers a, b, and c 
such that a==b, b==c, and a!=c.

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>Comment By: Tim Peters (tim_one)
Date: 2002-02-06 20:59

Message:
Logged In: YES 
user_id=31435

Oops!  I meant

"""
could lead to a different result than the explicit coercion 
in

somefloat == float(somelong)
"""


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Comment By: Tim Peters (tim_one)
Date: 2002-02-06 20:52

Message:
Logged In: YES 
user_id=31435

Since the coercion to float is documented and intended, 
it's not "a bug" (it's functioning as designed), although 
you may wish to argue for a different design, in which case 
making an incompatible change would first require a PEP and 
community debate.  Information loss in operations involving 
floats comes with the territory, and I don't see a reason 
to single this particular case out as especially 
surprising.  OTOH, I expect it would be especially 
surprising to a majority of users if the implicit coercion 
in

somefloat == somelong

could lead to a different result than the explicit coercion 
in

long(somefloat) == somelong

Note that the "long" type isn't unique here:  the same is 
true of mixing Python ints with Python floats on boxes 
where C longs have more bits of precision than C doubles 
(e.g., Linux for IA64, and Crays).

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