[ python-Bugs-1481296 ] long(float('nan'))!=0L

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Bugs item #1481296, was opened at 2006-05-03 20:39
Message generated for change (Comment added) made by ronaldoussoren
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Category: Python Interpreter Core
Group: Python 2.3
Status: Open
Resolution: None
Priority: 5
Submitted By: Erik Dahl (edahl)
Assigned to: Nobody/Anonymous (nobody)
Summary: long(float('nan'))!=0L

Initial Comment:
on all platforms I can test long(float('nan'))=0L

But on maxos X intel long(float('nan'))!=0L

it returns:

000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000L

This is a problem because:

>>> 344 - long(float('nan'))    
Objects/longobject.c:1645: failed assertion `borrow == 0'
Abort trap




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>Comment By: Ronald Oussoren (ronaldoussoren)
Date: 2006-05-27 20:34

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This could quite easily be fixed by testing for NaN in PyLong_FromDouble, such 
as the attached patch. This works correctly on OSX, but I have no idea if this will 
work on other platforms, that depends on how portable the already existing 
Py_IS_NAN macro is.  C99 also introduces an fpclassify and isnan functions that 
could be used (Py_IS_NAN uses neither because Python is written in C89 instead 
of C99).

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Comment By: Erik Dahl (edahl)
Date: 2006-05-10 14:36

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Just to be clear this does cause a crash in the interpreter.

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Comment By: Michael Hudson (mwh)
Date: 2006-05-09 20:47

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Nevertheless, assuming that a compiler implements the relavent standards (i.e. 
Appendix F of C99) it's pretty clear that what PyLong_FromDouble does with a 
nan is pretty random.  frexp(dval, &expo); for dval nan stores an unspecified 
value in expo which is then used to compute the length of the resulting long.  If 
the unspecified value is large-ish and positive, hilarity ensues.

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Comment By: Tim Peters (tim_one)
Date: 2006-05-03 21:09

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Try it on Windows and you'll get:

>>> long(float('nan'))
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
ValueError: invalid literal for float(): nan

Nothing about the behavior of NaNs, infinities, or signed
zeroes is defined or guaranteed by Python.  You use them at
your own risk, and their behavior does vary wildly in
practice (according to the HW, OS, C compiler, C library,
and even the C compiler flags specified when compiling Python).

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