[Patches] [ python-Patches-1675423 ] Make PyComplex_AsCComplex use __complex__

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Mon Mar 12 14:38:15 CET 2007

Patches item #1675423, was opened at 2007-03-07 03:03
Message generated for change (Comment added) made by gbrandl
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Category: Core (C code)
Group: Python 2.6
Status: Open
Resolution: None
Priority: 5
Private: No
Submitted By: Mark Dickinson (marketdickinson)
>Assigned to: Georg Brandl (gbrandl)
Summary: Make PyComplex_AsCComplex use __complex__

Initial Comment:
The functions in the math module have the (pleasantly) surprising and 
apparently undocumented property that they'll accept not just floats, but any Python object having a float method:

>>> class test1(object):
...     def __float__(self):
...             return 3. 
>>> from math import sqrt
>>> sqrt(test1())

Based on this, one might expect the functions in the complex math module cmath to have the same property with respect to __complex__.
But this isn't so:

>>> class test2(object):
...     def __complex__(self):
...             return -3 + 0j
>>> from cmath import sqrt as csqrt
>>> csqrt(test2())
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: a float is required

The real surprise is that the cmath functions *will* call the __float__ method, if it's available:

>>> csqrt(test1())

This patch expands the PyComplex_AsCComplex method so that it looks for a __complex__ method before looking for the __float__ method.  This `fixes' the above behaviour.

Should it be a documented feature that the math functions will make use of __float__?  If so, and if the patched behaviour seems desirable, I'll add suitable tests to test_math and test_cmath.


>Comment By: Georg Brandl (gbrandl)
Date: 2007-03-12 13:38

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I think it is the right thing to do.

Test cases would be welcome.


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