Make PyComplex_AsCComplex use __complex__

[mdickinson]

BPO	1675423
Nosy	@birkenfeld, @mdickinson
Files	complex_patch: Patch to PyComplex_AsCComplex in complexobject.py to allow conversion to complex use complex complex_patch.patch: Updated patch, including documentation and test cases

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/birkenfeld'
closed_at = <Date 2007-03-17.16:09:34.000>
created_at = <Date 2007-03-07.03:03:07.000>
labels = ['interpreter-core']
title = 'Make PyComplex_AsCComplex use __complex__'
updated_at = <Date 2007-03-17.16:09:34.000>
user = 'https://github.com/mdickinson'

bugs.python.org fields:

activity = <Date 2007-03-17.16:09:34.000>
actor = 'georg.brandl'
assignee = 'georg.brandl'
closed = True
closed_date = None
closer = None
components = ['Interpreter Core']
creation = <Date 2007-03-07.03:03:07.000>
creator = 'mark.dickinson'
dependencies = []
files = ['7829', '7830']
hgrepos = []
issue_num = 1675423
keywords = ['patch']
message_count = 5.0
messages = ['52070', '52071', '52072', '52073', '52074']
nosy_count = 2.0
nosy_names = ['georg.brandl', 'mark.dickinson']
pr_nums = []
priority = 'normal'
resolution = 'accepted'
stage = None
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue1675423'
versions = ['Python 2.6']

[mdickinson]

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())
1.7320508075688772

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())
(1.7320508075688772+0j)

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.

[birkenfeld]

I think it is the right thing to do.

Test cases would be welcome.

[birkenfeld]

Please include both old-style and new-style classes in the test case, and for each one test if the check for a complex return from __complex__ works.

[mdickinson]

File Added: complex_patch.patch

[birkenfeld]

OK, thank you! Committed the patch, new tests and docs in rev. 54421.