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Make PyComplex_AsCComplex use __complex__ #44670

mdickinson opened this issue Mar 7, 2007 · 5 comments

Make PyComplex_AsCComplex use __complex__ #44670

mdickinson opened this issue Mar 7, 2007 · 5 comments
interpreter-core (Objects, Python, Grammar, and Parser dirs)


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BPO 1675423
Nosy @birkenfeld, @mdickinson
  • complex_patch: Patch to PyComplex_AsCComplex in 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 = ''
    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 = '' 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 = ''
    versions = ['Python 2.6']

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    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.

    @mdickinson mdickinson added the interpreter-core (Objects, Python, Grammar, and Parser dirs) label Mar 7, 2007
    @mdickinson mdickinson added the interpreter-core (Objects, Python, Grammar, and Parser dirs) label Mar 7, 2007
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    I think it is the right thing to do.

    Test cases would be welcome.

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    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.

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    Member Author

    File Added: complex_patch.patch

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

    @ezio-melotti ezio-melotti transferred this issue from another repository Apr 10, 2022
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