New submission from Francesco Biscani:
The C++11/C99 standards define a complex infinity as a complex number in which
at least one of the components is inf. Consider the Python snippet:
>>> complex(float('inf'),float('nan'))*2
(nan+nanj)
This happens because complex multiplication in Python is implemented in the
most straightforward way, but the presence of a nan component "infects" both
components of the result and leads to a complex nan result. See also how
complex multiplication is implemented in Annex G.5.1.6 of the C99 standard.
It feels wrong that a complex infinity multiplied by a real number results in a
complex nan. By comparison, the result given here by C/C++ is inf+nan*j.
Note also this:
>>> complex(float('inf'),float('nan'))+2
(inf+nanj)
That is, addition has a different behaviour because it does not require mixing
up the components of the operands.
This behaviour has unexpected consequences when one interacts with math
libraries implemented in C/C++ and accessed via Python through some wrapping
tool. For instance, whereas 1./(inf+nan*j) is zero in C/C++, it becomes in
Python
>>> 1./complex(float('inf'),float('nan'))
(nan+nanj)
----------
messages: 253283
nosy: Francesco Biscani
priority: normal
severity: normal
status: open
title: Arithmetics with complex infinities is inconsistent with C/C++
type: behavior
versions: Python 2.7, Python 3.3
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Python tracker <rep...@bugs.python.org>
<http://bugs.python.org/issue25453>
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