[issue24438] Strange behaviour when multiplying imaginary inf by 1

[Steven D'Aprano]

Steven D'Aprano added the comment:

I've found the same behaviour going back to Python 1.5.

I think what happens here is that (0+∞j)*1 evaluates the multiplication by implicitly coercing the int to a complex:

    (0+∞j)*(1+0j)
    => (0*1 + ∞j*1 + 0*0j + ∞j*0j)
    => (0-NAN + ∞j+0j)
    => (NAN + ∞j)

rather than using the "simple" way (1*0 + 1*∞j) => (0+∞j).

The problem here is that while there is no mathematical difference between multiplying by 1 or (1+0j), once you involve NANs and INFs, it does. So even though they give different answers, both methods are correct, for some value of "correct".

I don't see that this is necessarily a bug to be fixed, it might count as a change in behaviour needing to go through a deprecation period first. Perhaps it should be discussed on python-ideas first?

My personal feeling is that Python should multiply a complex by a non-complex in the "simple" way, rather than implicitly converting the int to a complex. Anyone who wants the old behaviour can easily get it by explicitly converting 1 to a complex first.

So I guess this is a +1 to "fixing" this.

(Oh, the same applies to the / operator.)

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nosy: +steven.daprano

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<http://bugs.python.org/issue24438>
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