[issue17336] Complex number representation round-trip doesn't work with signed zero values

[Eric Wieser]

Eric Wieser  added the comment:

> BTW I don't want repr() of a complex number to use the complex(..., ...)

A compromise would be to only use this notation if signed zeros are involved.

---

Another option would be to use slightly unusual reprs for these complex 
numbers, which at least round-trip:

def check(s, v):
c = eval(s)
# use string equality, because it's the easiest way to compare signed 
zeros
cs = f"complex({c.real}, {c.imag})"
vs = f"complex({v.real}, {v.imag})"
assert vs == cs, f' expected {vs} got {cs}'

check("-(0+0j)", complex(-0.0, -0.0))
check("(-0.0-0j)", complex(-0.0, 0.0))  # non-intuitive
check("-(-0.0-0j)", complex(0.0, -0.0))  # non-intuitive

Which I suppose would extend to complex numbers containing just one signed zero

check("(-0.0-1j)", complex(-0.0, -1))
check("-(0.0-1j)", complex(-0.0, 1))
check("-(1+0j)", complex(-1, -0.0))
check("-(-1+0j)", complex(1, -0.0))

Only two of these reprs are misleading for users who don't understand what's 
going on, the rest will just strike users as odd.

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nosy: +Eric Wieser

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[issue17336] Complex number representation round-trip doesn't work with signed zero values

[Mark Dickinson]

Mark Dickinson  added the comment:

Also related: #40269

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[issue17336] Complex number representation round-trip doesn't work with signed zero values

[Mark Dickinson]

Mark Dickinson  added the comment:

The issue of changing the complex repr came up again in #41485, which has been 
closed as a duplicate of this issue.

See also https://bugs.python.org/issue23229#msg233963, where Guido says:

> BTW I don't want repr() of a complex number to use the complex(..., ...)
notation -- it's too verbose.

FWIW, I'd be +1 on changing the complex repr, but given Guido's opposition 
we're probably looking at a PEP to make that happen, and I don't have the 
bandwidth or the energy to push such a PEP through.

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[issue17336] Complex number representation round-trip doesn't work with signed zero values

[Manuel Jacob]

New submission from Manuel Jacob:

When evaluating, signed zero complex numbers aren't recovered correctly.
 -0j
(-0-0j)
 (-0-0j)
0j
 0j
0j

According to 
http://docs.python.org/dev/reference/datamodel.html#object.__repr__ the 
representation can be used to recreate an object with the same value. Shouldn't 
this also be possible with complex numbers?

When using complex('...'), round-trip works correctly. While this can be used 
to recover the exact number, i find it confusing that complex('...') isn't the 
same as eval('...').
 complex('-0j')
-0j
 complex('(-0-0j)')
(-0-0j)
 complex('0j')
0j

--
messages: 183313
nosy: mjacob
priority: normal
severity: normal
status: open
title: Complex number representation round-trip doesn't work with signed zero 
values
type: behavior
versions: Python 3.3

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[issue17336] Complex number representation round-trip doesn't work with signed zero values

[Ezio Melotti]

Changes by Ezio Melotti ezio.melo...@gmail.com:


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nosy: +ezio.melotti, mark.dickinson, skrah

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[issue17336] Complex number representation round-trip doesn't work with signed zero values

[Eric V. Smith]

Changes by Eric V. Smith e...@trueblade.com:


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[issue17336] Complex number representation round-trip doesn't work with signed zero values

[Mark Dickinson]

Mark Dickinson added the comment:

This is not easy to avoid, I'm afraid, and it's a consequence of Python's usual 
rules for mixed-type arithmetic:  (-0-0j) is interpreted as 0 - (0.0 + 0.0j) 
--- that is, the 0j is promoted to a complex instance (by giving it zero real 
part) before the subtraction is performed.  Then the real part of the result is 
computed as 0.0 - 0.0, which is 0.0.  Note that the first 0.0 comes from 
converting the *integer* 0 to a complex number.  If you do -0.0-0.0j you'll see 
a different result:

 -0.0-0.0j
(-0+0j)

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resolution:  - invalid
status: open - closed

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[issue17336] Complex number representation round-trip doesn't work with signed zero values

[Mark Dickinson]

Mark Dickinson added the comment:

An aside:  C99 gets around this problem by allowing an (optional) Imaginary 
type, separate from Complex.  Very few compilers support it, though.

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