add inf, nan, infj, nanj to cmath module

[mdickinson]

BPO	23229
Nosy	@mdickinson, @pitrou, @vstinner, @serhiy-storchaka
Files	cmath_inf_nan.patch

^{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/mdickinson'
closed_at = <Date 2016-08-29.12:59:24.714>
created_at = <Date 2015-01-13.09:37:07.756>
labels = ['extension-modules', 'type-feature']
title = 'add inf, nan, infj, nanj to cmath module'
updated_at = <Date 2016-08-29.12:59:24.713>
user = 'https://github.com/mdickinson'

bugs.python.org fields:

activity = <Date 2016-08-29.12:59:24.713>
actor = 'mark.dickinson'
assignee = 'mark.dickinson'
closed = True
closed_date = <Date 2016-08-29.12:59:24.714>
closer = 'mark.dickinson'
components = ['Extension Modules']
creation = <Date 2015-01-13.09:37:07.756>
creator = 'mark.dickinson'
dependencies = []
files = ['44181']
hgrepos = []
issue_num = 23229
keywords = ['patch']
message_count = 23.0
messages = ['233919', '233921', '233922', '233923', '233924', '233925', '233926', '233927', '233949', '233950', '233951', '233952', '233953', '233954', '233956', '233957', '233962', '233963', '233964', '233965', '233967', '273312', '273859']
nosy_count = 5.0
nosy_names = ['mark.dickinson', 'pitrou', 'vstinner', 'python-dev', 'serhiy.storchaka']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue23229'
versions = ['Python 3.6']

[mdickinson]

As pointed out by Guido in bpo-23185, the constants inf and nan should be added to the cmath module, too.

[vstinner]

What about cmath.nanj?

[mdickinson]

Guido also says:

"""
Also complex(0, math.nan) a value that is printed as "nanj" and complex("nanj") parses and returns such a value, so the point could be made that there should be a constant named complex.nanj.
"""

... and the same comments would apply to "infj".

[vstinner]

Oh, there is also infj:

>>> complex(0, float("inf"))
infj
>>> complex("infj")
infj
>>> complex(0, float("nan"))
nanj
>>> complex("nanj")
nanj

[mdickinson]

@Haypo: I'm not keen on either of infj or nanj, on a YAGNI basis. I expect they'd be used almost never, and for the few times that they're really needed, complex(0, inf) and complex(0, nan) seem like good enough spellings.

[mdickinson]

Note: following the precedent of cmath.e and cmath.pi, cmath.nan and cmath.inf should have type *float*. Let's not get into the business of deciding *which* complex infinities and nans to represent.

[serhiy-storchaka]

There are other names which exist only in math, but not in cmath.

>>> sorted(set(dir(math)) - set(dir(cmath)))
['atan2', 'ceil', 'copysign', 'degrees', 'erf', 'erfc', 'expm1', 'fabs', 'factorial', 'floor', 'fmod', 'frexp', 'fsum', 'gamma', 'hypot', 'inf', 'ldexp', 'lgamma', 'log1p', 'log2', 'modf', 'nan', 'pow', 'radians', 'trunc']

May be complex equivalents of all functions should be added for the same reasons?

[mdickinson]

May be complex equivalents of all functions should be added for the same reasons?

(1) This is off-topic for this issue; please open a separate one.

(2) Many of those functions simply don't make sense for complex numbers (for example floor, degrees, etc.), or it's not obvious how to extend them. For those that do make sense (erf, for example), implementing and testing a library-quality version would take a lot of time and effort, and no-one would be interested in the result. It's not worth it.

[gvanrossum]

Note: following the precedent of cmath.e and cmath.pi, cmath.nan and
cmath.inf should have type *float*. Let's not get into the business of
deciding *which* complex infinities and nans to represent.

Agreed.

[gvanrossum]

Note, the reason I proposed nanj (and infj) is that these are produced by
repr() of complex numbers. Having them in the cmath module provides a place
to document them which will then be searchable.

On Tue, Jan 13, 2015 at 8:46 AM, Guido van Rossum <report@bugs.python.org>
wrote:

Guido van Rossum added the comment:

> Note: following the precedent of cmath.e and cmath.pi, cmath.nan and
cmath.inf should have type *float*. Let's not get into the business of
deciding *which* complex infinities and nans to represent.

Agreed.

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Python tracker <report@bugs.python.org>
<http://bugs.python.org/issue23229\>

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[serhiy-storchaka]

Note, the reason I proposed nanj (and infj) is that these are produced by
repr() of complex numbers. Having them in the cmath module provides a place
to document them which will then be searchable.

Another solution would be to change repr() of complex if imaginary component is not finite number to the form complex(x, y).