[issue23229] add inf, nan, infj, nanj to cmath module

[Mark Dickinson]

Changes by Mark Dickinson :


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[issue23229] add inf, nan, infj, nanj to cmath module

[Mark Dickinson]

Changes by Mark Dickinson :


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[issue23229] add inf, nan, infj, nanj to cmath module

[Roundup Robot]

Roundup Robot added the comment:

New changeset 4b25da63d1d0 by Mark Dickinson in branch 'default':
Issue 23229: add cmath.inf, cmath.nan, cmath.infj and cmath.nanj.
https://hg.python.org/cpython/rev/4b25da63d1d0

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[issue23229] add inf, nan, infj, nanj to cmath module

[Mark Dickinson]

Changes by Mark Dickinson :


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versions: +Python 3.6 -Python 3.5

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[issue23229] add inf, nan, infj, nanj to cmath module

[Mark Dickinson]

Mark Dickinson added the comment:

Here's a patch that adds inf, infj, nan and nanj.

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Added file: http://bugs.python.org/file44181/cmath_inf_nan.patch

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[issue23229] add inf, nan, infj, nanj to cmath module

[Mark Dickinson]

Mark Dickinson added the comment:

 Having them in the cmath module provides a place
to document them which will then be searchable.

Okay, makes sense.

One of the reasons I'm a bit unhappy with the idea of adding infj and nanj is 
that it seems like an encouragement to expect eval(repr(z)) to work (i.e., 
recover the original value of z), and because Python doesn't have strict 
imaginary numbers (i.e., numbers with no real part at all), that fails:

 from math import inf, nan
 infj = complex(0.0, inf)
 nanj = complex(0.0, nan)
 z = complex(0.0, -inf)
 w = eval(repr(z))
 z
-infj
 w  # real part has the wrong sign
(-0-infj)

But that's a pretty hollow objection, because this really has nothing to do 
with inf and nan; it's a problem with finite values, too:

 z = complex(0.0, -3.4)
 w = eval(repr(z))
 z
-3.4j
 w
(-0-3.4j)

So I'll add infj and nanj if the consensus is that they're useful.

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[issue23229] add inf, nan, infj, nanj to cmath module

[Guido van Rossum]

Guido van Rossum added the comment:

I don't understand why w ends up having -0 as the real part. For floats, at
least, we've done a lot of work to make eval(repr()) roundtrip correctly in
all cases. Is the issue with complex superficial (we should fix eval or
repr) or deep (if we fixed it something else would be broken)?

On Tue, Jan 13, 2015 at 9:27 AM, Mark Dickinson rep...@bugs.python.org
wrote:


 Mark Dickinson added the comment:

  Having them in the cmath module provides a place
 to document them which will then be searchable.

 Okay, makes sense.

 One of the reasons I'm a bit unhappy with the idea of adding infj and nanj
 is that it seems like an encouragement to expect eval(repr(z)) to work
 (i.e., recover the original value of z), and because Python doesn't have
 strict imaginary numbers (i.e., numbers with no real part at all), that
 fails:

  from math import inf, nan
  infj = complex(0.0, inf)
  nanj = complex(0.0, nan)
  z = complex(0.0, -inf)
  w = eval(repr(z))
  z
 -infj
  w  # real part has the wrong sign
 (-0-infj)

 But that's a pretty hollow objection, because this really has nothing to
 do with inf and nan; it's a problem with finite values, too:

  z = complex(0.0, -3.4)
  w = eval(repr(z))
  z
 -3.4j
  w
 (-0-3.4j)

 So I'll add infj and nanj if the consensus is that they're useful.

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[issue23229] add inf, nan, infj, nanj to cmath module

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

 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).

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[issue23229] add inf, nan, infj, nanj to cmath module

[Mark Dickinson]

Mark Dickinson added the comment:

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

That wouldn't help with the str(), though, unless you're proposing to change 
that, too.

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[issue23229] add inf, nan, infj, nanj to cmath module

[Mark Dickinson]

Mark Dickinson added the comment:

 Is the issue with complex superficial

Unfortunately not: something like this is fairly inescapable.  The problem is 
that when you do (for example)  5 - 6j  you're in effect subtracting 
complex(0.0, 6.0) from complex(5.0, 0.0): you've invented a real part of 0.0 
for the second term and an imaginary part of 0.0 for the first term.  And since 
0.0 is *not* an identity for addition (-0.0 + 0.0 is 0.0, not -0.0, under the 
usual rounding modes), signs of zeros tend to get lost.

So the safe way to construct a complex number is to avoid any actual arithmetic 
by using complex(real_part, imag_part) rather than real_part + imag_part*1j.  I 
rather like Serhiy's idea of making the complex repr have this form, except 
that the change would probably break existing code.  (And the str should really 
stay in the current expected human-readable format, so the problems with infj 
and nanj don't go away.)

Another possible fix is to introduce a new 'imaginary' type, such that the 
type of an imaginary literal is now 'imaginary' rather than 'complex', and 
arithmetic operations like addition can special-case the addition of a float to 
an 'imaginary' instance to produce a complex number with exactly the right 
bits. The C standardisation folks already tried this: C99 introduces optional 
imaginary types and a new _Imaginary keyword, but last time I looked almost 
none of the popular compilers supported those types.  (I think Intel's icc 
might be an exception.)  While this works as a technical solution, IMO the cure 
is worse than the disease; I don't want to think about the user-confusion that 
would result from having separate complex and imaginary types.

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[issue23229] add inf, nan, infj, nanj to cmath module

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

 Another possible fix is to introduce a new 'imaginary' type, such that the 
 type of an imaginary literal is now 'imaginary' rather than 'complex', and 
 arithmetic operations like addition can special-case the addition of a float 
 to an 'imaginary' instance to produce a complex number with exactly the right 
 bits. The C standardisation folks already tried this: C99 introduces optional 
 imaginary types and a new _Imaginary keyword, but last time I looked almost 
 none of the popular compilers supported those types.  (I think Intel's icc 
 might be an exception.)  While this works as a technical solution, IMO the 
 cure is worse than the disease; I don't want to think about the 
 user-confusion that would result from having separate complex and 
 imaginary types.

This type should exist only at compile time. Peephole optimizer should replace 
it with complex after folding constants.

Or may be repr() (and str()) should keep zero real part if imaginary part is 
negative and output period if real part is zero. For now:

 z = complex(0.0, 3.4); z; eval(repr(z))
3.4j
3.4j
 z = complex(0.0, -3.4); z; eval(repr(z))
-3.4j
(-0-3.4j)
 z = complex(-0.0, 3.4); z; eval(repr(z))
(-0+3.4j)
3.4j
 z = complex(-0.0, -3.4); z; eval(repr(z))
(-0-3.4j)
-3.4j

But all this perhaps is offtopic here.

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[issue23229] add inf, nan, infj, nanj to cmath module

[Mark Dickinson]

Mark Dickinson added the comment:

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

Okay, fair enough.

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[issue23229] add inf, nan, infj, nanj to cmath module

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

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?

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[issue23229] add inf, nan, infj, nanj to cmath module

[STINNER Victor]

Changes by STINNER Victor victor.stin...@gmail.com:


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title: add inf and nan to cmath module - add inf, nan, infj, nanj to cmath 
module

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[issue23229] add inf, nan, infj, nanj to cmath module

[Mark Dickinson]

Changes by Mark Dickinson dicki...@gmail.com:


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[issue23229] add inf, nan, infj, nanj to cmath module

[Mark Dickinson]

Mark Dickinson 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.

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[issue23229] add inf, nan, infj, nanj to cmath module

[Mark Dickinson]

Mark Dickinson added the comment:

 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.

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[issue23229] add inf, nan, infj, nanj to cmath module

[Mark Dickinson]

Mark Dickinson added the comment:

@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.

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[issue23229] add inf, nan, infj, nanj to cmath module

[Guido van Rossum]

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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[issue23229] add inf, nan, infj, nanj to cmath module

[Guido van Rossum]

Guido van Rossum added the comment:

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 rep...@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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[issue23229] add inf, nan, infj, nanj to cmath module

[Antoine Pitrou]

Antoine Pitrou added the comment:

 I don't understand why w ends up having -0 as the real part.

Because -3.4j is interpreted as -complex(0, 3.4).

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[issue23229] add inf, nan, infj, nanj to cmath module

[Guido van Rossum]

Guido van Rossum added the comment:

OK, let's not try to resolve that issue, we can just note it in the docs.
BTW I don't want repr() of a complex number to use the complex(..., ...)
notation -- it's too verbose.

On Tue, Jan 13, 2015 at 11:38 AM, Serhiy Storchaka rep...@bugs.python.org
wrote:


 Serhiy Storchaka added the comment:

  Another possible fix is to introduce a new 'imaginary' type, such that
 the type of an imaginary literal is now 'imaginary' rather than 'complex',
 and arithmetic operations like addition can special-case the addition of a
 float to an 'imaginary' instance to produce a complex number with exactly
 the right bits. The C standardisation folks already tried this: C99
 introduces optional imaginary types and a new _Imaginary keyword, but
 last time I looked almost none of the popular compilers supported those
 types.  (I think Intel's icc might be an exception.)  While this works as a
 technical solution, IMO the cure is worse than the disease; I don't want to
 think about the user-confusion that would result from having separate
 complex and imaginary types.

 This type should exist only at compile time. Peephole optimizer should
 replace it with complex after folding constants.

 Or may be repr() (and str()) should keep zero real part if imaginary part
 is negative and output period if real part is zero. For now:

  z = complex(0.0, 3.4); z; eval(repr(z))
 3.4j
 3.4j
  z = complex(0.0, -3.4); z; eval(repr(z))
 -3.4j
 (-0-3.4j)
  z = complex(-0.0, 3.4); z; eval(repr(z))
 (-0+3.4j)
 3.4j
  z = complex(-0.0, -3.4); z; eval(repr(z))
 (-0-3.4j)
 -3.4j

 But all this perhaps is offtopic here.

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[issue23229] add inf, nan, infj, nanj to cmath module

[Mark Dickinson]

Mark Dickinson added the comment:

 This type should exist only at compile time.

But then after doing x = 5j, -0.0 + 5j and -0.0 + x would have different 
values.  Yuck!

 repr() (and str()) should keep zero real part if imaginary part is negative 
 and output period if real part is zero

Sorry, I don't see how this helps.  What do you want the repr of (for example) 
complex(-0.0, 5.0) to be, and why?  What about the cases with 0 in the 
imaginary part?

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[issue23229] add inf, nan, infj, nanj to cmath module

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

 Sorry, I don't see how this helps.  What do you want the repr of (for 
 example) complex(-0.0, 5.0) to be, and why?  What about the cases with 0 in 
 the imaginary part?

Ah, it doesn't help in this case. It helps only when the imaginary part is 
negative.

 eval('(-0.0-5j)')
(-0-5j)
 eval('(-0-5j)')
-5j

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[issue23229] add inf, nan, infj, nanj to cmath module

[Guido van Rossum]

Changes by Guido van Rossum gu...@python.org:


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