Why doesn t Python have a sign function

Question

I can t understand why Python doesn t have a sign function  It has an abs builtin  which I consider sign s sister   but no sign   In python 2 6 there is even a copysign function  in math   but no sign  Why bother to write a copysign x y  when you could just write a sign and then get the copysign directly from abs x    sign y   The latter would be much more clear  x with the sign of y  whereas with copysign you have to remember if it s x with the sign of y or y with the sign of x   Obviously sign x  does not provide anything more than cmp x 0   but it would be much more readable that this too  and for a greatly readable language like python  this would have been a big plus     If I were a python designer  I would been the other way arond  no cmp builtin  but a sign  When you need cmp x y   you could just do a sign x-y   or  even better for non-numerical stuff  just a x y - of course this should have required sorted accepting a boolean instead of an integer comparator   This would also be more clear  positive when x gt y  whereas with cmp you have to remember the convention positive when the first is bigger  but it could be the other way around   Of course cmp makes sense in its own for other reasons  e g  when sorting non-numerical things  or if you want the sort to be stable  which is not possible using with simply a boolean   So  the question is  why did the Python designer s  decide to leave the sign function out of the language  Why the heck bother with copysign and not its parent sign   Am I missing something   EDIT - after Peter Hansen comment   Fair enough that you didn t use it  but you didn t say what you use python for  In 7 years that I use python  I needed it countless times  and the last is the straw that broke the camel s back   Yes  you can pass cmp around  but 90  of the times that I needed to pass it was in an idiom like  lambda x y  cmp score x  score y   that would have worked with sign just fine   Finally  I hope you agree that sign would be more useful than copysign  so even if I bought your view  why bother about defining that in math  instead of sign  How can copysign be so much useful than sign

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Another one liner for sign    sign   lambda x   1  -1  x lt 0    If you want it to return 0 for x   0   sign   lambda x  x and  1  -1  x lt 0

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In Python 2  cmp   returns an integer  there s no requirement that the result be -1  0  or 1  so sign x  is not the same as cmp x 0   In Python 3  cmp   has been removed in favor of rich comparison   For cmp    Python 3 suggests this  def cmp a  b       return  a  gt  b  -  a  lt  b   which is fine for cmp    but again can t be used for sign   because the comparison operators need not return booleans  To deal with this possibility  the comparison results must be coerced to booleans   def sign x       return bool x  gt  0  - bool x  lt  0   This works for any type which is totally ordered  including special values like NaN or infinities

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You dont need one  you can just use  if not number    0      sig   number abs number  else      sig   0  Or create a function as described by others  sign   lambda x  bool x  gt  0  - bool x  lt  0   def sign x       return bool x  gt  0  - bool x  lt  0

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Try running this  where x is any number  int sign   bool x  gt  0  - bool x  lt  0    The coercion to bool   handles the possibility that the comparison operator doesn t return a boolean

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copysign  is defined by IEEE 754  and part of the C99 specification  That s why it s in Python  The function cannot be implemented in full by abs x    sign y  because of how it s supposed to handle NaN values    gt  gt  gt  import math  gt  gt  gt  math copysign 1  float  nan    1 0  gt  gt  gt  math copysign 1  float  -nan    -1 0  gt  gt  gt  math copysign float  nan    1  nan  gt  gt  gt  math copysign float  nan    -1  nan  gt  gt  gt  float  nan     -1 nan  gt  gt  gt  float  nan     1 nan  gt  gt  gt     That makes copysign   a more useful function than sign     As to specific reasons why IEEE s signbit x  is not available in standard Python  I don t know  I can make assumptions  but it would be guessing   The math module itself uses copysign 1  x  as a way to check if x is negative or non-negative  For most cases dealing with mathematical functions that seems more useful than having a sign x  which returns 1  0  or -1 because there s one less case to consider  For example  the following is from Python s math module   static double m atan2 double y  double x            if  Py IS NAN x     Py IS NAN y                   return Py NAN          if  Py IS INFINITY y                     if  Py IS INFINITY x                             if  copysign 1   x     1                                      atan2  -inf   inf      -pi 4                                    return copysign 0 25 Py MATH PI  y                           else                                    atan2  -inf  -inf      -pi 3 4                                    return copysign 0 75 Py MATH PI  y                                        atan2  -inf  x      -pi 2 for finite x                    return copysign 0 5 Py MATH PI  y     There you can clearly see that copysign   is a more effective function than a three-valued sign   function   You wrote      If I were a python designer  I would been the other way around  no cmp   builtin  but a sign     That means you don t know that cmp   is used for things besides numbers   cmp  This    That   cannot be implemented with a sign   function   Edit to collate my additional answers elsewhere   You base your justifications on how abs   and sign   are often seen together  As the C standard library does not contain a  sign x   function of any sort  I don t know how you justify your views  There s an abs int  and fabs double  and fabsf float  and fabsl long  but no mention of sign  There is  copysign    and  signbit    but those only apply to IEEE 754 numbers   With complex numbers  what would sign -3 4j  return in Python  were it to be implemented  abs -3 4j  return 5 0  That s a clear example of how abs   can be used in places where sign   makes no sense   Suppose sign x  were added to Python  as a complement to abs x   If   x  is an instance of a user-defined class which implements the   abs   self  method then abs x  will call x   abs      In order to work correctly  to handle abs x  in the same way then Python will have to gain a sign x  slot   This is excessive for a relatively unneeded function  Besides  why should sign x  exist and nonnegative x  and nonpositive x  not exist  My snippet from Python s math module implementation shows how copybit x  y  can be used to implement nonnegative    which a simple sign x  cannot do   Python should support have better support for IEEE 754 C99 math function  That would add a signbit x  function  which would do what you want in the case of floats  It would not work for integers or complex numbers  much less strings  and it wouldn t have the name you are looking for   You ask  why   and the answer is  sign x  isn t useful   You assert that it is useful  Yet your comments show that you do not know enough to be able to make that assertion  which means you would have to show convincing evidence of its need  Saying that NumPy implements it is not convincing enough  You would need to show cases of how existing code would be improved with a sign function   And that it outside the scope of StackOverflow  Take it instead to one of the Python lists

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EDIT   Indeed there was a patch which included sign   in math  but it wasn t accepted  because they didn t agree on what it should return in all the edge cases    -0    -nan  etc   So they decided to implement only copysign  which  although more verbose  can be used to delegate to the end user the desired behavior for edge cases - which sometimes might require the call to cmp x 0      I don t know why it s not a built-in  but I have some thoughts   copysign x y   Return x with the sign of y    Most importantly  copysign is a superset of sign   Calling copysign with x 1 is the same as a sign function   So you could just use copysign and forget about it    gt  gt  gt  math copysign 1  -4  -1 0  gt  gt  gt  math copysign 1  3  1 0   If you get sick of passing two whole arguments  you can implement sign this way  and it will still be compatible with the IEEE stuff mentioned by others    gt  gt  gt  sign   functools partial math copysign  1    either of these  gt  gt  gt  sign   lambda x  math copysign 1  x    two will work  gt  gt  gt  sign -4  -1 0  gt  gt  gt  sign 3  1 0  gt  gt  gt  sign 0  1 0  gt  gt  gt  sign -0 0  -1 0  gt  gt  gt  sign float  nan    -1 0   Secondly  usually when you want the sign of something  you just end up multiplying it with another value   And of course that s basically what copysign does   So  instead of   s   sign a  b   b   s   You can just do   b   copysign b  a    And yes  I m surprised you ve been using Python for 7 years and think cmp could be so easily removed and replaced by sign   Have you never implemented a class with a   cmp   method   Have you never called cmp and specified a custom comparator function   In summary  I ve found myself wanting a sign function too  but copysign with the first argument being 1 will work just fine   I disagree that sign would be more useful than copysign  as I ve shown that it s merely a subset of the same functionality

User · Answer

It just doesn t   The best way to fix this is   sign   lambda x  bool x  gt  0  - bool x  lt  0

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numpy has a sign function  and gives you a bonus of other functions as well   So    import numpy as np x   np sign y    Just be careful that the result is a numpy float64     gt  gt  gt  type np sign 1 0    lt type  numpy float64  gt    For things like json  this matters  as json does not know how to serialize numpy float64 types   In that case  you could do    float np sign y     to get a regular float

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Since cmp has been removed  you can get the same functionality with  def cmp a  b       return  a  gt  b  -  a  lt  b   def sign a       return  a  gt  0  -  a  lt  0    It works for float  int and even Fraction  In the case of float  notice sign float  nan    is zero   Python doesn t require that comparisons return a boolean  and so coercing the comparisons to bool   protects against allowable  but uncommon implementation   def sign a       return bool a  gt  0  - bool a  lt  0

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Yes a correct sign   function should be at least in the math module - as it is in numpy  Because one frequently needs it for math oriented code   But math copysign   is also useful independently   cmp   and obj   cmp         have generally high importance independently  Not just for math oriented code  Consider comparing sorting tuples  date objects       The dev arguments at http   bugs python org issue1640 regarding the omission of math sign   are odd  because    There is no separate -NaN sign nan     nan  without worry  like exp nan    sign -0 0     sign 0 0     0 without worry sign -inf     -1 without worry   -- as it is in numpy

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The reason  sign  is not included is that if we included every useful one-liner in the list of built-in functions  Python wouldn t be easy and practical to work with anymore  If you use this function so often then why don t you do factor it out yourself  It s not like it s remotely hard or even tedious to do so

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Only correct answer compliant with the Wikipedia definition The definition on Wikipedia reads   Hence  sign   lambda x  -1 if x  lt  0 else  1 if x  gt  0 else  0 if x    0 else NaN    Which for all intents and purposes may be simplified to  sign   lambda x  -1 if x  lt  0 else  1 if x  gt  0 else 0   This function definition executes fast and yields guaranteed correct results for 0  0 0  -0 0  -4 and 5  see comments to other incorrect answers   Note that zero  0  is neither positive nor negative

[python] Why doesn't Python have a sign function?

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