Python if not vs if

Question

What is the difference between these two lines of code   if not x     val     and   if x     val     Is one more efficient than the other   Would it be better to use  if x     val       pass else

User · Answer

jonrsharpe has an excellent explanation of what s going on  I thought I d just show the difference in time when running each of the 3 options 10 000 000 times  enough for a slight difference to show    Code used   def a x       if x     val           pass   def b x       if not x     val           pass   def c x       if x     val           pass     else          pass   x   1 for i in range 10000000       a x      b x      c x    And the cProfile profiler results     So we can see that there is a very minute difference of  0 7  between if not x     val   and if x     val    Of these  if x     val   is the fastest  However  most surprisingly  we can see that   if x     val           pass     else    is in fact the fastest  and beats if x     val   by  0 3   This isn t very readable  but I guess if you wanted a negligible performance improvement  one could go down this route

User · Answer

Using dis to look at the bytecode generated for the two versions   not       4           0 LOAD FAST                0  foo                3 LOAD FAST                1  bar                6 COMPARE OP               2                    9 UNARY NOT                         10 RETURN VALUE            4           0 LOAD FAST                0  foo                3 LOAD FAST                1  bar                6 COMPARE OP               3                    9 RETURN VALUE      The latter has fewer operations  and is therefore likely to be slightly more efficient      It was pointed out in the commments  thanks   Quincunx  that where you have if foo    bar vs  if not foo    bar the number of operations is exactly the same  it s just that the COMPARE OP changes and POP JUMP IF TRUE switches to POP JUMP IF FALSE   not        2           0 LOAD FAST                0  foo                3 LOAD FAST                1  bar                6 COMPARE OP               2                    9 POP JUMP IF TRUE        16         2           0 LOAD FAST                0  foo                3 LOAD FAST                1  bar                6 COMPARE OP               3                    9 POP JUMP IF FALSE       16   In this case  unless there was a difference in the amount of work required for each comparison  it s unlikely you d see any performance difference at all     However  note that the two versions won t always be logically identical  as it will depend on the implementations of   eq   and   ne   for the objects in question  Per the data model documentation      There are no implied relationships among the comparison operators  The   truth of x  y does not imply that x  y is false    For example    gt  gt  gt  class Dummy object       def   eq   self  other           return True     def   ne   self  other           return True    gt  gt  gt  not Dummy      Dummy   False  gt  gt  gt  Dummy      Dummy   True     Finally  and perhaps most importantly  in general  where the two are logically identical  x    y is much more readable than not x    y

User · Answer

gt  gt  gt  from dis import dis  gt  gt  gt  dis compile  not 10    20        exec      1           0 LOAD CONST               0  10                3 LOAD CONST               1  20                6 COMPARE OP               2                    9 UNARY NOT              10 POP TOP              11 LOAD CONST               2  None               14 RETURN VALUE  gt  gt  gt  dis compile  10    20        exec      1           0 LOAD CONST               0  10                3 LOAD CONST               1  20                6 COMPARE OP               3                    9 POP TOP              10 LOAD CONST               2  None               13 RETURN VALUE   Here you can see that not x    y has one more instruction than x    y  So the performance difference will be very small in most cases unless you are doing millions of comparisons and even then this will likely not be the cause of a bottleneck

User · Answer

I want to expand on my readability comment above   Again  I completely agree with readability overriding other  performance-insignificant  concerns   What I would like to point out is the brain interprets  positive  faster than it does  negative    E g    stop  vs   do not go   a rather lousy example due to the difference in number of words    So given a choice   if a    b      do this  else      do that    is preferable to the functionally-equivalent   if a    b      do that  else      do this    Less readability understandability leads to more bugs   Perhaps not in initial coding  but the  not as smart as you   maintenance changes

User · Answer

In the first one Python has to execute one more operations than necessary instead of just checking not equal to it has to check if it is not true that it is equal  thus one more operation   It would be impossible to tell the difference from one execution  but if run many times  the second would be more efficient  Overall I would use the second one  but mathematically they are the same

User · Answer

It s about your way of reading it  not operator is dynamic  that s why you are able to apply it in   if not x     val     But    could be read in a better context as an operator which does the opposite of what    does

User · Answer

An additional note  since the other answers answered your question mostly correctly  is that if a class only defines   eq     and not   ne      then your COMPARE OP      will run   eq     and negate it  At that time  your third option is likely to be a tiny bit more efficient  but should only be considered if you NEED the speed  since it s difficult to understand quickly

[python] Python if not == vs if !=

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