Python syntax for if a or b or c but not all of them

Question

I have a python script that can receive either zero or three command line arguments   Either it runs on default behavior or needs all three values specified    What s the ideal syntax for something like   if a and  not b or not c  or b and  not a or not c  or c and  not b or not a

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I d go for   conds   iter  a  b  c   if any conds  and not any conds         okay      I think this should short-circuit fairly efficiently  Explanation  By making conds an iterator  the first use of any will short circuit and leave the iterator pointing to the next element if any item is true  otherwise  it will consume the entire list and be False  The next any takes the remaining items in the iterable  and makes sure than there aren t any other true values    If there are  the whole statement can t be true  thus there isn t one unique element  so short circuits again   The last any will either return False or will exhaust the iterable and be True   note  the above checks if only a single condition is set    If you want to check if one or more items  but not every item is set  then you can use   not all conds  and any conds

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This question already had many highly upvoted answers and an accepted answer  but all of them so far were distracted by various ways to express the boolean problem and missed a crucial point       I have a python script that can receive either zero or three command   line arguments   Either it runs on default behavior or needs all three   values specified    This logic should not be the responsibility of your code in the first place  rather it should be handled by argparse module   Don t bother writing a complex if statement  instead prefer to setup your argument parser something like this      usr bin env python import argparse as ap parser   ap ArgumentParser   parser add argument  --foo   nargs 3  default   x    y    z    args   parser parse args   print args foo    And yes  it should be an option not a positional argument  because it is after all optional       edited   To address the concern of LarsH in the comments  below is an example of how you could write it if you were certain you wanted the interface with either 3 or 0 positional args   I am of the opinion that the previous interface is better style  because optional arguments should be options  but here s an alternative approach for the sake of completeness   Note the overriding kwarg usage when creating your parser  because argparse will auto-generate a misleading usage message otherwise       usr bin env python import argparse as ap parser   ap ArgumentParser usage    prog s  -h   a b c  n   parser add argument  abc   nargs      help  specify 3 or 0 items   default   x    y    z    args   parser parse args   if len args abc     3    parser error  expected 3 arguments   print args abc    Here are some usage examples     default case wim wim-zenbook  tmp    three or none py    x    y    z      explicit case wim wim-zenbook  tmp    three or none py 1 2 3   1    2    3      example failure mode wim wim-zenbook  tmp    three or none py 1 2  usage  three or none py  -h   a b c  three or none py  error  expected 3 arguments

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When every given bool is True  or when every given bool is False    they all are equal to each other     So  we just need to find two elements which evaluates to different bools to know that there is at least one True and at least one False   My short solution   not bool a   bool b   bool c    I belive it short-circuits  cause AFAIK a  b  c equals a  b and b  c   My generalized solution   def  any but not all first  iterable    doing dirty work     bool first bool first      for x in iterable          if bool x  is not bool first              return True     return False  def any but not all arg   args    takes any amount of args convertable to bool     return  any but not all arg  args   def v any but not all iterable    takes iterable or iterator     iterator iter iterable      return  any but not all next iterator   iterator    I wrote also some code dealing with multiple iterables  but I deleted it from here because I think it s pointless  It s however still available here

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If you don t mind being a bit cryptic you can simly roll with 0  lt   a   b   c   lt  3 which will return true if you have between one and two true statements and false if all are false or none is false   This also simplifies if you use functions to evaluate the bools as you only evaluate the variables once and which means you can write the functions inline and do not need to temporarily store the variables   Example  0  lt    a x    b x    c x     lt  3

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This returns True if one and only one of the three conditions is True  Probably what you wanted in your example code   if sum 1 for x in  a b c  if x     1

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To be clear  you want to made your decision based on how much of the parameters are logical TRUE  in case of string arguments - not empty    argsne    1 if a else 0     1 if b else 0     1 if c else 0    Then you made a decision   if   0  lt  argsne  lt  3     doSth      Now the logic is more clear

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This is basically a  some  but not all   functionality  when contrasted with the any   and all   builtin functions    This implies that there should be Falses and Trues among the results  Therefore  you can do the following   some   lambda ii  frozenset bool i  for i in ii  issuperset  True  False      one way to test this is    test   lambda iterable   any iterable  and  not all iterable      see also http   stackoverflow com a 16522290 541412    Some test cases    assert some        False          all   is true  and any   is false assert some  False       False    any   is false assert some  True       False     any   and all   are true  assert some  False False      False  assert some  True True      False  assert some  True False      True  assert some  False True      True    One advantage of this code is that you only need to iterate once through the resulting  booleans  items   One disadvantage is that all these truth-expressions are always evaluated  and do not do short-circuiting like the or and operators

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The English sentence          if a or b or c but not all of them      Translates to this logic    a or b or c  and not  a and b and c    The word  but  usually implies a conjunction  in other words  and    Furthermore   all of them  translates to a conjunction of conditions  this condition  and that condition  and other condition  The  not  inverts that entire conjunction   I do not agree that the accepted answer  The author neglected to apply the most straightforward interpretation to the specification  and neglected to apply De Morgan s Law to simplify the expression to fewer operators    not a or not b or not c  - gt   not  a and b and c    while claiming that the answer is a  minimal form

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If you work with an iterator of conditions  it could be slow to access  But you don t need to access each element more than once  and you don t always need to read all of it  Here s a solution that will work with infinite generators      usr bin env python3 from random import randint from itertools import tee  def generate random        while True          yield bool randint 0 1    def any but not all2 s     elegant     t1  t2   tee s      return False in t1 and True in t2   could also use  not all      and any        def any but not all s     simple     hadFalse   False     hadTrue   False     for i in s          if i              hadTrue   True         else              hadFalse   True         if hadTrue and hadFalse              return True     return False   r1  r2   tee generate random    assert any but not all r1  assert any but not all2 r2   assert not any but not all  True  True   assert not any but not all2  True  True    assert not any but not all     assert not any but not all2      assert any but not all  True  False   assert any but not all2  True  False

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How about   conditions    a  b  c  if any conditions  and not all conditions            Other variant   if 1  lt   sum map bool  conditions    lt   2

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If you mean a minimal form  go with this   if  not a or not b or not c  and  a or b or c     Which translates the title of your question   UPDATE  as correctly said by Volatility and Supr  you can apply De Morgan s law and obtain equivalent   if  a or b or c  and not  a and b and c     My advice is to use whichever form is more significant to you and to other programmers  The first means  there is something false  but also something true   the second  There is something true  but not everything   If I were to optimize or do this in hardware  I would choose the second  here just choose the most readable  also taking in consideration the conditions you will be testing and their names   I picked the first

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As I understand it  you have a function that receives 3 arguments  but if it does not it will run on default behavior  Since you have not explained what should happen when 1 or 2 arguments are supplied I will assume it should simply do the default behavior  In which case  I think you will find the following answer very advantageous    def method a None  b None  c None       if all  a  b  c              received 3 arguments     else            default behavior   However  if you want 1 or 2 arguments to be handled differently   def method a None  b None  c None       args    a  b  c      if all args             received 3 arguments     elif not any args             default behavior     else            some args  raise exception     note  This assumes that  False  values will not be passed into this method

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And why not just count them    import sys a   sys argv if len a    1           No arguments were given  the program name count as one elif len a    4         Three arguments were given else         another amount of arguments was given

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The question states that you need either all three arguments  a and b and c  or none of them  not  a or b or c    This gives    a and b and c  or not  a or b or c

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What about   unique condition   if  bool a    bool b    bool c     1     Notice  if you allow two conditions too you could do that  if  bool a    bool b    bool c  in  1 2

[python] Python syntax for "if a or b or c but not all of them"

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