Does Python support short-circuiting

Question

Does Python support short-circuiting in boolean expressions

User · Accepted Answer

Yep  both and and or operators short-circuit -- see the docs

User · Answer

Yes  Try the following in your python interpreter   and   gt  gt  gt False and 3 0 False  gt  gt  gt True and 3 0 ZeroDivisionError  integer division or modulo by zero   or   gt  gt  gt True or 3 0 True  gt  gt  gt False or 3 0 ZeroDivisionError  integer division or modulo by zero

User · Answer

Yes  Python does support Short-circuit evaluation  minimal evaluation  or McCarthy evaluation for Boolean operators  It is used to reduce the number of evaluations for computing the output of boolean expression  Example - Base Functions def a x       print  a       return x  def b x       print  b       return x   AND if a True  and b True        print 1 end   n n    if a False  and b True        print 2 end   n n     AND-OUTPUT a b 1  a   OR if a True  or b False        print 3 end   n n    if a False  or b True        print 4 end   n n     OR-OUTPUT a 3  a b 4

User · Answer

Short-circuiting behavior in operator and  or  Let s first define a useful function to determine if something is executed or not  A simple function that accepts an argument  prints a message and returns the input  unchanged   gt  gt  gt  def fun i           print  quot executed quot          return i       One can observe the Python s short-circuiting behavior of and  or operators in the following example   gt  gt  gt  fun 1  executed 1  gt  gt  gt  1 or fun 1       due to short-circuiting   quot executed quot  not printed 1  gt  gt  gt  1 and fun 1      fun 1  called and  quot executed quot  printed  executed 1  gt  gt  gt  0 and fun 1      due to short-circuiting   quot executed quot  not printed  0  Note  The following values are considered by the interpreter to mean false          False    None    0     quot  quot                      Short-circuiting behavior in function  any    all    Python s any   and all   functions also support short-circuiting  As shown in the docs  they evaluate each element of a sequence in-order  until finding a result that allows an early exit in the evaluation  Consider examples below to understand both  The function any   checks if any element is True  It stops executing as soon as a True is encountered and returns True   gt  gt  gt  any fun i  for i in  1  2  3  4       bool 1    True executed True  gt  gt  gt  any fun i  for i in  0  2  3  4      executed                                 bool 0    False executed                                 bool 2    True True  gt  gt  gt  any fun i  for i in  0  0  3  4   executed executed executed True  The function all   checks all elements are True and stops executing as soon as a False is encountered   gt  gt  gt  all fun i  for i in  0  0  3  4   executed False  gt  gt  gt  all fun i  for i in  1  0  3  4   executed executed False  Short-circuiting behavior in Chained Comparison  Additionally  in Python  Comparisons can be chained arbitrarily  for example  x  lt  y  lt   z is equivalent to x  lt  y and y  lt   z  except that y is evaluated only once  but in both cases z is not evaluated at all when x  lt  y is found to be false     gt  gt  gt  5  gt  6  gt  fun 3       same as   5  gt  6 and 6  gt  fun 3  False                   5  gt  6 is False so fun   not called and  quot executed quot  NOT printed  gt  gt  gt  5  lt  6  gt  fun 3       5  lt  6 is True  executed                fun 3  called and  quot executed quot  printed True  gt  gt  gt  4  lt   6  gt  fun 7      4  lt   6 is True   executed                fun 3  called and  quot executed quot  printed False  gt  gt  gt  5  lt  fun 6   lt  3      only prints  quot executed quot  once executed False  gt  gt  gt  5  lt  fun 6  and fun 6   lt  3   prints  quot executed quot  twice  because the second part executes it again executed executed False  Edit  One more interesting point to note  - Logical and  or operators in Python returns an operand s value instead of a Boolean  True or False   For example   Operation x and y gives the result if x is false  then x  else y  Unlike in other languages e g   amp  amp      operators in C that return either 0 or 1  Examples   gt  gt  gt  3 and 5      Second operand evaluated and returned  5                     gt  gt  gt  3  and        gt  gt  gt     and 5     Second operand NOT evaluated as first operand    is  false                  so first operand returned   Similarly or operator return left most value for which bool value     True else right most false value  according to short-circuiting behavior   examples   gt  gt  gt  2 or 5      left most operand bool 2     True 2      gt  gt  gt  0 or 5      bool 0     False and bool 5     True 5  gt  gt  gt  0 or        So  how is this useful  One example is given in Practical Python By Magnus Lie Hetland  Let   s say a user is supposed to enter his or her name  but may opt to enter nothing  in which case you want to use the default value   lt Unknown gt    You could use an if statement  but you could also state things very succinctly  In  171   name   raw input  Enter Name     or   lt Unknown gt   Enter Name    In  172   name Out 172     lt Unknown gt    In other words  if the return value from raw input is true  not an empty string   it is assigned to name  nothing changes   otherwise  the default   lt Unknown gt   is assigned to name

[python] Does Python support short-circuiting?

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