Short Circuit Evaluation

Notes

The and and or are the short circuit operators. They stop evaluating when they have sufficient information instead of evaluating all of its operands.

Examples 1

if 1==1 or this_function_does_not_exist():

    print("surprise!")

#You may expect that an error may occur because the program calls a function this_function_does_not_exist() that is not defined; 

#but surprisingly the program print the words "surprise!" successfully. Why?

#Well because 1==1 is true. For the or operator, it determines that it has enough information to produce a true output, so it is not necessary to evaluate the this_function_does_not_exist() function.

Examples 2

if not (1==0 and this_function_does_not_exist()):

    print("surprise!")

#Again, You may expect that an error may occur because the program calls a function this_function_does_not_exist() that is not defined; 

#but surprisingly the program print the words "surprise!" successfully. Why?

#Well because 1==0 is false. For the and operator, it determines that it has enough information to produce a false output, so it is not necessary to evaluate the this_function_does_not_exist() function.