When Python is processing a logical expression such as x >= 2 and (x/y) >2, it evaluates the expression from left-to-right. Because of the definition of and, if x is less than 2, the expression x >= 2 is False and so the whole expression is False regardless of whether (x/y) > 2 evaluates to True or False.
When Python detects that there is nothing to be gained by evaluating the rest of a logical expression, it stops its evaluation and does not do the computations in the rest of the logical expression. When the evaluation of a logical expression stops because the overall value is already known, it is called short-circuiting the evaluation.
While this may seem like a fine point, the short circuit behavior leads to a clever technique called the guardian pattern. Consider the following code sequence in the Python interpreter:
>>> x = 6>>> y = 2>>> x >= 2 and (x/y) > 2True>>> x = 1>>> y = 0>>> x >= 2 and (x/y) > 2False>>> x = 6>>> y = 0>>> x >= 2 and (x/y) > 2Traceback (most recent call last): File "<stdin>", line 1, in <module>ZeroDivisionError: integer division or modulo by zero>>>
The third calculation failed because Python was evaluating (x/y) and y was zero which causes a runtime error. But the second example did not fail because the first part of the expression x >= 2 evaluated to False so the (x/y) was not ever executed due to the short circuit rule and there was no error.
We can construct the logical expression to strategically place a guard evaluation just before the evaluation that might cause an error as follows:
>>> x = 1>>> y = 0>>> x >= 2 and y != 0 and (x/y) > 2False>>> x = 6>>> y = 0>>> x >= 2 and y != 0 and (x/y) > 2False>>> x >= 2 and (x/y) > 2 and y != 0Traceback (most recent call last): File "<stdin>", line 1, in <module>ZeroDivisionError: integer division or modulo by zero>>>
In the first logical expression, x >= 2 is False so the evaluation stops at the and.
In the second logical expression x >= 2 is True but y != 0 is False so we never reach (x/y).
In the third logical expression, the y != 0 is after the (x/y) calculation so the expression fails with an error.
In the second expression, we say that y != 0 ac ts as a guard to insure that we only execute (x/y) if y is non-zero.