4.8. Short-circuit evaluation of logical expressionsΒΆ
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:
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:
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 acts as a
guard to insure that we only execute (x/y)
if y is non-zero.
- x >= 2
- Try again. This element is making sure x matches a condition.
- y != 0
- This element is a guard, making sure the code does not try to divide by 0.
- (x/y) > 2
- Try again. This element is making sure x/y matches a condition.
- and
- Try again. This is a logical operator for the boolean expression.
csp-10-2-3: Which of the following is a guard for the code below?
x >= 2 and y != 0 and (x/y) > 2
csp-10-2-4: True or False? The guard should be placed before an expression is evaluated.
-
csp-10-2-5: Match each term with its meaning.
What is the purpose of each term?
- guardian pattern
- A logical expression with additional comparisons to take advantage of the short circuit behavior.
- short circuit
- When Python is part-way through evaluating a logical expression and stops the evaluation because Python knows the final value for the expression without needing to evaluate the rest of the expression.
- guard
- A comparison put in place to cause short circuit behavior and avoid a runtime error.