# 3.8: Short-Circuit Evaluation of Logical Expressions

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- Chuck Severance
- Clinical Associate Professor (School of Information) at University of Michigan

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) > 2
True
>>> x = 1
>>> y = 0
>>> x >= 2 and (x/y) > 2
False
>>> x = 6
>>> y = 0
>>> x >= 2 and (x/y) > 2
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ZeroDivisionError: division 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) > 2
False
>>> x = 6
>>> y = 0
>>> x >= 2 and y != 0 and (x/y) > 2
False
>>> x >= 2 and (x/y) > 2 and y != 0
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ZeroDivisionError: division 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`

acts as a *guard* to insure that we only execute `(x/y)`

if `y`

is non-zero.