# 8.3. Logical operators¶

There are three logical operators: `and`, `or`, and `not`. All three operators take boolean operands and produce boolean values. The semantics (meaning) of these operators is similar to their meaning in English:

• `x and y` is `True` if both `x` and `y` are `True`. Otherwise, `and` produces `False`.

• `x or y` yields `True` if either `x` or `y` is `True`. Only if both operands are `False` does `or` yield `False`.

• `not x` yields `False` if `x` is `True`, and vice versa.

Look at the following example. See if you can predict the output. Then, Run to see if your predictions were correct:

Although you can use boolean operators with simple boolean literals or variables as in the above example, they are often combined with the comparison operators, as in this example. Again, before you run this, see if you can predict the outcome:

The expression `x > 0 and x < 10` is `True` only if `x` is greater than 0 and at the same time, x is less than 10. In other words, this expression is `True` if x is between 0 and 10, not including the endpoints.

Common Mistake!

There is a very common mistake that occurs when programmers try to write boolean expressions. For example, what if we have a variable `number` and we want to check to see if its value is 5 or 6. In words we might say: “number equal to 5 or 6”. However, if we translate this into Python, `number == 5 or 6`, it will not yield correct results. The `or` operator must have a complete equality check on both sides. The correct way to write this is `number == 5 or number == 6`. Remember that both operands of `or` must be booleans in order to yield proper results.

## 8.3.1. Smart Evaluation¶

Python is “smart” about the way it evaluates expressions using boolean operators. Consider the following example:

```answer = input('Continue?')
print('Continuing!')
```

There are two operands for the `or` operator here: `answer == 'Y'` and `'answer == 'y'`. Python evaluates from left to right, and if the first operand for `or` evaluates to `True`, Python doesn’t bother evaluating the second operand, because it knows the result must be `True` (recall that if either operand for `or` is `True`, the result is `True`). So, if the user enters `Y`, Python first evaluates ```answer == 'Y'```, determines that it is `True`, and doesn’t bother to check to see if `answer == 'y'` is `True`; it just concludes that the entire condition is `True` and executes the print statement.

In a similar fashion, with the `and` operator, if the first operand evaluates to `False`, Python doesn’t check the second operand’s value, because it can conclude that the result must be `False`.

This behavior, in which Python in some cases skips the evaluation of the second operand to `and` and `or`, is called short-circuit boolean evaluation. You don’t have to do anything to make Python do this; it’s the way Python works. It saves a little processing time. And, as a special bonus, you can take advantage of Python’s short-circuiting behavior to shorten your code. Consider the following example:

This code checks to see if the average weight of a given number of pieces of luggage is greater than 50 pounds. However, there is a potential crash situation here. If the user enters `0` for `num_pieces`, the program will crash with a divide by zero error. Try it out to see it happen.

To prevent the crash, you might add an extra if statement to check for zero:

```if num_pieces != 0:
if total_weight / num_pieces > 50:
print('Average weight is greater than 50 pounds -> \$100 surcharge.')
```

Now, the division will not occur if `num_pieces` is zero, and a potential runtime crash has been averted. Good job!

We can shorten this example to a single `if` statement if we do it carefully. Anytime you have two nested `if` statements as in the example above, you can combine them into a single `if` statement by joining the conditions using the `and` operator. Consider the version below, and think about why this `if` statement is equivalent in its behavior to the previous version with two nested `if` statements:

But wait a minute: is this code safe? Try running the program and entering the value `500` for `total_weight` and the value `5` for num_pieces. Then, try it again using the value `0` for num_pieces. There should be no crash.

Next, try altering the code and reversing the order of the `if` conditions:

```if total_weight / num_pieces > 50 and num_pieces != 0:
print('Average weight is greater than 50 pounds -> \$100 surcharge.')
```

Run the program again, performing the same two tests. This time, you should observe a crash when you enter `0` for num_pieces. Can you analyze why the first version did not crash, but the second one does?

In the second version, when evaluating left-to-right, the division by zero occurs before Python evaluates the comparison `num_pieces != 0`. When joining two `if` statements into a single `if` statement, you must be sure to put the condition from the first `if` statement on the left-hand side of the `and` operator, and the other condition on the right-hand side, in order to get the same effect.

To summarize this discussion on smart evaluation, keep in mind that when you are performing potentially dangerous operations in an `if` statement or `while` loop using boolean logic with `and` or `or`, order matters!