# Short circuit evaluation

Categories: python language intermediate python

Python has a useful feature called short circuit evaluation that acts on logical operators `or`

and `and`

.

## Short circuiting with the or operator

Consider this code:

```
x = 5
y = 10
if x > 6 or y < 20:
print('OK')
```

When Python reaches the if statement, it starts evaluating the terms from left to right:

`x > 6`

is false`y < 20`

is true- therefore the result of the
`or`

expression is true - the print statement executes

Now compare this code:

```
x = 7
y = 10
if x > 6 or y < 20:
print('OK')
```

`x > 6`

is true- therefore the result of the
`or`

expression is true - the print statement executes

In this case, as soon as Python discovers that `x`

is greater than 6, it knows that the whole `or`

statement is true, so it doesn't bother checking `y`

. It doesn't matter whether `y`

is less than 20 or not, it won't affect the outcome because an `or`

statement is always true if either of the terms is true.

Python

guaranteesnot to evaluate the second term if the first term is true.

## Short circuiting with the and operator

Short circuiting with the `and`

operator works in a similar way.

```
x = 5
y = 10
if x > 6 and y < 20:
print('OK')
```

In this case:

`x > 6`

is false- therefore the result of the
`and`

expression is false - the print statement doesn't execute

In this case, Python never evaluates `y < 20`

because it already knows the final result will be false.

In summary:

- An
`or`

statement stops as soon as if encounters the first`True`

case - An
`and`

statement stops as soon as if encounters the first`False`

case

## Short circuiting to avoid errors

We can often use short circuit evaluation to avoid errors. A common case is checking for a value being 0 before performing a calculation:

```
n = 3
m = 2
if m < 1/n:
print('m < 1/n')
```

This code looks alright at first glance. But what happens if `n`

is zero? Dividing by zero is an error, so our code would thrown an exception and stop working.

A quick way to avoid this is to use short circuiting, with the `and`

operator:

```
n = 0
m = 2
if n and m < 1/n:
print('m < 1/n')
```

With this new code, we have an `and`

expression. The first term in `n`

. If `n`

is 0, which of course counts as false, Python knows that the whole an expression is false, so it never evaluates `1/n`

and there is no error.

If `n`

is not zero, Python evaluates the second expression as normal.

You need to take care with this technique. Although we have successfully avoided throwing a divide by zero error, we have also ignored the fact that

`n`

is zero. This could lead to further errors in other parts of the code.

## The value of an or/and expression

You might expect the value of a logical expression to always be `True`

or `False`

, but in fact this isn't the case. Python does something more useful.

```
a = 0
b = 2
c = 3
x = c or a:
print(x)
```

You may be surprised to see it print 3.

What actually happens is that Python prints the last value it evaluates. In this case, Python evaluates `c`

, which has a value 3. Since this counts as true, it doesn't need to evaluate `a`

, so it returns the value 3.

Here are some more examples, using the same values of `a`

, `b`

and `c`

:

```
x = a or b:
print(x) #prints 2
x = b or c:
print(x) #prints 2
x = 0 or a:
print(x) #prints 0 (the result is false because 0 and a are both false)
x = a or b+c or b+a:
print(x) #prints 5 (b+c, the first true value)
```

Here is a real life example:

```
username=''
username = username or input('Enter username')
```

In this case, the `or`

statement first evaluates `username`

, which is an empty string, so it counts as false. It then evaluates the `input`

statement - that is, it calls the `input`

function, waits for the user to type in their name, and returns that value.

If we were to execute this statement again, later, when `username`

has a value, the `or`

expression would simply return the current value, and would not call `input()`

at all.

## Getting a true or false value

It doesn't usually matter that `a`

or `b`

gives a value of 2 rather than `True`

, because it works fine in an if statement.

If you really need to, you can always use the `bool()`

function. `bool()`

converts *any* value to `True`

or `False`

.

## See also

- List comprehensions
- Objects and variables
- Objects and identity
- Immutable objects
- Global variables
- Data types
- Lists vs tuples
- Sequences
- Named tuples
- Operators
- Walrus Operator
- For loops
- For loop using range vs iterables
- Changing the loop order
- Using enumerate in a for loop
- Using zip in a for loop
- Looping over multiple items (old article)
- Looping over selected items
- Functions
- Declaring functions
- Calling functions
- Function objects and lambdas
- Function decorators
- With statements
- Exception handling
- String functions
- Built-in functions
- Optimisation
- Optimisation good practice
- Low level code optimisation
- Structural optimisation

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