# Conditions and Conditionals¶

## Introduction¶

**Conditions** are things that can be True or False. For instance, the
expression `x < 3`

which is True if `x`

is the name of a number less than 3
and False otherwise.

**Conditionals** are things that depend on conditions. The most important kind
of conditional in Python is the `if`

statement, which lets you do one thing
or another depending on whether a condition is True.

This sheet tells you about conditions and conditionals.

## Conditions¶

Try typing these things in. We haven’t shown the answers, because you’ll learn better if you try them.

```
>>> 1 < 2 # 1 is less then 2, so this condition is True
[CENSORED]
>>> 1 > 2 # 1 is not greater than 2, so this condition is False
[CENSORED]
```

`True`

and `False`

(be sure to capitalize the first letter) represent the
truth value of conditions. They are special Python values used for things that
are either True or False.

You can actually use other things as truth values. We don’t recommend this, though; it’s just likely to be confusing.

## Comparisons¶

Most conditions are comparisons of one object with another. Here’s a brief list of ways to compare things in Python.

`a < b`

- True if
`a`

is less than`b`

`a <= b`

- True if
`a`

is less than or equal to`b`

`a > b`

- True if
`a`

is greater than`b`

`a >= b`

- True if
`a`

is greater than or equal to`b`

`a == b`

- True if
`a`

is equal to`b`

`a != b`

- True if
`a`

is not equal to`b`

It’s pretty obvious what these things mean when `a`

and `b`

are numbers.
But they make sense for other sorts of objects, too. For instance, strings are
compared in something rather like alphabetical order. In fact, you can compare
*any* two objects, though many of the possible comparisons are *very silly*.
For instance, it turns out that Python thinks that 3 is less than `'silly'`

.

Be careful, by the way, to notice the difference between `=`

and `==`

. You
use `=`

for setting a variable (i.e., giving a name to an object), and `==`

for testing whether two things are equal.

### Combining comparisons¶

You can say things like `1 < x < 2`

, meaning 1 is less than `x`

, and `x`

is less than 2 .

## Other conditions¶

Here are some other useful conditions.

`0`

- Always False
- any non-zero number
- Always True
`x in y`

- True if
`x`

is equal to some element of`y`

`x not in y`

- True if
`x`

is not equal to any element of`y`

- an empty list, tuple, or dictionary (
`[]`

,`()`

, or`{}`

) - Always False
- a list, tuple, or dictionary that is not empty
- Always True

(*note*: For `in`

and `not in`

, `y`

should be a sequence: that is, a list
or a tuple or a string.)

## Combining conditions¶

You can join conditions together using the words `and`

, `or`

and `not`

So, for instance, `x < 3 and y > 6`

is a condition.

## The `if`

statement¶

So, now we know about conditions. One very important thing we can do with them
is to use them in an `if`

statement. This is pretty straightforward:

```
if x < 3:
print("x is less than 3. I'm setting it to 3.")
x = 3
```

Often, you want to do one thing if a condition is True and another thing if the
condition is False. To do this, use the magic word `else`

:

```
if x < 3:
print("x is less than 3.")
else:
print("x is not less than 3.")
```

And, less often, you want to test a whole bunch of conditions and do something
according to the first one that comes out True. For this, you need the strange
word `elif`

, which is short for `else if`

:

```
if x < 3:
print("x is less than 3")
elif x < 4:
print("x is not less than 3, but it's less than 4.")
else:
print("x isn't even less than 4.")
```