# # Comparisons

## # Chain Comparisons

You can compare multiple items with multiple comparison operators with chain comparison. For example

```
x > y > z
```

is just a short form of:

```
x > y and y > z
```

This will evaluate to `True`

only if both comparisons are `True`

.

The general form is

```
a OP b OP c OP d ...
```

Where `OP`

represents one of the multiple comparison operations you can use, and the letters represent arbitrary valid expressions.

Note that `0 != 1 != 0`

evaluates to `True`

, even though `0 != 0`

is `False`

. Unlike the common mathematical notation in which `x != y != z`

means that `x`

, `y`

and `z`

have different values. Chaining `==`

operations has the natural meaning in most cases, since equality is generally transitive.

### # Style

There is no theoretical limit on how many items and comparison operations you use as long you have proper syntax:

```
1 > -1 < 2 > 0.5 < 100 != 24
```

The above returns `True`

if each comparison returns `True`

. However, using convoluted chaining is not a good style. A good chaining will be "directional", not more complicated than

```
1 > x > -4 > y != 8
```

### # Side effects

As soon as one comparison returns `False`

, the expression evaluates immediately to `False`

, skipping all remaining comparisons.

Note that the expression `exp`

in `a > exp > b`

will be evaluated only once, whereas in the case of

```
a > exp and exp > b
```

`exp`

will be computed twice if `a > exp`

is true.

## # Comparison by `is`

vs `==`

A common pitfall is confusing the equality comparison operators `is`

and `==`

.

`a == b`

compares the value of `a`

and `b`

.

`a is b`

will compare the **identities** of `a`

and `b`

.

To illustrate:

```
a = 'Python is fun!'
b = 'Python is fun!'
a == b # returns True
a is b # returns False
a = [1, 2, 3, 4, 5]
b = a # b references a
a == b # True
a is b # True
b = a[:] # b now references a copy of a
a == b # True
a is b # False [!!]
```

Basically, `is`

can be thought of as shorthand for `id(a) == id(b)`

.

Beyond this, there are quirks of the run-time environment that further complicate things. Short strings and small integers will return `True`

when compared with `is`

, due to the Python machine attempting to use less memory for identical objects.

```
a = 'short'
b = 'short'
c = 5
d = 5
a is b # True
c is d # True
```

But longer strings and larger integers will be stored separately.

```
a = 'not so short'
b = 'not so short'
c = 1000
d = 1000
a is b # False
c is d # False
```

You should use `is`

to test for `None`

:

```
if myvar is not None:
# not None
pass
if myvar is None:
# None
pass
```

A use of `is`

is to test for a “sentinel” (i.e. a unique object).

```
sentinel = object()
def myfunc(var=sentinel):
if var is sentinel:
# value wasn’t provided
pass
else:
# value was provided
pass
```

## # Greater than or less than

```
x > y
x < y
```

These operators compare two types of values, they're the less than and greater than operators. For numbers this simply compares the numerical values to see which is larger:

```
12 > 4
# True
12 < 4
# False
1 < 4
# True
```

For strings they will compare lexicographically, which is similar to alphabetical order but not quite the same.

```
"alpha" < "beta"
# True
"gamma" > "beta"
# True
"gamma" < "OMEGA"
# False
```

In these comparisons, lowercase letters are considered 'greater than' uppercase, which is why `"gamma" < "OMEGA"`

is false. If they were all uppercase it would return the expected alphabetical ordering result:

```
"GAMMA" < "OMEGA"
# True
```

Each type defines it's calculation with the `<`

and `>`

operators differently, so you should investigate what the operators mean with a given type before using it.

## # Not equal to

```
x != y
```

This returns `True`

if `x`

and `y`

are not equal and otherwise returns `False`

.

```
12 != 1
# True
12 != '12'
# True
'12' != '12'
# False
```

## # Equal To

```
x == y
```

This expression evaluates if `x`

and `y`

are the same value and returns the result as a boolean value. Generally both type and value need to match, so the int `12`

is not the same as the string `'12'`

.

```
12 == 12
# True
12 == 1
# False
'12' == '12'
# True
'spam' == 'spam'
# True
'spam' == 'spam '
# False
'12' == 12
# False
```

Note that each type has to define a function that will be used to evaluate if two values are the same. For builtin types these functions behave as you'd expect, and just evaluate things based on being the same value. However custom types could define equality testing as whatever they'd like, including always returning `True`

or always returning `False`

.

## # Comparing Objects

In order to compare the equality of custom classes, you can override `==`

and `!=`

by defining `__eq__`

and `__ne__`

methods. You can also override `__lt__`

(`<`

), `__le__`

(`<=`

), `__gt__`

(`>`

), and `__ge__`

(`>`

). Note that you only need to override two comparison methods, and Python can handle the rest (`==`

is the same as `not <`

and `not >`

, etc.)

```
class Foo(object):
def __init__(self, item):
self.my_item = item
def __eq__(self, other):
return self.my_item == other.my_item
a = Foo(5)
b = Foo(5)
a == b # True
a != b # False
a is b # False
```

Note that this simple comparison assumes that `other`

(the object being compared to) is the same object type. Comparing to another type will throw an error:

```
class Bar(object):
def __init__(self, item):
self.other_item = item
def __eq__(self, other):
return self.other_item == other.other_item
def __ne__(self, other):
return self.other_item != other.other_item
c = Bar(5)
a == c # throws AttributeError: 'Foo' object has no attribute 'other_item'
```

Checking `isinstance()`

or similar will help prevent this (if desired).

## # Common Gotcha: Python does not enforce typing

In many other languages, if you run the following (Java example)

```
if("asgdsrf" == 0) {
//do stuff
}
```

... you'll get an error.
You can't just go comparing strings to integers like that. In Python, this is a perfectly legal statement - it'll just resolve to `False`

.

A common gotcha is the following

```
myVariable = "1"
if 1 == myVariable:
#do stuff
```

This comparison will evaluate to `False`

without an error, every time, potentially hiding a bug or breaking a conditional.

#### # Syntax

#### # Parameters

Parameter | Details |
---|---|

x | First item to be compared |

y | Second item to be compared |

← Conditionals Loops →