85 lines
3.7 KiB
Markdown
85 lines
3.7 KiB
Markdown
---
|
|
title: Python Comparisons
|
|
---
|
|
[Python Docs - Comparisons](https://docs.python.org/3/library/stdtypes.html#comparisons)
|
|
|
|
There are eight comparison operations in Python. They all have the same priority (which is higher than that of the Boolean operations). Comparisons can be chained arbitrarily; for example, `x < y <= z` is equivalent to `x < y and y <= z`, except that `y` is evaluated only once (but in both cases `z` is not evaluated at all when `x < y` is found to be false).
|
|
|
|
This table summarizes the comparison operations:
|
|
|
|
Operation | Meaning
|
|
--------- | -----------------------
|
|
`<` | strictly less than
|
|
`<=` | less than or equal to
|
|
`>` | strictly greater than
|
|
`>=` | greater than or equal to
|
|
`==` | equal to
|
|
`!=` | not equal to
|
|
`is` | object identity
|
|
`is not` | negated object identity
|
|
|
|
Objects of different types, except different numeric types, never compare equal. Furthermore, some types (for example, function objects) support only a degenerate notion of comparison where any two objects of that type are unequal. The `<`, `<=`, `>` and `>=` operators will raise a `TypeError` exception when comparing a complex number with another built-in numeric type, when the objects are of different types that cannot be compared, or in other cases where there is no defined ordering.
|
|
|
|
Non-identical instances of a class normally compare as non-equal unless the class defines the `__eq__()` method.
|
|
|
|
Instances of a class cannot be ordered with respect to other instances of the same class, or other types of object, unless the class defines enough of the methods `__lt__()`, `__le__()`, `__gt__()`, and `__ge__()` (in general, `__lt__()` and `__eq__()` are sufficient, if you want the conventional meanings of the comparison operators).
|
|
|
|
The behavior of the `is` and `is not` operators cannot be customized; also they can be applied to any two objects and never raise an exception.
|
|
|
|
We can also chain `<` and `>` operators together. For instance, `3 < 4 < 5` will return `True`, but `3 < 4 > 5` will not. We can also chain the equality operator. For instance, `3 == 3 < 5` will return `True` but `3 == 5 < 5` will not.
|
|
|
|
### Equality Comparisons - "is" vs "=="
|
|
|
|
In Python, there are two comparison operators which allow us to check to see if two objects are equal. The `is` operator and the `==` operator. However, there is a key difference between them!
|
|
|
|
The key difference between 'is' and '==' can be summed up as:
|
|
* `is` is used to compare <b>identity</b>
|
|
* `==` is used to compare <b>equality</b>
|
|
|
|
## Example
|
|
First, create a list in Python.
|
|
|
|
```python
|
|
myListA = [1,2,3]
|
|
```
|
|
|
|
Next, create a copy of that list.
|
|
|
|
```python
|
|
myListB = myListA
|
|
```
|
|
|
|
If we use the '==' operator or the 'is' operator, both will result in a <b>True</b> output.
|
|
|
|
```python
|
|
>>> myListA == myListB # both lists contains similar elements
|
|
True
|
|
>>> myListB is myListA # myListB contains the same elements
|
|
True
|
|
```
|
|
This is because both myListA and myListB are pointing to the same list variable, which I defined at beginning of my Python program. Both lists are exactly the same, both in identity and in content.
|
|
|
|
However, what if I now create a new list?
|
|
|
|
```python
|
|
myListC = [1,2,3]
|
|
```
|
|
|
|
Performing the `==` operator still shows that both lists are the same, in terms of content.
|
|
|
|
```python
|
|
>>> myListA == myListC
|
|
True
|
|
```
|
|
|
|
However, performing the `is` operator will now produce a `False` output. This is because myListA and myListC are two different variables, despite containing the same data. Even though they look the same, they are <b>different</b>.
|
|
|
|
```python
|
|
>>> myListA is myListC
|
|
False # both lists have different reference
|
|
```
|
|
|
|
To sum up:
|
|
* An `is` expression outputs `True` if both variables are pointing to the same reference
|
|
* An `==` expression outputs `True` if both variables contain the same data
|