Comparisons

Syntax

• != - Is not equal to

• == - Is equal to

• `>` - greater than

• `<` - less than

• `>=` - greater than or equal to

• `<=` - less than or equal to

• `is` - test if objects are the exact same object

• is not = test if objects are not the exact same object

Parameters

ParameterDetails
xFirst item to be compared
ySecond item to be compared

Chain Comparisons

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

is just a short form of:

This will evaluate to `True` only if both comparisons are `True`.

The general form is

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:

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

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

`exp` will be computed twice if `a > exp` is true.

Common Gotcha: Python does not enforce typing

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

... 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

This comparison will evaluate to `False` without an error, every time, potentially hiding a bug or breaking a conditional.

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.)

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:

Checking `isinstance()` or similar will help prevent this (if desired).

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:

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.

But longer strings and larger integers will be stored separately.

You should use `is` to test for `None`:

A use of `is` is to test for a “sentinel” (i.e. a unique object).

Equal To

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'`.

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`.

Greater than or less than

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:

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

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:

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

This returns `True` if `x` and `y` are not equal and otherwise returns `False`.