Python Comparisons


title: Python Comparisons

Python Docs – 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).

You can also give two condition and check for only one condition using or like x > y or x >= y will check for either of the two conditions. If one of the condition is true the program will proceed. This is the functionality of or.

This table summarizes the comparison operations:

OperationMeaning
<strictly less than
<=less than or equal to
>strictly greater than
>=greater than or equal to
==equal to
!=not equal to
isobject identity
is notnegated 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 identity
  • == is used to compare equality

Example

First, create a list in Python.

myListA = [1,2,3]

Next, create a copy of that list.

myListB = myListA

If we use the == operator or the is operator, both will result in a True output.

>>> 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?

myListC = [1,2,3]

Performing the == operator still shows that both lists are the same, in terms of content.

>>> 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 different.

>>> 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

However, interestingly, there are a few special cases in Python with is.

>>> a = 5 >>> b = 5 >>> a is b True >>> a = 1000 >>> b = 1000 >>> a is b False

This is due to how Python is implemented. In Python, small integers (from -5 up to 256, a fast test shows) are cached. You shouldn’t rely on this detail – larger integers do not evaluate to true when you use is. For more information on this topic, view this StackOverflow thread.

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