Is vs Equality

The main pitfall when using is vs == in Python is that they test for different things:

1. is checks for object identity (if two variables point to the exact same object in memory)

2. == checks for value equality (if two objects have the same value)

# Example 1: Integers
a = 1000
b = 1000
print(a == b)  # True
print(a is b)  # False

# Example 2: Small integers
x = 5
y = 5
print(x == y)  # True
print(x is y)  # True

# Example 3: Lists
list1 = [1, 2, 3]
list2 = [1, 2, 3]
print(list1 == list2)  # True
print(list1 is list2)  # False

# Example 4: Strings
str1 = "hello"
str2 = "hello"
print(str1 == str2)  # True
print(str1 is str2)  # True

# Example 5: None
print(None == None)  # True
print(None is None)  # True

True
False
True
True
True
False
True
True
True
True

Why so weird, let’s explain them.

1. For large integers, == returns True because the values are the same, but is returns False because Python creates separate objects for larger integers.

2. For small integers (-5 to 256), Python often reuses the same object for optimization, so both == and is return True.

3. For lists with the same content, == returns True because the values are equal, but is returns False because they are distinct objects in memory.

4. For strings, Python often interns (reuses) string literals, so both == and is might return True. However, this behavior isn’t guaranteed and can vary between Python implementations.

5. For None, both == and is always return True because None is a singleton object in Python.

The main pitfall occurs when programmers use is to compare values, expecting it to behave like ==. This can lead to unexpected results, especially with numbers, strings, or custom objects.

So the best practices are:

1. Use == for value comparisons.

2. Use is only for comparing to None or when you explicitly want to check for object identity.

None Is A Singleton Object

None is a singleton object in Python, meaning there is only one instance of it in memory. This is why None == None and None is None both evaluate to True.

p = None
q = None
print(p == q)  # True
print(p is q)  # True

True
True

References And Further Readings

• What is the difference between “is None” and “== None”