How to compare values to None in Python

Compare values to None in Python #

Use the is operator to compare to None in Python, e.g. if my_var is None:. The is operator returns True if the two values point to the same object (it checks for identity), whereas the double equals == operator checks if the two values are equal.

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my_var = None

if my_var is None:
    # 👇️ this runs
    print('variable stores None')
else:
    print('variable does NOT store None')

You should use the is operator when you need to check for an object's identity.

The pep 8 style guide mentions that comparison to singletons like None should always be done with is or is not, and never the equality operators.

When using is, we check if the values are the same object (same location in memory), not if they are equal.

Here is an example that better illustrates checking for identity (is) vs checking for equality (==).

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my_first_list = ['a', 'b', 'c']

my_second_list = my_first_list # 👈️ same list as above

print(my_first_list is my_second_list)  # 👉️ True

print(my_first_list == my_second_list)  # 👉️ True

We declared 2 variables that store the same list.

We set the second variable to the first, so both variables point to the same list object in memory.

Now, let's create a shallow copy of the list and assign it to the second variable.

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my_first_list = ['a', 'b', 'c']

my_second_list = my_first_list.copy() # 👈️ copy created

print(my_first_list is my_second_list)  # 👉️ False

print(my_first_list == my_second_list)  # 👉️ True

When we use the double equals operator, Python calls the __eq__() method on the object.

That is x==y calls x.__eq__(y). In theory this method could be implemented in an unpredictable way, so comparing to None with the is operator is more direct.

You can use the id() function to get the identity of an object.

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my_first_list = ['a', 'b', 'c']
print(id(my_first_list))  # 👉️ 139944523741504

my_second_list = my_first_list.copy()
print(id(my_second_list))  # 👉️ 139944522293184

print(id(my_first_list) == id(my_second_list))  # 👉️ False

The function returns an integer, which is guaranteed to be unique and constant for the object's lifetime.

The id() function returns the address of the object in memory in CPython.

If the two variables refer to the same object, the id() function will produce the same result.

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my_first_list = ['a', 'b', 'c']
print(id(my_first_list))  # 👉️ 140311440685376

my_second_list = my_first_list
print(id(my_second_list))  # 👉️ 140311440685376

print(id(my_first_list) == id(my_second_list))  # 👉️ True

Passing a None value to the id() function is always going to return the same result because there is only one instance of None in a Python program.

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print(id(None))  # 👉️ 9817984

print(id(None))  # 👉️ 9817984