Check if a variable is not None in Python

Check if a variable is not None in Python #

Use the is not operator to check if a variable is not None in Python, e.g. if my_var is not None:. The is not operator returns True if the values on the left-hand and right-hand sides don't point to the same object (same location in memory).

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my_var = None

# ✅ check if variable is NOT none
if my_var is not None:
    print('variable does NOT store None')

# ✅ check if variable is None
if my_var is None:
    print('variable stores None')

You should use the is not operator when you need to check if a variable doesn't store a None value.

When we use is or is not, we check for the 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.

Here is an example that better illustrates checking for identity (is and is not) vs checking for equality (== and !=).

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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 not equals != operator, Python calls the __ne__() method on the object.

That is x!=y calls x.__ne__(y). In theory this method could be implemented in an unpredictable way, so checking for None with the is and is not operators 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 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

You might also see examples online that check for truthyness and falsyness.

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my_var = None

# 👇️ checks if variable stores a falsy value
if not my_var:
    # 👇️ this runs
    print('variable is falsy')

# 👇️ checks if variable stores a truthy value
if my_var:
    print('variable is truthy')

All values that are not truthy are considered falsy. The falsy values in Python are:

• constants defined to be falsy: None and False.
• 0 (zero) of any numeric type
• empty sequences and collections: "" (empty string), () (empty tuple), [] (empty list), {} (empty dictionary), set() (empty set), range(0) (empty range).

If you check if a variable is falsy, you are checking if the variable is any of the aforementioned falsy values (not just None).