Check if a variable is not None in Python

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Borislav Hadzhiev

Wed Jun 15 20223 min read

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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).

main.py
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')
The first if statement checks if the variable doesn't store a None value, and the second - if the variable stores a None value.

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.

Use the equality operators (equals == and not equals != ) when you need to check if a value is equal or is not equal to another value, e.g. 'a' != 'b'.

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

main.py
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.

main.py
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
Notice that the identity check failed. Even though the two lists store the same values, in the same order, they point to different locations in memory (they are not the same object).

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.

main.py
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.

main.py
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.

main.py
print(id(None)) # 👉️ 9817984 print(id(None)) # 👉️ 9817984

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

main.py
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')
However, this is very different than explicitly checking if a variable doesn't store a None value because there are many other falsy values that are not None.

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).

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