Borislav Hadzhiev
Wed Jun 15 2022·3 min read
Photo by Katie McBroom
Use the is
operator to check if a variable is None in Python, e.g.
if my_var is None:
. The is
operator returns True
if the values on the
left-hand and right-hand sides point to the same object and should be used when
checking for singletons like None
.
my_var = None # ✅ Check if variable is None if my_var is None: # 👇️ this runs print('variable is None') # ✅ Check if variable is NOT None if my_var is not None: print('variable is not None')
if
statement checks if the variable stores a None
value, and the second - if the variable doesn't store a None
value.You should use the is
operator when you need to check if a variable stores a
None
value.
When we use is
, 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.
==
and not equals !=
) when you need to check if a value is equal to another value, e.g. 'hello' == 'hello'
.Here is an example that better illustrates checking for identity (is
) vs
checking for equality (==
).
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.
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 checking for None
with the is
operator is more
direct.
You can use the id() function to get the identity of an object.
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.
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.
print(id(None)) # 👉️ 9817984 print(id(None)) # 👉️ 9817984
You might also see examples online that check for truthyness.
my_var = None # 👇️ checks if variable stores a truthy value if my_var: print('variable is truthy') # 👇️ checks if variable stores a falsy value if not my_var: # 👇️ this runs print('variable is falsy')
However, this is very different than explicitly checking if a variable stores 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:
None
and False
.0
(zero) of any numeric type""
(empty string), ()
(empty tuple), []
(empty list), {}
(empty dictionary), set()
(empty set), range(0)
(empty
range).If you check if a variable is truthy, you are checking if the variable is not
any of the aforementioned falsy values, not just None
.