In Python, the is not None syntax is used to check if a variable or expression is not equal to None.
The None keyword in Python represents the absence of a value or a null value.
The is not operator returns True when the left-hand operator is not equal to the right-hand operator:
x = None
if x is not None:
print("x is not None")
else:
print("x is None") # ✅
In the above example, the variable x is assigned the value of None. The if statement checks if x is not equal to None by using the is not operator.
In this case, the code will print x is None because x is assigned the value of None.
You can also use the equality comparison operator != to replace the is not operator as follows:
x = None
if x != None:
print("x is not None")
else:
print("x is None") # ✅
Since None is a singleton object, it is possible to use the is not operator to check if a variable is None instead of using the != operator.
The is or is not operator checks for object identity, which is faster than the == or != operator that checks for the equality of the object values.
What’s more, the == result is always True when is returns True (but not the opposite)
Let’s see an example that shows the differences:
x = [1, 2, 3]
y = [1, 2, 3]
print(x == y) # True
print(x is y) # False
When you check if the value of x and y is equal using the == operator, Python returns True because they have the same value of [1, 2, 3].
But when you check if the object identity of x is the same as y, Python returns False because they are different objects, stored at different memory points.
You can use the id() function to check for the identification number that points to the object in memory as follows:
x = [1, 2, 3]
y = [1, 2, 3]
print(id(x)) # 4341386624
print(id(y)) # 4342199296
The ID numbers may be different when you run the code above, but the x and y numbers will never be the same.
On the other hand, the ID numbers for the None object is always the same:
x = None
y = None
print(id(x)) # 4375967240
print(id(y)) # 4375967240
It’s recommended to use the is not operator instead of != operator when checking for None.
Next, you can also use the is not None operator in a function that returns a value only if the input is not None:
def add_numbers(x, y):
if x is not None and y is not None:
return x + y
else:
return None
result = add_numbers(2, 3)
print(result) # 5
result = add_numbers(None, 3)
print(result) # None
In this example, the function add_numbers takes two arguments and returns the sum of the two numbers if both arguments are not None. Otherwise, it returns None.
Finally, it’s also possible to use the None keyword in the condition of a ternary operator as follows:
x = None
result = "x is not None" if x is not None else "x is None"
print(result) # x is None
Conclusion
To conclude, the is not None syntax is a way to check if a variable or expression is not equal to None in Python.
Combined with the if statement, this logic can be useful in situations where you have a function that requires a certain variable to be not None.
It can also be used when you want to return a value only if the input is not None. It’s recommended to use the is operator instead of the == operator when checking for None.