Python provides two different operators for comparison:
==(Equality Operator) → Checks if two values are equal.is(Identity Operator) → Checks if two variables refer to the same object in memory.
Misusing is and == can lead to logical errors that may be difficult to detect. Let’s explore this issue step by step.
1. Understanding == vs is
Using == (Equality Operator)
- Compares values of two objects.
- Works correctly for numbers, strings, lists, tuples, etc.
Example:
a = [1, 2, 3]
b = [1, 2, 3]
print(a == b) # True (Values are equal)
print(a is b) # False (Different memory locations)
== is correct here because we only care about the content of the lists.
Using is (Identity Operator)
- Checks whether two variables point to the same object in memory.
- Should be used when checking identity, not equality.
- Works correctly for singletons like
None,True,False.
Example:
a = None
b = None
print(a is b) # True (Both refer to the same `None` object)
Use is for None checks instead of ==:
if x is None: # Correct
print("x is None")
Avoid == with None:
if x == None: # Not recommended
print("x is None")
2. Common Mistakes When Using == Instead of is
Mistake 1: Using is for Value Comparison
Using is instead of == can cause incorrect results, especially with strings and numbers.
a = "hello"
b = "hello"
print(a == b) # True (Values are equal)
print(a is b) # True (Might be True due to interning, but not guaranteed)
Fix: Use == for comparing string values.
Mistake 2: Using is to Compare Integers
Python interns small integers (usually between -5 and 256), meaning they might have the same memory address.
x = 100
y = 100
print(x == y) # True (Same values)
print(x is y) # True (Might be True due to interning)
However, for large numbers:
a = 500
b = 500
print(a == b) # True (Same value)
print(a is b) # False (Different memory locations)
Fix: Always use == for numerical comparisons.
Mistake 3: Using == Instead of is for None
x = None
if x == None: # Not recommended
print("x is None")
Fix: Use is for None checks.
if x is None: # Recommended
print("x is None")
3. Correct Use Cases for is
Use is for Singleton Objects (None, True, False)
if value is None:
print("No value assigned")
Use is to Compare Object Identity
a = [1, 2, 3]
b = a
print(a is b) # True (Same object in memory)
✅ Use == for Content Comparison
pythonCopyEditlist1 = [1, 2, 3]
list2 = [1, 2, 3]
print(list1 == list2) # True (Same content)
print(list1 is list2) # False (Different objects in memory)
4. Summary: When to Use == and is
| Scenario | Use == (Value Comparison) | Use is (Identity Check) |
|---|---|---|
| Comparing numbers/strings | Yes | No |
| Comparing lists/dictionaries | Yes | No |
Checking None | No | Yes |
| Checking if two variables refer to the same object | No | Yes |
Comparing Boolean values (True, False) | No | Yes |