Identity and Equality¶

Understanding the difference between is (identity) and == (equality) operators.

Identity (`is`)¶

The is operator checks if two names refer to the same object in memory:

a = [1, 2, 3]
b = a

print(a is b)  # True (same object)
print(id(a) == id(b))  # True (same id)

Different Objects¶

a = [1, 2, 3]
b = [1, 2, 3]

print(a is b)  # False (different objects)
print(id(a))   # 140234567890
print(id(b))   # 140234567999

Even though a and b have the same values, they are different objects in memory.

Equality (`==`)¶

The == operator checks if two objects have the same value:

a = [1, 2, 3]
b = [1, 2, 3]

print(a == b)  # True (same values)
print(a is b)  # False (different objects)

When to Use Each¶

Use `is` for Singletons¶

# For None
if x is None:
    print("x is None")

if x is not None:
    print("x has a value")

# For True/False (rare, but valid)
if flag is True:
    pass

if flag is False:
    pass

Use `==` for Values¶

# For comparing values
if x == 42:
    pass

if name == "Alice":
    pass

if items == [1, 2, 3]:
    pass

Common Pitfall: Integer Caching¶

Python caches small integers (-5 to 256), which can cause confusion:

# Small integers are cached
a = 256
b = 256
print(a is b)  # True (cached)

# Large integers are not
a = 257
b = 257
print(a is b)  # False (different objects)
print(a == b)  # True (same value)

Rule: Always use == for comparing values, even integers.

String Interning¶

Similar caching happens with some strings:

a = "hello"
b = "hello"
print(a is b)  # True (interned)

a = "hello world!"
b = "hello world!"
print(a is b)  # May be False
print(a == b)  # True (same value)

Special Cases¶

Augmented Assignment¶

Augmented assignment behaves differently based on mutability:

# Mutable: modifies in place
x = [1, 2]
y = x
x += [3, 4]
print(x is y)  # True (same object)

# Immutable: creates new object
x = 10
y = x
x += 5
print(x is y)  # False (different objects)

None Comparison¶

Always use is for None:

# Correct
if result is None:
    pass

# Incorrect (works but not recommended)
if result == None:
    pass

Boolean Context¶

For boolean checks, often no comparison needed:

# Instead of
if flag == True:
    pass

# Prefer
if flag:
    pass

# Instead of
if len(items) == 0:
    pass

# Prefer
if not items:
    pass

Summary Table¶

Operator	Checks	Use For	Example
`is`	Identity (same object)	`None`, singletons	`x is None`
`==`	Equality (same value)	Values, objects	`x == 42`
`is not`	Not same object	`None` check	`x is not None`
`!=`	Not same value	Value comparison	`x != 0`

Key Takeaways¶

is checks if two names point to the same object
== checks if two objects have the same value
Use is for None, True, False
Use == for all value comparisons
Don't rely on integer/string caching
Identity implies equality, but not vice versa