is, is not

Overview

'is' checks whether two variables point to the exact same object in memory (same id()), unlike == which checks value equality. Mastering this distinction prevents bugs and demonstrates Python internals knowledge in interviews.

Syntax

python

# 'is' — identity check (same object)
x = [1, 2, 3]
y = x
z = [1, 2, 3]

x is y    # True  (same object)
x is z    # False (equal value, different objects)
x == z    # True  (value equality)

# Correct use: compare to None
result = None
result is None     # True (correct idiom)
result == None     # works but not idiomatic

Common Pitfalls

Never use 'is' to compare integers, strings, or other value types — CPython caches small integers (-5 to 256) and interned strings, giving misleading True results.
PEP 8 says always use 'is' and 'is not' when comparing to None, True, or False.
Interview tip: id(obj) returns the object's memory address — two objects with the same id() are the same object.

Real-World Example

Safe None checking in a data processing pipeline

example

python

def process(value):
    if value is None:
        return "missing"
    if value is not None and value != "":
        return f"Valid: {value}"
    return "empty string"

print(process(None))   # missing
print(process(""))     # empty string
print(process("hello")) # Valid: hello

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