Membership and Identity Operators

Membership Operators — in, not in

Test whether a value exists within a sequence (string, list, tuple, dict, set).

fruits = ["apple", "banana", "mango"]
print("banana" in fruits)        # True
print("grape" not in fruits)     # True

text = "Python Programming"
print("Prog" in text)            # True
print("prog" in text)            # False (case sensitive)

marks = {"Amit": 85, "Riya": 92}
print("Amit" in marks)           # True -- checks keys, not values
print(85 in marks.values())      # True -- explicit values() check

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Identity Operators — is, is not

Test whether two variables refer to the same object in memory (not just equal values).

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

print(a == b)     # True  -- same contents
print(a is b)     # False -- different objects in memory
print(a is c)     # True  -- c points to the same object as a
print(a is not b) # True

== vs is — the key distinction

• == checks value equality (do they contain the same data?)
• is checks identity (are they literally the same object?)

x = None
if x is None:      # correct, idiomatic way to check for None
    print("x has no value")

Always use is / is not when comparing against None, never ==.