What is the difference between a list and a tuple?

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Both list and tuple are ordered sequences of heterogeneous Python objects, but they differ in mutability , syntax , performance , and idiomatic intent . Mutability — the primary difference: List is mutable — you can append, extend, insert, remove, sort, reverse, or reassign elements in place. Tuple is immutable — once created, its contents cannot be changed. (If a tuple contains a mutable object like a list, that inner object can still be mutated, but the tuple’s own references are fixed.) Syntax: lst = [1, 2, 3] # list literal: square brackets tup = (1, 2, 3) # tuple literal: parentheses (optional) one = (1,) # single-element tuple NEEDS the trailing comma also = 1, 2, 3 # comma, not parens, creates a tuple Performance: tuples are slightly smaller and faster to construct and iterate because the interpreter can allocate a fixed-size object and skip mutation bookkeeping. The difference is modest but matters in tight loops and large data structures. Hashability: a tuple of hashable items is itself hashable , so it can be a dict key or a set element; a list cannot. d = { (x, y): "point" for x in range(3) for y in range(3) } # OK d = { [1, 2]: "nope" } # TypeError: unhashable type: 'list' Interview-ready summary: lists are mutable, variable-length sequences used for collections you modify; tuples are immutable, fixed-shape, hashable sequences used for records, keys, and return values. Reach for a tuple when the shape is fixed and you want safety or dict-key behavior; reach for a list when you need to change the sequence over time. Key difference: mutability. Know that tuples are hashable (dict keys, set elements) and lists are not. The memory/performance advantage of tuples and the namedtuple pattern show deeper knowledge.