What is a lambda function in Python and when is it appropriate to use one?

A lambda is an anonymous, single-expression function defined inline. The syntax is lambda parameters: expression. It returns the value of the expression automatically — no return keyword needed. It can have any number of parameters, including defaults and *args.

double = lambda x: x * 2
print(double(5))  # 10

# Most common use: as a sort key
people = [('Alice', 30), ('Bob', 25), ('Carol', 35)]
people.sort(key=lambda p: p[1])  # sort by age
print(people)  # [('Bob', 25), ('Alice', 30), ('Carol', 35)]

# With filter and map
evens  = list(filter(lambda x: x % 2 == 0, range(10)))
doubled = list(map(lambda x: x * 2, [1, 2, 3]))

When is a lambda appropriate? Use it for short, throwaway callables passed to sorted(), filter(), map(), or event handlers — situations where naming the function would make the code wordier without adding clarity. The PEP 8 style guide explicitly discourages assigning a lambda to a variable (like double = lambda x: x*2) because a def statement is clearer and gives the function a proper name visible in tracebacks.

Lambdas are limited to a single expression — no statements, no multi-line logic, no assignments. If the logic is even slightly complex, write a named function with def.

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