Python lambda

The lambda keyword in Python is used to create small, anonymous functions. These functions are defined without a name and consist of a single expression. lambda functions are often used as a concise way to write short, throwaway functions.

Example

square = lambda x: x ** 2

print(square(4))

Output:

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Syntax

lambda arguments: expression

• arguments: A comma-separated list of arguments that the function takes.
• expression: A single expression whose result is returned.

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Why Use lambda?

1. Conciseness: Write short functions in a single line of code.
2. Throwaway Functions: Ideal for functions that are used once or for a short duration.
3. Used in Higher-Order Functions: Commonly used with functions like map(), filter(), and sorted().

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Common Examples

1. Basic Lambda Function

add = lambda x, y: x + y

print(add(3, 5))

Output:

8

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2. Using Lambda with map()

numbers = [1, 2, 3, 4]

squares = map(lambda x: x ** 2, numbers)

print(list(squares))

Output:

[1, 4, 9, 16]

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3. Using Lambda with filter()

numbers = [10, 15, 20, 25, 30]

divisible_by_5 = filter(lambda x: x % 5 == 0, numbers)

print(list(divisible_by_5))

Output:

[10, 15, 20, 25, 30]

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4. Using Lambda with sorted()

points = [(1, 2), (3, 1), (5, 0)]

sorted_points = sorted(points, key=lambda x: x[1])

print(sorted_points)

Output:

[(5, 0), (3, 1), (1, 2)]

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5. Inline Lambda in Function Calls

print((lambda x, y: x * y)(4, 5))

Output:

20

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Key Notes

1. Anonymous Functions:
   • lambda functions are unnamed and often used for short, one-off tasks.
   • They are not a replacement for def but a tool for specific use cases.
2. Single Expression Only:
   • A lambda function can only contain a single expression, unlike regular functions defined with def.
3. Readable Use:
   • Overusing lambda can make code harder to read. Use them judiciously and prefer def for complex logic.

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When to Use lambda?

1. Short-Lived Functions:
   • For single-use, small functions, like sorting keys or applying transformations in a list.
2. Higher-Order Functions:
   • When using map(), filter(), or reduce() to apply a specific transformation or condition.
3. Simpler Syntax:
   • To avoid defining full functions when only a single expression is required.