Lambda Functions

A Lambda Function is a small, anonymous function defined without a name. While standard functions are defined using the def keyword, lambdas use the lambda keyword.

In Python, lambdas are a “syntactic sugar” for creating simple, one-off functions, primarily used as arguments for higher-order functions.

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1. Syntax & Restrictions

Syntax: lambda arguments: expression

The Rules of Lambda

1. Anonymous: They don’t have a name unless you assign them to a variable.
2. Single Expression: A lambda can only contain one expression. It cannot contain statements (like if-else blocks, for loops, or return).
3. Implicit Return: The result of the expression is automatically returned.

lambda_vs_def.py

# Standard
def square(x):
    return x * x

# Lambda
sq = lambda x: x * x

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2. Functional Patterns: Map, Filter, and Sort

Lambdas shine when passed into functions that expect another function as an argument.

Case A: Custom Sorting

You can tell the .sort() method exactly how to compare items.

sorting.py

data = [("Alice", 88), ("Bob", 95), ("Charlie", 82)]

# Sort by the score (index 1 of the tuple)
data.sort(key=lambda item: item[1])
# Result: [('Charlie', 82), ('Alice', 88), ('Bob', 95)]

Case B: map() and filter()

• map(): Applies the lambda to every item in an iterable.
• filter(): Keeps only items where the lambda returns True.

functional.py

nums = [1, 2, 3, 4, 5]

evens = list(filter(lambda x: x % 2 == 0, nums))
squares = list(map(lambda x: x ** 2, nums))

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3. Under the Hood: Function Objects

Is a lambda different from a normal function? No.

When Python executes a lambda expression, it creates a standard Function Object in memory, exactly like def does. You can even inspect its properties:

f = lambda x: x + 1
print(type(f)) # <class 'function'>
print(f.__name__) # <lambda>

The only difference is that the __name__ attribute is set to <lambda> and the object doesn’t have a docstring or type annotations.

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4. The “Anti-Pattern” Warning

While lambdas are powerful, they are often overused by beginners.

When to avoid Lambdas

If you are assigning a lambda to a variable name, you should probably just use def.

Bad: add = lambda x, y: x + y Good: def add(x, y): return x + y

def provides better error messages (tracebacks), supports docstrings, and allows for type hints, making your code significantly more maintainable.

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5. Summary Table

Feature	def Function	lambda Function
Syntax	Multi-line.	Single-line expression.
Name	Explicitly named.	Anonymous.
Capability	Any statement/logic.	Expression only.
Return	Requires return.	Implicit return.
Docstrings	Supported.	Not supported.
Tracebacks	Shows function name.	Shows <lambda>.