Python Lambda Expressions

Lambda. A method is data. It can be stored as a variable, just like a string or number.

Python provides a clear syntax to create these methods.

A keyword. With "lambda," we use higher-order procedures. A lambda contains no statements. It returns a single expression. This limitation leads to simpler logic.

In this program, two lambdas are created. The first is called "square" and it receives one argument "n" and returns "n * n". The second is called "cube".

Square: This method receives one value. It returns the value multiplied by itself.

Cube: This method too receives one value. It returns the value cubed (with an exponent of 3).

Syntax: Lambda expression syntax uses the lambda keyword. The arguments are specified after.

Based on:

Python 3

Python program that uses lambda argument

def apply(f, n):
    print(f(n))

# Create two lambdas.
square = lambda n: n * n
cube = lambda n: n * n * n

# Pass lambdas to apply method.
apply(square, 4)
apply(cube, 3)

Output

16
27

Lambda usage. Here we define a lambda that receives no arguments. It simply returns an expression. Here it returns the sum of the numbers 1, 2 and 3 (which is 6).

Variable: We can assign a variable to a lambda expression and then invoke the lambda with parentheses. We call it like any other function.

Here: To get the value 6 in this program, we invoke x. We then print the value it returns (stored in y).

So: Lambdas can be passed to other methods. They can be stored in variables. And those variables can be called like methods.

Python program that uses lambda, no arguments

# Assign variable to lambda expression.
x = lambda: sum(range(1, 4))

# Invoke lambda expression.
y = x()
print(y)

Output

6

None. Many statements, like print(), return None. This is a valid return value for a lambda. We can specify a lambda with side effects, and a None return value.

NonePrint

Python program that uses None, lambda

# This lambda has a side effect.
# ... Print returns None.
p = lambda x: print(x)

p("Hello")
p("World")

Output

Hello
World

Nested. A lambda can call another. This can simplify complex computations—we assign names to parts of a computation. The order the lambdas appear in the file does not matter.

With nested lambdas: Recursion can occur. This may result in a RuntimeError: maximum recursion depth exceeded error.

Python program that uses lambda in lambda

add_two = lambda n: n + 2
multiply_add_two = lambda n: add_two(n * 2) # Call lambda in lambda.

print(multiply_add_two(3))
print(multiply_add_two(5))

Output

8
12

Limitations. The lambda syntax in Python has serious limitations. But these limitations make sense. You cannot have multiple statements in a lambda expression.

Tip: If you want many statements, please instead use a def. For complex or involved methods, def is a better option.

Def

Concepts. A lambda is an expression of behavior. It is a small function meant to do a well-defined task. We have no need to write an entire program in lambda syntax.

And: Lambda supplements, but does not replace, the def method syntax. It should be used when it improves clarity.

Greek letter: The word "lambda" is simply the name of a letter in the Greek alphabet. It is similar to the letter "L."

Performance research. In some languages lambdas cause performance loss. In the Python documentation I found that lambdas should not cause this problem. They behave just like def methods.

The expression lambda yields a function object. The unnamed object behaves like a function object defined with [def].

Expressions: python.org

Performance. Here we test lambda performance. The square() method above is rewritten in the def method syntax. We then benchmark the methods against each other.

First: The program tests the performance of the def method call. We call it ten million times.

Second: The program times the lambda expression method call. We also call it ten million times.

Note: The results show that methods written with the def keyword and with the lambda keyword are close in performance.

Python program that times lambda expressions

import time

# Method.
def square1(n):
    return n ** 2

# Lambda method.
square2 = lambda n: n ** 2

print(time.time())

# Use def method.
i = 0
while i < 10000000:
    square1(1)
    i += 1

print(time.time())

# Use lambda method.
i = 0
while i < 10000000:
    square2(1)
    i += 1

print(time.time())

Output

1346613154.399
1346613158.919 (Def =    4.52 s)
1346613163.397 (Lambda = 4.48 s)

Macros. In older languages like C a macro is an easy way to combine code statements. It reduces repetitive typing. A lambda can be used in this way.

Here: We use a lambda X to combine two string method calls. This is like a macro for those invocations. Typing is reduced.

Python program that uses lambda as macro

line1 = "A cat, a dog  "
line2 = "  a bird, a mountain"

# Use X as an alias for two methods.
x = lambda s: s.strip().upper()

# Call the lambda to shorten the program's source.
line1b = x(line1)
line2b = x(line2)

print(line1b)
print(line2b)

Output

A CAT, A DOG
A BIRD, A MOUNTAIN

A summary. Function objects provide many possibilities. We specify these objects with the lambda syntax form. And when we pass them to other functions, we develop higher-order procedures.

In functional programming, we construct functions that act on other functions. We emphasize less the order of statements and loops. We focus on the input, and output, of functions.

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