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Python Lambda ExpressionsUse lambda expression syntax. Pass a lambda expression to another method.
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Lambda. Python programs often use the lambda keyword. Methods like map() can receive a lambda to specify their behavior. In this way we can specify behavior as arguments.
Lambda limitations. We cannot have multiple statements in a lambda expression. If we require many statements in a complex method, consider a def.
Def
First example. Lambda expression syntax uses the lambda keyword. The arguments are specified after. A lambda is an object: we can pass it to a def that receives arguments.
Part 1 Here 2 lambdas are created. The first is called "square" and it receives one argument "n" and returns "n * n".
Part 2 We pass the lambdas as an argument to our apply() function. Our lambda expressions are objects.
Part 3 In apply, we call the lambda argument like a function. The logic in the lambda expressions is executed.
Python program that uses lambda argument
def apply(f, n): # Part 3: invoke the lambda. result = f(n) print("RESULT:", result) # Part 1: create 2 similar lambdas. square = lambda n: n * n cube = lambda n: n * n * n # Part 2: pass lambdas to apply method. apply(square, 4) apply(cube, 3)
RESULT: 16 RESULT: 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)
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.
None
Console, print
Python program that uses None, lambda
# This lambda has a side effect. # ... Print returns None. p = lambda x: print(x) p("Hello") p("World")
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))
8 12
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)
A CAT, A DOG A BIRD, A MOUNTAIN
Avoiding lambda. Sometimes developers use a lambda with one argument and one result, but the lambda is not needed. Here we can use a function name (like math.sqrt) instead of a lambda.
And When we avoid the lambda, the code becomes clearer and shorter. The lambda does not add much in this case.
Note Sometimes a program can benefit from lambda when the lambda increases the "symmetry" of the program—other places also use lambda.
Python program that avoids lambda when possible
import math values = [10, 20, 30] # Apply sqrt to all elements in the list with map. result1 = map(math.sqrt, values) # We can use a lambda, but it is not needed. result2 = map(lambda x: math.sqrt(x), values) print("values:", values) print("math.sqrt:", list(result1)) print("lambda:", list(result2))
values: [10, 20, 30] math.sqrt: [3.1622776601683795, 4.47213595499958, 5.477225575051661] lambda: [3.1622776601683795, 4.47213595499958, 5.477225575051661]
Lambda, benchmark. Here we test lambda performance. The square() method is rewritten in the def method syntax. We then benchmark the methods against each other.
Version 1 The program tests the performance of the def method call. We call it ten million times.
Version 2 The program times the lambda expression method call. We also call it ten million times.
Result We find 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()) # Version 1: use def method. i = 0 while i < 10000000: square1(1) i += 1 print(time.time()) # Version 2: use lambda method. i = 0 while i < 10000000: square2(1) i += 1 print(time.time())
1346613154.399 1346613158.919 (Def = 4.52 s) 1346613163.397 (Lambda = 4.48 s)
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."
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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