Overuse of lambda expressions in Python

What are lambda expressions?
A lambda expression is a special syntax to create functions without names. These functions are called lambda functions. These lambda functions can have any number of arguments but only one expression along with an implicit return statement. Lambda expressions return function objects. For Example consider the lambda expression:

lambda (arguments) : (expression)

This lambda expression defines an unnamed function, which accepts two arguments and returns the sum of the two arguments. But how do we call an unnamed function? The above defined unnamed lambda function can be called as:

(lambda x, y: x + y)(1, 2)

Code 1:

filter_none

edit
close

play_arrow

link
brightness_4
code

# Python program showing a use
# lambda function
  
# performing a addition of three number
x1 = (lambda x, y, z: (x + y) * z)(1, 2, 3)
print(x1)
  
# function using a lambda fumction      
x2 = (lambda x, y, z: (x + y) if (z == 0) else (x * y))(1, 2, 3)
print(x2)       

chevron_right


Output:

9
2

Though it is not encouraged, the function object returned by a lambda expression can be assigned to a variable. See the example below in which a variable sum is assigned a function object returned by a lambda expression.

filter_none

edit
close

play_arrow

link
brightness_4
code

# Python program showing
# variable is storing lambda
# expression
  
# assingned to a variable
sum = lambda x, y: x + y
print(type(sum))
  
x1 = sum(4, 7)
print(x1)

chevron_right


Output:

11

 
Common uses of lambda expressions :

  • Since lambda functions are anonymous and do not require a name to be assigned, they are usually used to call functions(or classes) which require a function object as an argument. Defining separate functions for such function arguments is of no use because, the function definition is usually short and they are required only once or twice in the code. For example, the key argument of the inbuilt function, sorted().
    filter_none

    edit
    close

    play_arrow

    link
    brightness_4
    code

    # Python program showing
    # using of normal function
    def Key(x):
        return x%2
    nums = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
    sort = sorted(nums, key = Key)
    print(sort)

    chevron_right

    
    

    Output:

    [0, 2, 4, 6, 8, 1, 3, 5, 7, 9]
    
    filter_none

    edit
    close

    play_arrow

    link
    brightness_4
    code

    # Python program showing use
    # of lambda function
      
    nums = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
    sort_lambda = sorted(nums, key = lambda x: x%2)
    print(sort_lambda)

    chevron_right

    
    

    Output:

    [0, 2, 4, 6, 8, 1, 3, 5, 7, 9]
    
  • Lambda functions are inline functions and hence they are used whenever there is a need of repetitive function calls to reduce execution time. Some of the examples of such scenarios are the functions: map(), filter() and sorted(). For example,
    filter_none

    edit
    close

    play_arrow

    link
    brightness_4
    code

    # Python program showing a use
    # of lambda function
      
    nums = [1, 2, 3, 4, 5, 6, 7, 8, 9, 0]
      
    # using map() function
    squares = map(lambda x: x * x, nums)
    print(list(squares))
      
    # using filter() function
    evens = filter(lambda x: True if (x % 2 == 0)
                                  else False, nums)
    print(list(evens))

    chevron_right

    
    

 
Pros and Cons of lambda functions :
Pros of lambda functions:

  • Being anonymous, lambda functions can be easily passed without being assigned to a variable.
  • Lambda functions are inline functions and thus execute comparatively faster.
  • Many times lambda functions make code much more readable by avoiding the logical jumps caused by function calls. For example, read the following blocks of code.
    filter_none

    edit
    close

    play_arrow

    link
    brightness_4
    code

    # Python program performing
    # operation using def()
    def fun(x, y, z):
        return x*y+z
    a = 1
    b = 2
    c = 3
      
    # logical jump
    d = fun(a, b, c)
    print(d)     

    chevron_right

    
    

    Output:

    5
    
    filter_none

    edit
    close

    play_arrow

    link
    brightness_4
    code

    # Python program performing
    # operation using lambda
      
    d = (lambda x, y, z: x*y+z)(1, 2, 3)
    print(d)

    chevron_right

    
    

    Output:

    5
    

Cons on lambda functions:

  • Lambda functions can have only one expression.
  • Lambda functions cannot have a docstring.
  • Many times lambda functions make code difficult to read. For example, see the blocks of code given below.
    filter_none

    edit
    close

    play_arrow

    link
    brightness_4
    code

    def func(x):
        if x == 1:
            return "one"
        elif x == 2:
            return "two"
        elif x == 3:
            return "three"
        else:
            return "ten"
    num = func(3)
    print(num)

    chevron_right

    
    

    Output:

    three
    
    filter_none

    edit
    close

    play_arrow

    link
    brightness_4
    code

    # Python program showing use 
    # of lambda function
    num = (lambda x: "one" if x == 1 else( "two" if x == 2 
                           else ("three" if x == 3 else "ten")))(3)
    print(num)

    chevron_right

    
    

    Output:

    three
    

 
Misuse of Lambda expressions :

  • Assignment of lambda expressions : The official python style guide PEP8, strongy discourages the assignment of lambda expressions as shown in the example below.
    func = lambda x, y, z: x*y + z
    

    Instead, it is recommended to write a one-liner function as,

    def func(x, y, z): return x*y + z 
    

    While the second method encourages the fact that lambda functions are anonymous, it is also useful for tracebacks during debugging. Run the code below to see how def makes tracebacks much useful.

    filter_none

    edit
    close

    play_arrow

    link
    brightness_4
    code

    func = lambda x, y, z: x * y + z
    print(func)
    def func(x, y, z): return x * y + z
    print(func)

    chevron_right

    
    

  • Wrapping lambda expressions around functions : Many times, lambda expressions are needlessly wrapped around functions as shown below.
    nums = [-2, -1, 0, 1, 2]
    sort = sorted(nums, key=lambda x: abs(x))
    

    While the above syntax is absolutely correct, programmers must understand that all functions in python can be passed as function objects. Hence the same code can(and should) be written as,

    sort = sorted(nums, key=abs)
    
  • Passing functions unnecessarily : Many times, programmers pass functions which perform only a single operation. See the following code.
    nums = [1, 2, 3, 4, 5]
    summation = reduce(lambda x, y: x + y, nums)
    

    The lambda function passed above performs only a single operation, adding the two arguments. The same result can be obtained by the using the built-in function sum, as shown below.

    nums = [1, 2, 3, 4, 5]
    summation = sum(nums)
    

    Programmers should avoid using lambda expressions for common operations, because it is highly probable to have a built-in function providing the same results.

 
Overuse of lambda expressions :

  • Using lambda for non-trivial functions :Sometimes, simple functions can be non-trivial. See the code below.
    filter_none

    edit
    close

    play_arrow

    link
    brightness_4
    code

    details = [{'p':100, 'r':0.01, 'n':2, 't':4},
               {'p':150, 'r':0.04, 'n':1, 't':5},
               {'p':120, 'r':0.05, 'n':5, 't':2}] 
    sorted_details = sorted(details, 
                            key=lambda x: x['p']*((1 + x['r']/
                                    x['n'])**(x['n']*x['t'])))
    print(sorted_details)

    chevron_right

    
    

    Output:

    [{‘n’: 2, ‘r’: 0.01, ‘t’: 4, ‘p’: 100}, {‘n’: 5, ‘r’: 0.05, ‘t’: 2, ‘p’: 120}, {‘n’: 1, ‘r’: 0.04, ‘t’: 5, ‘p’: 150}]

    Here, we are sorting the dictionaries on the basis of the compound interest. Now, see the code written below, which uses def.

    filter_none

    edit
    close

    play_arrow

    link
    brightness_4
    code

    details = [{'p':100, 'r':0.01, 'n':2, 't':4},
               {'p':150, 'r':0.04, 'n':1, 't':5},
               {'p':120, 'r':0.05, 'n':5, 't':2}] 
    def CI(det):
        '''sort key: coumpound interest, P(1 + r/n)^(nt)'''
        return det['p']*((1 + det['r']/det['n'])**(det['n']*det['t']))
    sorted_details = sorted(details, key=CI)
    print(sorted_details)

    chevron_right

    
    

    Output:

    [{‘n’: 2, ‘r’: 0.01, ‘t’: 4, ‘p’: 100}, {‘n’: 5, ‘r’: 0.05, ‘t’: 2, ‘p’: 120}, {‘n’: 1, ‘r’: 0.04, ‘t’: 5, ‘p’: 150}]

    Though both codes do the same thing, the second one which uses def is much more readable. The expression written here under the lambda might be simple, but it has a meaning(formula for compound interest). Hence, the expression is non-trivial and deserves a name. Using lambda expressions for non-trivial functions reduces the readability of the code.

  • Using lambdas when multiple lines would help :If using a multiple line function makes the code more readable, using lambda expressions to reduce some lines of code is not worth it. For example, see the code below.

    people = [(‘sam’, ‘M’, 18), (‘susan’, ‘F’, 22), (‘joy’, ‘M’, 21), (‘lucy’, ‘F’, 12)]
    sorted_people = sorted(people, key=lambda x: x[1])

    Also see the following code which uses def.

    def Key(person):
        name, sex, age = person
        return sex
    sorted_people = sorted(people, key=Key)
    

    See how tuple unpacking in the second block of code makes it much more readable and logical. Readibility of the code should the be utmost priority of a programmer, working in a collaborative environment.

  • Using lambda expressions for map and filter : Lambdas are very commonly used with map() and filter() as shown.
    filter_none

    edit
    close

    play_arrow

    link
    brightness_4
    code

    nums = [0, 1, 2, 3, 4, 5]
    mapped = map(lambda x: x * x, nums)
    filtered = filter(lambda x: x % 2, nums)
    print(list(mapped))
    print(list(filtered))

    chevron_right

    
    

    Following is another block of code which uses generator expressions to achieve similar results.

    filter_none

    edit
    close

    play_arrow

    link
    brightness_4
    code

    nums = [0, 1, 2, 3, 4, 5]
    mapped = (x * x for x in nums)
    filtered = (x for x in nums if x % 2 == 1)
    print(list(mapped))
    print(list(filtered))

    chevron_right

    
    

    Unlike map() and filter(), generator expressions are general purpose features of python language. Thus generators enhance the readability of the code. While, map() and filter() require prior knowledge of these functions.

  • Use of higher order functions : The functions which accept other function objects as arguments are called higher order functions (i.e. map() and filter()), which are common in functional programming. As stated above, lambda expressions are commonly used as the function arguments of higher order functions. Compare the two code blocks shown below.
    Using high order function reduce()

    nums = [1, 2, 3, 4, 5]
    product = reduce(lambda x, y: x*y, nums, 1)
    

    Without using high order function

    nums = [1, 2, 3, 4, 5]
    def multiply(nums):
        prod = 1
        for number in nums:
            prod *= number
        return prod
    product = multiply(nums)
    

    While the first block uses fewer lines of code and is not that difficult to understand, programmers with no background of functional programming will find the second block of code much readable. Unless practicing proper functional programming paradigm, passing one function to another function is not appreciated, as it can reduce readability.



  • My Personal Notes arrow_drop_up

    Python enthusisast

    If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

    Please Improve this article if you find anything incorrect by clicking on the "Improve Article" button below.




    Article Tags :

    2


    Please write to us at contribute@geeksforgeeks.org to report any issue with the above content.