with & without lambda in python
#python#tutorial#functional
dillan teagle

dillan teagle @teaglebuilt

About: Principal Engineer at VertikalRMS

Location:
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Joined:
Aug 19, 2019

with & without lambda in python

Publish Date: Nov 29 '19
7 4

A quick understanding of using functional methods like map, filter, & sorted with and without lambda functions. A great example of how lambda functions work in python.

use the filter function to remove items from a list

without lambda 


def filterFunc(x):
    if x % 2 == 0:
        return False
    return True


nums = (1, 8, 4, 5, 12, 26, 381, 58, 47)
odds = list(filter(filterFunc, nums))
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[1, 5, 381, 47]

with lambda 

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

odds = list(filter((lambda x: x % 2 != 0), mylist))
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[1, 5, 381, 47]

remove uppercase from string

without lambda 

def filterFunc2(x):
    if x == x.upper():
        return False
    return True

chars  = "aBcDeFghoiJk"

lower = list(filter(filterFunc2, chars))
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['a', 'c', 'e', 'g', 'h', 'o', 'i', 'k']

with lambda 
lower = list(filter((lambda x: x == x.upper()), chars))
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['a', 'c', 'e', 'g', 'h', 'o', 'i', 'k']

return new list with index squared

without lambda 

def squareFunc(x):
    return x ** 2

ints = [1, 2, 3, 4, 5]
squared = list(map(squareFunc, ints))
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[1, 4, 9, 16, 25]

with lambda 

squared = list(map(lambda x: x ** 2, ints))
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[1, 4, 9, 16, 25]

use sorted and map to change numbers to grades 


def toGrade(x):
    if x  >= 90:
        return "A"
    elif x >= 80:
        return "B"
    elif x >= 70:
        return "C"
    elif x >= 60:
        return "D"
    else:
        return "F"

grades = (81, 89, 94, 78, 61, 99, 74, 90)

order = sorted(grades)
final = list(map(toGrade, order))
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['D', 'C', 'C', 'B', 'B', 'A', 'A', 'A']

Challenge:

Is using a lambda function a good choice here? If so, how will you do it?

Comments 4 total

  • 3cor
    3corNov 30, 2019

    Thank you for a great quick write up!

    I found a typo here.

    ...
    use the filter function to remove items from a list
    with lambda
    ...

    Should be...
    without lambda
    right?

  • David Nehme
    David NehmeDec 2, 2019

    Lambda expressions are great in spots. The most common use is when you need a short function to pass as an argument to another function. However, list comprehensions remove the need for a lambda in a call to map. In the first example

    odds = list(filter(filterFunc, nums))

    could be replaced with

    [num for num in nums if num % 2]

    You probably want to avoid lambda when the function is long (like in your toGrade example), when the function is likely to be used elsewhere in your code or if creating a named function helps with documenting your intentions. Using your first example, defining the function isEven, or isLower in the second would tilt the scales against using lambda.

    • dillan teagle
      dillan teagleDec 2, 2019

      good point, the odds example is definitely overkill when a list comprehension is a possible solution. I think this also answers, the question of using lambda's for multi conditional solutions.

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