Python Use lambda to create a function for simple logic

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Many functions or classes require a callback as a parameter. For example, map requires a callback as the first parameter. Creating a function might be too much in this case if the logic is too simple. Lambda can be used for this purpose to pass a callback with simple logic without a function name.

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The syntax is simple.

lambda arguments: "logic here"

It’s not possible to write multiple lines.

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Use lambda without argument

If it’s not necessary to use any arguments, omit them and write the logic only at the right side.

fn = lambda: print("Hello World!")
# Hello World!

Note that it’s not a good design to assign a lambda function to a variable. Pylint shows a warning on the line.

Lambda expression assigned to a variable. Define a function using the "def" keyword instead.pylint(unnecessary-lambda-assignment)

Lambda should be used where a function requires a callback.

Lambda with multiple arguments

If multiple arguments are needed, add the parameters to the left side.

fn2 = lambda x, y, z: print(f"(x, y, z): ({x}, {y}, {z})")
fn2(11, 22, 33)
# (x, y, z): (11, 22, 33)

Lambda with named parameter and keyword parameter

Named parameters and keyword parameters are also possible to use.

fn3 = lambda x, *args, y, **kwargs: print(
    f"x: {x}, args: {args}, y: {y}, kwargs: {kwargs}"
fn3(11, 12, 13, 14, y=55, example_arg1=1, example_arg2=22)
# x: 11, args: (12, 13, 14), y: 55, kwargs: {'example_arg1': 1, 'example_arg2': 22}

Lambda with ternary operator

The logic needs to be written in one line but the ternary operator can be used.

fn4 = lambda found: print("found") if found else print("not found")
fn4(True)  # found
fn4(False) # not found

Lambda with Multiple lines

I mentioned that the logic needs to be written in a single line. That’s true. However, we could have a trick here. If it is a list, multiple things can be done in the lambda function.

fn5 = lambda x: [res := x + x, res := res + res + x, print(res)]
fn5(1)  # 5

It does 3 things. If I write it in 3 lines, it looks like this.

res = x + x
res = res + res + x

I don’t recommend this way because it looks ugly. It’s not readable. If multiple lines are needed, create a named function instead.

Can original value be updated in a lambda function?

What if we pass an object as a parameter? Is it updated in the function? It’s not.

fn6 = lambda data: [data := [3, 4], data := [data[0] + data[1]], print(data)]
data = [1, 2]
# [7]
# [1, 2]

The original data variable keeps the same values.

It’s not possible to assign a value to the origin parameter.

# Parsing failed: 'cannot assign to subscript here. Maybe you meant '==' instead of '='? (<unknown>, line 36)'pylint(syntax-error)
fn7 = lambda data: [data[0] = 1, print(data)]

If it is a function, the original variable could be updated. Using lambda for simple logic is better to avoid this mistake.

def update_list_data(data):
    data[0] = 11

data = [1, 2]
# [11, 2]

When is lambda actually used?

We learned how to define lambda but when can we actually use it? We can use it if a function requires a callback.

data_list = [1, 2, 3, 4, 5]
result = map(lambda x: x * x, data_list)
# [1, 4, 9, 16, 25]

data_list2 = [("AAA", 35), ("BBB", 12), ("CCC", 92), ("DDD", 22)]
data_list2.sort(key=lambda data: data[1])
# [('BBB', 12), ('DDD', 22), ('AAA', 35), ('CCC', 92)]

Lambda is a good choice if the logic is clear and simple enough.


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