I don't understand lambdas functions in comprehension lists-CodePudding

I can't understand how lambdas in comprehension lists works.

Please help me undestand this mechanism, and sorry but I'm learning right now and the explications that I can find online are not really clear to me.

def times_two(a): return a * 2
def times_three(a): return a * 3

numbers = [1, 2, 3, 4]

both_functions = [times_two, times_three]

for number in numbers:
  print(list(map((lambda x: x(number)), both_functions)))

What does (lambda x: x(number)) do exacly, why is there a second x, what kind of parameter is?

Other example:

is_even_list = [lambda a=x: a * 10 for x in range(0, 6)]
for item in is_even_list:
    print(item())

Why here there is an a=x, usually lambdas doesn't have only one parameter?

The last one:

A = [1, 2, 3, 4]
B = [lambda: _ for _ in A]
C = [_() for _ in B]
print(A)
print(B)
print(C)

Here what does _() do?

CodePudding user response：

The lists you're iterating over contain functions. Inside the iteration, you call the function in the current iteration item.

lambda x: x(number)

means that it receives a function in x, and then calls that function with number as the argument.

for item in is_even_list:
    print(item())

loops over the functions in is_even_list, and calls each of those functions.

[_() for _ in B]

_ is just a variable name, although conventionally it's only used for iterations where you don't use the iteration item. B contains a list of functions, so this calls each of those functions and returns a list of the results.

CodePudding user response：

list(map((lambda x: x(number)), both_functions))

map loops over the items defined in both_functions which means for each number it loops over times_two and times_three function. ie: when number is 1, it produces output [times_two(1), times_three(1)] equals [2, 3]

CodePudding user response：

What does (lambda x: x(number)) do exactly

In terms of the comprehension, nothing specific. It is an argument of map()

why is there a second x, what kind of parameter is?

It is a callable function object.

Why here there is an a=x

It seems to be defining a partial function with a parameter for the range(). That syntax is new to me...

If you remove =x, then you get an error missing 1 required positional argument: 'a', which is the same error if you had defined a regular function def foo(a): and just called foo() in the loop.

usually lambdas doesn't have only one parameter

They are in-line functions, so no, they can have as many as necessary.

what does _() do

_ is just a variable. It can be assigned to a callable. _() calls a function named _.