list comprehension for lambdas in python

Question

I don't get 4 different functions but only 4 equal for last value of ops

https://code.sololearn.com/civo7fUYiP71/?ref=app

Answer 1

Oma Falk here is a better explanation, after reading more about generators.

In list comprehensions, all evaluations are done before actually returning the list. Which means you must wait for the evaluations to finish before you access it.

None of the functions are returned, and they are only assigned after all evaluations are done, thus all of them having the same last operator.

A generator on the other hand allows you to access each element independetly. Which means that at first, it doesn't evaluate anything at all. So when it runs the first time, it returns the first value of "op", which was "+", and return it as a function.

After that, it revaluates the output, and this time even when the variable "op" changes, it will not affect the first "add" function because it has already been defined and finished. Same goes for the two remaining functions.

(I'm not sure if you understood, I can think of a better way to explain this if you wish to. This can be quite confusing at first).)

Answer 2

This is a common problem with lambda and loops. When you define lambda function inside a loop, any variable that changes inside the loop will be modified, and will also modify the lambda function itself.

To avoid this, you can pass the variables that change within the loop as arguments of the function:
https://code.sololearn.com/cKZTZw8M5td9/?ref=app

Answer 3

Aymane Boukrouh
I hoped using tuples do it.
Whats the difference to

https://code.sololearn.com/cNU52yK69BSj/?ref=app

Answer 4

Aymane Boukrouh but doesnt that makes the operator accessible from parameter ?

like add(2,3,"+4*") would change the function entirely.

i can only came up with this, but its not one liner
https://code.sololearn.com/cZqfscD4x2fU/?ref=app

Answer 5

Oma Falk It's a generator.

To create tuple comprehension you can use:
tuple([...])

But this creates a generator:
(...)

Answer 6

I found this
https://stackoverflow.com/questions/6076270/lambda-function-in-list-comprehensions/6076304

Based on it I wrote another version of "function comprehension" with partial()
https://code.sololearn.com/cqug6Ml0O7dI/?ref=app

Answer 7

You somehow created a shared local variable for the functions, but where is it stored?

Very annoying problem.

Answer 8

Oma Falk
This is not a tuple:
(i for i in range(10))

Answer 9

Seb TheS but omas is a tuple
edit:nope 🤔

Answer 10

Oma Falk interesting, I didn't know you can do it like this as well. First of all, as Seb TheS mentioned, this is a generator not a tuple.

Anyway, here is my interpretation:
A generator saves the results in memory, without actually doing the calculations. In this case, I would say that for each loop it saves the lambda function as a new one, until being called.

Answer 11

Taste yes, it does make it accessible from outside the function. However, in python 3.8, I believe there is a new syntax that prevents the user from accessing certain default values of parameters. Not sure tho.

Answer 12

Aymane Boukrouh
I got it a bit.
It is midnight. have to sleep about it