🔒 Class variable causes NameError in comprehension

[sinful grail]

Does anyone have insight on why this happens?
My class looks like this and works:

class Breakfast:
    food_types = ["Spam", "Ham", "Eggs"]
    food = {
        f: 1 for f in food_types
    }

However, when I extend with like this:

class Breakfast:
    food_types = ["Spam", "Ham", "Eggs"]
    dislike = ["Spam"]  # new
    food = {
        f: 1 for f in food_types
        if f not in dislike  # new
    }

I get an error

NameError: name 'dislike' is not defined

Is the if-clause evaluated in a different scope or something like that?
Please don't suggest alternative ways to implement this, I am just trying to find out why this happens.

[timber egret]

@sinful grail

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[sinful grail]

!e

class Breakfast:
    food_types = ["Spam", "Ham", "Eggs"]
    food = {
        f: 1 for f in food_types
    }
print(Breakfast.food)

[timber egret]

:white_check_mark: Your 3.12 eval job has completed with return code 0.

{'Spam': 1, 'Ham': 1, 'Eggs': 1}

[sinful grail]

!e

class Breakfast:
    food_types = ["Spam", "Ham", "Eggs"]
    dislike = ["Spam"]  # new
    food = {
        f: 1 for f in food_types
        if f not in dislike  # new
    }
print(Breakfast.food)

[timber egret]

:x: Your 3.12 eval job has completed with return code 1.

001 | Traceback (most recent call last):
002 |   File "/home/main.py", line 1, in <module>
003 |     class Breakfast:
004 |   File "/home/main.py", line 6, in Breakfast
005 |     if f not in dislike  # new
006 |                 ^^^^^^^
007 | NameError: name 'dislike' is not defined

[sinful grail]

Class variable causes NameError in comprehension

[solemn python]

!e

food_types = ["Spam", "Ham", "Eggs"]
dislike = ["Spam"]
print([1 if f not in dislike else 0 for f in food_types])

Testing

[timber egret]

:white_check_mark: Your 3.12 eval job has completed with return code 0.

[0, 1, 1]

[solemn python]

yeah do the if else first then the for loop

[quasi thicket]

It’s a peculiarity with how class scopes work. Unlike functions or global variables, variables defined at the class level aren’t visible from inside functions.

[sinful grail]

I wonder why food_types works tho

Inside functions, you say? Is the if clause implemented as filter or so?

[quasi thicket]

In 3.11 and earlier, a comprehension is implemented as a function, that way the loop variables don't escape to outer scopes. In 3.12 that's changed for performance reasons, but the class variables are still not accessible.

The reason {f: 1 for f in food_types} works is that the outermost iterable is evaluated at the class scope and passed in, not in the comprehension's scope.

[sinful grail]

Ah, I see, interesting

Good to know, thanks a lot!

[quasi thicket]

Probably the easiest solution is just to make everything module-global.

[sinful grail]

I'd prefer the attributes in the namespace but it's not too bad to take out the "dislike" at least

Thanks a lot for the help in figuring this quirk out 😄

[solemn python]

Probably better if its in a function

a class function

[quasi thicket]

Well, then it's being recalculated every time, makes stuff more complicated.

[solemn python]

!e

class Breakfast:
    food_types = ["Spam", "Ham", "Eggs"]
    dislike = ["Spam"]  # new

    def food(self):
        food = {
            f: 1 if f not in self.dislike else 0 for f in self.food_types
        }
        return food
    
breakfast = Breakfast()
print(breakfast.food())

[timber egret]

:white_check_mark: Your 3.12 eval job has completed with return code 0.

{'Spam': 0, 'Ham': 1, 'Eggs': 1}

[sinful grail]

!close

[timber egret]

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