Why is one class variable not defined in list comprehension but another is?

Question

I just read the answer to this question: Accessing class variables from a list comprehension in the class definition

It helps me to understand why the following code results in NameError: name 'x' is not defined:

class A:
    x = 1
    data = [0, 1, 2, 3]
    new_data = [i + x for i in data]
    print(new_data)

The NameError occurs because x is not defined in the special scope for list comprehension. But I am unable to understand why the following code works without any error.

class A:
    x = 1
    data = [0, 1, 2, 3]
    new_data = [i for i in data]
    print(new_data)

I get the output [0, 1, 2, 3]. But I was expecting this error: NameError: name 'data' is not defined because I was expecting just like in the previous example the name x is not defined in the list comprehension's scope, similarly, the name data would not be defined too in the list comprehension's scope.

Could you please help me to understand why x is not defined in the list comprehension's scope but data is?

Answer 1

data is the source of the list comprehension; it is the one parameter that is passed to the nested scope created.

Everything in the list comprehension is run in a separate scope (as a function, basically), except for the iterable used for the left-most for loop. You can see this in the byte code:

>>> def foo():
...     return [i for i in data]
... 
>>> dis.dis(foo)
  2           0 LOAD_CONST               1 (<code object <listcomp> at 0x105390390, file "<stdin>", line 2>)
              3 LOAD_CONST               2 ('foo.<locals>.<listcomp>')
              6 MAKE_FUNCTION            0
              9 LOAD_GLOBAL              0 (data)
             12 GET_ITER
             13 CALL_FUNCTION            1 (1 positional, 0 keyword pair)
             16 RETURN_VALUE

The <listcomp> code object is called like a function, and iter(data) is passed in as the argument (CALL_FUNCTION is executed with 1 positional argument, the GET_ITER result).

The <listcomp> code object looks for that one argument:

>>> dis.dis(foo.__code__.co_consts[1])
  2           0 BUILD_LIST               0
              3 LOAD_FAST                0 (.0)
        >>    6 FOR_ITER                12 (to 21)
              9 STORE_FAST               1 (i)
             12 LOAD_FAST                1 (i)
             15 LIST_APPEND              2
             18 JUMP_ABSOLUTE            6
        >>   21 RETURN_VALUE

The LOAD_FAST call refers to the first and only positional argument passed in; it is unnamed here because there never was a function definition to give it a name.

Any additional names used in the list comprehension (or set or dict comprehension, or generator expression, for that matter) are either locals, closures or globals, not parameters.