Accessing class variables from a list comprehension in the class definition

Question

How do you access other class variables from a list comprehension within the class definition? The following works in Python 2 but fails in Python 3:

class          


        

Answer 1

This is a bug in Python. Comprehensions are advertised as being equivalent to for loops, but this is not true in classes. At least up to Python 3.6.6, in a comprehension used in a class, only one variable from outside the comprehension is accessible inside the comprehension, and it must be used as the outermost iterator. In a function, this scope limitation does not apply.

To illustrate why this is a bug, let's return to the original example. This fails:

class Foo:
    x = 5
    y = [x for i in range(1)]

But this works:

def Foo():
    x = 5
    y = [x for i in range(1)]

The limitation is stated at the end of this section in the reference guide.