Scope of class variable with list comprehension [duplicate]

Question

This question already has answers here:

Why is one class variable not defined in list comprehension but another is? (2 answers)

Closed 5 years ago.

have a look at the following piece of code:

class a:
    s = 'python'
    b = ['p', 'y']
    c = [x for x in s]

the output:

>>> a.c
['p', 'y', 't', 'h', 'o', 'n']

but when i try to limit the list with if:

class a:
    s = 'python'
    b = ['p', 'y']
    c = [x for x in s if x in b]

Shows the following exception:

Traceback (most recent call last):
  File "<pyshell#22>", line 1, in <module>
    class a:
  File "<pyshell#22>", line 4, in a
    c = [x for x in s if x in b]
  File "<pyshell#22>", line 4, in <listcomp>
    c = [x for x in s if x in b]
NameError: global name 'b' is not defined

If a make global b it works, why this is happening?

Answer 1

This isn't so much about the variable scope in list comprehensions as about the way classes work. In Python 3 (but not in Python 2!), list comprehensions don't affect the scope around them:

>>> [i for i in range(10)]
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> i
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'i' is not defined
>>> i = 0
>>> [i for i in range(10)]
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> i
0

However, when you do that in a class, it won't look for b among the class attributes the way it would in the local scope of a module or function. To do what you are trying to do, use the @property decorator:

>>> class a:
...   s = 'python'
...   b = 'py'
...   @property
...   def c(self):
...     return [x for x in self.s if x in self.b]
... 
>>> A = a()
>>> A.c
['p', 'y']

Also, remember that strings are iterable too (they're just lists of their component characters), so no need to explicitly make b a list.