List comprehension at class-level, Python 3

Asked Apr 02 '19 at 13:28

Active Apr 02 '19 at 13:33

Viewed 28 times

Consider this code:

class MyMegaClass(object):
    excluded_fields = {1, 2}

    all_fields = list(range(10))
    print(excluded_fields)
    valid_fields = [itm for itm in all_fields if itm not in excluded_fields]
cl = MyMegaClass()
print(cl.valid_fields)

This code works fine on Python 2.7. But Python 3.7 raises an exception on the line with the list comprehension. It doesn't see the "excluded_fields" field. Although, "print(excluded_files)" works! The "excluded_field" is not visible only after "if" in a list comprehension. Any ideas?

The traceback:

Traceback (most recent call last):
File "C:/Users/S2400244/PycharmProjects/enumtest/classfieldtest.py", line 1, in <module>
class MyMegaClass(object):
File "C:/Users/S2400244/PycharmProjects/enumtest/classfieldtest.py", line 6, in MyMegaClass
valid_fields = [itm for itm in all_fields if itm > len(excluded_fields)]
File "C:/Users/S2400244/PycharmProjects/enumtest/classfieldtest.py", line 6, in <listcomp>
valid_fields = [itm for itm in all_fields if itm > len(excluded_fields)]
NameError: name 'excluded_fields' is not defined

edited Apr 02 '19 at 13:33

asked Apr 02 '19 at 13:28

dondublon

Don't you need an `__init__`? – Elmex80s Apr 02 '19 at 13:31
@Elmex80s, I don't want to use it, because logically values should be at the class-level. They aren't connected with an object instance. – dondublon Apr 02 '19 at 13:34
Plausible explanation, but then why use a class and not just a module? – Elmex80s Apr 02 '19 at 13:36
@Aran-Fey, yes, you're right, that is the case. – dondublon Apr 02 '19 at 13:38
@Elmex80s, because they have to be visible only in class, not in the entire module. – dondublon Apr 02 '19 at 13:39
It apears to be a bug (or not desired by the creators of the language). The suggested `__init__` by me would be an acceptable walk-around. – Elmex80s Apr 02 '19 at 13:46

List comprehension at class-level, Python 3

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