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The following code does work on python 2.7 but fails on python 3.7. The code seems correct.
class A:
 class B:
 def __init__(self, param):
 self.param = param
 
 l = [B(i) for i in range(10)]
The result on:
3.7.3 (v3.7.3:ef4ec6ed12, Mar 25 2019, 22:22:05) [MSC v.1916 64 bit (AMD64)]
Traceback (most recent call last):
 File "test.py", line 6, in <module>
 class A:
 File "test.py", line 11, in A
 l = [B(i) for i in range(10)]
 File "test.py", line 11, in <listcomp>
 l = [B(i) for i in range(10)]
NameError: name 'B' is not defined
B can be accessed if not inside a generator expression