NameError: name 'Class_Name' is not defined

Question

I have an issue which is I keep getting the error of NameError: name 'Class_Name' is not defined. Which I understand. The tricky part is that my code looks something like this:

class FirstClass():
  second_class: SecondClass
  def __init__(self):
      """SOME CODE HERE"""

class SecondClass(firsClass: FirstClass):
  def __init__(self):
      self.first_class = firstClass

So the problem is that no matter how I arrange the code, I will always have one above the other so it will always say that is not defined. How can I solve this?

------------------------SOLUTION----------------------------

I found the solution to this. If you use the import

from __future__ import annotations

It will read the file and you will be able to call the classes even if they are defined later in the code.

Is this a learning exercise? Because you should never write classes like this. — Code-Apprentice, Nov 12 '22 at 04:12
Does this answer your question? [Type hints: solve circular dependency](https://stackoverflow.com/questions/33837918/type-hints-solve-circular-dependency) — Code-Apprentice, Nov 12 '22 at 04:13

JL Peyret · Answer 1 · 2022-11-12T05:56:58.913


from pprint import pp

class FirstClass():

  #for typing, if you want to indicate a class variable
  second_class: type["SecondClass"]

  #for typing, if you want to indicate an instance variable
  second_class_instance: "SecondClass"

  def __init__(self):
      """SOME CODE HERE"""

      self.second_class_instance = self.second_class()

#this looks off in the inheritance declaration - dont specify types here
#class SecondClass(firsClass: FirstClass):

class SecondClass(FirstClass):
  def __init__(self):
      #this code will error, watch your cases!
      #self.first_class = firstClass
      ...

#for doing something with the second_class attribute, later in the runtime
#however, no tricks with "SecondClass" quoting allowed, you need
#to wait till both FirstClass and SecondClass EXIST, which is why 
#this line is here, with this indentation
FirstClass.second_class = SecondClass

first = FirstClass()
pp(first)
pp(vars(first))

Is doing this a good idea? Probably not, but may there is special, special, justification for tracking a second_class class reference, say to call a class constructor without hardcoding the SecondClass in FirstClass methods.

And, in this scheme, I am even less convinced by having SecondClass subclass FirstClass, but...

output:

% py test_429_declare.py
<__main__.FirstClass object at 0x10841f220>
{'second_class_instance': <__main__.SecondClass object at 0x10841f250>}

and mypy

% mypy test_429_declare.py
Success: no issues found in 1 source file

Actually, I found the solution for this. If we use the import `from __future__ import annotations` it will read the file and it won't have that problem, so we can reference different things in the order that we want. — Reponic, Nov 20 '22 at 05:45
@Reponic if you fixed with that approach, you should probably post it, and accept it, as an answer instead of a comment in your Q. — JL Peyret, Nov 20 '22 at 19:39

score 0 · Accepted Answer · answered Nov 30 '22 at 04:48

Actually, I found the solution to this.

If we use the import from __future__ import annotations it will read the file and it won't have that problem, so we can reference different things in the order that we want. Taking the example:

class FirstClass():
  second_class: SecondClass
  def __init__(self):
      """SOME CODE HERE"""

class SecondClass(firsClass: FirstClass):
  def __init__(self):
      self.first_class = firstClass

This will be resolved as:

from __future__ import annotations

class FirstClass():
  second_class: SecondClass
  def __init__(self):
      """SOME CODE HERE"""

class SecondClass(firsClass: FirstClass):
  def __init__(self):
      self.first_class = firstClass

You can find more details in the python documentation: https://docs.python.org/3/library/__future__.html

NameError: name 'Class_Name' is not defined

2 Answers2

output: