How to access another object

Question

I've been trying to create a class with an __init__ method which will gather some of it's data from another object's attributes. The following code is a bare bone demonstration of my problem.

main.py

import foo
import rat
Archive = foo.Foo()
A = rat.Rat()

rat.py

import foo

class Rat:

    def __init__(self):
        self.name = "Hi there"
        self.birthday = Archive.tick

foo.py

import rat
class Foo:

    def __init__(self):
        self.tick = 5

As you can guess, this doesn't work and it tells me:

in __init__
    self.birthday = Archive.tick
NameError: name 'Archive' is not defined

I don't understand why it tells me Archive is not defined. Why can't it access the instance I just created? Why does this happen and how can I go around this?

PS: I know it's generally a terrible idea to access an object's attributes in such a public way, but the foo class is here just to have a general data gathering object, nothing serious.

score 3 · Accepted Answer · edited May 23 '17 at 12:29

Archive is not defined in the scope of rat.py.

It's not enough that you define Archive somewhere in a module that gets loaded - you also need to make it available everywhere you use it by either importing it, or assign it to some other name.

So you've got two options:

Import Archive from main in rat.py
Pass Archive to Rat()

I would advise against importing it however. It seems you already have circular imports (foo importing rat and vice-versa). This can sometimes work, but usually it's a bad idea and should be avoided. Instead you should design and structure your code in a way where you make concious decision about what class needs to know about what, and, as a result of that, what imports the module it lives in actually needs. (Why does rat import foo? It doesn't do anything with it).

So this is what I would suggest:

rat.py

class Rat(object):

    def __init__(self, archive):
        self.archive = archive
        self.name = "Hi there"
        self.birthday = self.archive.tick

main.py

from rat import Rat
from foo import Foo

archive = Foo()
rat = Rat(archive)

Additional changes, unrelated to your problem:

Use new style classes by inheriting from object
Only use CamelCase for class names according to PEP8

score 1 · Answer 2 · answered Sep 17 '14 at 22:57

1

rat.py

class Rat:
    def __init__(self, archive):
        self.name = "Hi there"
        self.birthday = archive.tick

main.py

Archive = foo.Foo()
A = rat.Rat(Archive)

answered Sep 17 '14 at 22:57

Anton Savin

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Can you explain the error in the OP's code, and how this avoids it? – John Kugelman Sep 17 '14 at 22:58
Simple and clear. Would indeed like a more complete answer as Kugelman asked. – Antoine Ender Ezechiel Sep 17 '14 at 23:14

How to access another object

main.py

rat.py

foo.py

2 Answers2