Use global variables defined by one class within the class

Question

I'm trying to use global variables LAND which is an object of class Type within the class Type.

The following code gives me an error: SyntaxError: invalid syntax because LAND is not defined:

# Define a class named Type
class Type():
    def __init__(self, id, symbol):
        self.id = id
        self.symbol = symbol
    def is_free(self):
        return self.id = LAND.id

# Define variables of this class
LAND = Type(-1, '.')

I can't just move LAND before the definition of Type, because that will also give me the error SyntaxError: invalid syntax because Type is not defined.

Can somebody tell me if it is possible? Or is there any alternative way to do this?

I have asked a question with a bad title Python class forward declaration, it was marked as duplicate with two questions, but as I explained in my original post, I don't think my question is the same as those two:

I have seen the post Is it possible to forward-declare a function in Python?, but I don't think it's the same issue. Because the question is about forward declaration of function, while I'm asking about class.

I also read this post Does Python have class prototypes (or forward declarations)?, but it was asking about refer to a class in another class, while I'm asking about refer to variables of its own type within the class. I don't see my solution in this post, or can somebody who thinks it's duplicate explain a little bit?

If `LAND.id` is always -1, can't `is_free()` just check for that exact value? — John Gordon, Apr 23 '18 at 03:30
@JohnGordon It can, but that will make code difficult to read. Consider `LAND` as a const variable. — zhm, Apr 23 '18 at 03:32
@MartinZhai which version of python you are executing code? Actually in python2.7 code executed correctly. — Manoj Jadhav, Apr 23 '18 at 03:37
Please show the full error. With stack trace. Something is missing here. — Mad Physicist, Apr 23 '18 at 03:42

timgeb · Accepted Answer · 2018-04-23T03:54:13.440

Following code gives me error, SyntaxError: invalid syntax because LAND is not defined

No, you get a SyntaxError because an assignment a = b (in your specific case
self.id = LAND.id) is not an expression, wich means that it does not produce a value which you would be able to return.

Your code works just fine when you change the = to a ==, which is what I assume you want. Alternatively (I'm a bit unsure what is supposed to happen in your code), you could write two lines:

self.id = LAND.id
return self.id

Anyway, the problem here is not that LAND is not defined when is_free is created. That's not a problem, because nobody is calling is_free yet. The problem is that you probably thought (because you seem to come from a C background ) that an assignment with = produces a value, which is not the case in Python.

Use global variables defined by one class within the class

1 Answers1