Issue with class declaration in python. Two classes with fields of each other

Question

My code is something like this:

class className1:
    fieldClassName2 = className2()

class className2:
    fieldClassName1 = className1()

and i have NameError: name 'className2' is not defined. I don't know if there is a declaration in python like in c++, and you can call classes in any order. Please help me...

full code:

    class Speciality(models.Model):
    title = models.CharField(max_length=70)
    subject1 = models.ManyToManyField(Subject, verbose_name='Перший предмет', related_name='+')
    subject2 = models.ManyToManyField(Subject, verbose_name='Другий предмет', related_name='+')
    subject3 = models.ManyToManyField(Subject, verbose_name='Третій предмет', related_name='+')
    coefficient1 = models.FloatField()
    coefficient2 = models.FloatField()
    coefficient3 = models.FloatField()
    coefficient4 = models.FloatField()
    amount = models.IntegerField()
    budget_amount = models.IntegerField()
    department = models.ForeignKey(Department, verbose_name='Факультет', on_delete=models.CASCADE)
    code = models.IntegerField(default=0)
    abit = models.ManyToManyField(AdmissionList, verbose_name='Заявки', on_delete=models.CASCADE)

    def __str__(self):
        return str(self.code)

# there some another models

class AdmissionList(models.Model):
    user = models.ForeignKey(User, verbose_name='Абітурієнт', on_delete=models.CASCADE)
    speciality = models.ForeignKey(Speciality, verbose_name='Спеціальність', on_delete=models.CASCADE)
    subject = models.ForeignKey(Subject, on_delete=models.CASCADE, null=True)
    firstRate = models.IntegerField(default=0)
    secondRate = models.IntegerField(default=0)
    thirdRate = models.IntegerField(default=0)
    avgRate = models.FloatField(default=0)
    summaryRate = models.FloatField(default=0)
    is_approved = models.BooleanField(default=False)

    def __str__(self):
        return self.user.full_name

field Speciality.abit is calls AdmissionList and field AdmissionList.speciality calls Speciality.

You should put these two fields in the `__init__` definition body instead of the class definition body. — Mechanic Pig, Sep 07 '22 at 12:25
class 1 instantiates class2, which instantiates class 1 which instantiates class 2... that looks problematic — , Sep 07 '22 at 12:28
`class` statements are executed in order; `className2` hasn't been defined yet when the `class` statement defining `className1` tries to use it. — chepner, Sep 07 '22 at 12:31
What library or framework are you using? I vaguely remember seeing something in SQLAlchemy(?) which let you define keys in one class from another, avoiding this kind of circular reference. Maybe something similar exists in yours. — chepner, Sep 07 '22 at 12:45

score 2 · Answer 1 · answered Sep 07 '22 at 12:40

you could try something like this:

class className2:
    def __init__(self, fieldClassName1=False):
        self.fieldClassName1 = fieldClassName1

class className1:
    fieldClassName2 = className2()


var1 = className1()
var2 = className2(var1)


print(type(var2.fieldClassName1))

It distorts the current structure of your code, but it avoids circular dependency

score 0 · Answer 2 · answered Sep 07 '22 at 12:33

Python is an interpreted language, contrary to C++, which is a compiled language. Therefore, in Python, contrary to C++, the interpreter reads your code line by line. That is why when it gets to the first class definition, it doesn't know className2.

What you want to do is called a circular dependency. A similar problem has been described in this question : Circular dependency between python classes

If we adapt the solution to your problem, the result would be :

class className1:
    pass

class className2:
    pass

className1.fieldClassName2 = className2()
className2.fieldClassName1 = className1()

Django is not allow that solution. – Илья Власюк Sep 07 '22 at 12:57 — Илья Власюк, Sep 07 '22 at 12:57

score 0 · Accepted Answer · answered Sep 07 '22 at 13:16

I think i avoid that problem by adding quotes into class call

abit = models.ManyToManyField(AdmissionList, verbose_name='Заявки', on_delete=models.CASCADE)

abit = models.ManyToManyField('AdmissionList', verbose_name='Заявки', on_delete=models.CASCADE)

Issue with class declaration in python. Two classes with fields of each other

3 Answers3