I'm trying to run the following code but "full_name" gives me the following error.

TypeError: unsupported operand type(s) for : 'CharField' and 'str'

from django.db import models

class user(models.Model):
    first_name = models.CharField(max_length=200)
    last_name = models.CharField(max_length=200)

    full_name = first_name   ' '   last_name

    def __str__(self):
        return self.full_name

When I change the "full_name" from a variable to a function, the TypeError goes away and my code works fine.

from django.db import models

class user(models.Model):
    first_name = models.CharField(max_length=200)
    last_name = models.CharField(max_length=200)
    
    def full_name(self):
        return self.first_name   ' '   self.last_name

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

Why does the variable give me a TypeError but the function doesn't? Don't they do the same thing?

CodePudding user response：

It gives error because full_name is actually a variable in user class. But fucntion is returning a string. Class user never actually runs so you cant's define a variable which doesn't subclass django.models.

CodePudding user response：

Statements on class level are evaluated on start up. Statements in methods are evaluated when the method is called.

In the first example the expression is evaluated on start up; against the class attribute values. Effectively, you are doing models.CharField(max_length=200) ' ' models.CharField(max_length=200), but you can't concatenate a CharField with a string.

In the second example the expression is performed when the method is called on an instance; against the values of both fields, which are strings.