python django why class method can be used like a variable?

Question

I see this in the UserCreationForm:

def clean_username(self):
    # Since User.username is unique, this check is redundant,
    # but it sets a nicer error message than the ORM. See #13147.
    username = self.cleaned_data["username"]
    try:
        User._default_manager.get(username=username)
    except User.DoesNotExist:
        return username
    raise forms.ValidationError(self.error_messages['duplicate_username'])

This method can be used in this way:

def register_me(request):
    if request.method == 'POST':
        form = UserCreationForm(request.POST)
        uname = form.clean_username

See the last line above . The method is without the ()... Why???

Answer 1

This is because python is not strongly typed (so you can assign any kind of object to any variable) and because functions are first class objects: you can assign them to variables like any normal object.

In your example after assigning *form.clean_username* to uname you can use the uname variable like this: uname() and it will do the same as *form.clean_username()* because you assigned to uname the "runnable" that was referenced by *form.clean_username*