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???

yes, can be a property, but.. are you using the instance method? — Leandro
– Leandro, Commented Jan 24, 2014 at 12:53
@rednaw I got just a character '>'.. why? I am new to python django.. so please bear with me for these silly questions.. — user3108063
– user3108063, Commented Jan 24, 2014 at 13:00

Matteo Scotuzzi · Accepted Answer · 2014-01-24 13:26:10Z

2

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*

edited Jan 24, 2014 at 13:26

answered Jan 24, 2014 at 13:00

Matteo Scotuzzi

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1 Comment

Matteo Scotuzzi Over a year ago

in this case yes, you are only creating a reference to the method, that didn't run yet

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