Qfyilyi

I don’t understand how to do assignment checking to a variable, as it is done in Kivy.
I know how this is done for class properties, and it looks like this

#!/usr/bin/python3.6    

class Foo:

   var = property()

   def __init__(self):

      self._var = 0

   @var.setter

   def var(self, value):

      self._var = value

      # code setter

      pass



   @var.getter

   def var(self):

      # code getter

      print('Getter method')

      return self._var



a = Foo()

a.var = 5

print(a.var)

# Getter method

# 5    

In Kivy can do:

class LabelBase(Label):

msg = StringProperty('t')



   def __init__(self, **kwargs):

       super(LabelBase, self).__init__(**kwargs)

       self.msg = 5

I take

     Traceback (most recent call last):

   File "/home/Python/Prj/View/Main.py", line 83, in <module>

     Start().build()

   File "/home/Python/Prj/View/Main.py", line 73, in build

     GUI().run()

   File "/usr/lib/python3/dist-packages/kivy/app.py", line 800, in run

     root = self.build()

   File "/home/Python/Prj/View/Main.py", line 65, in build

     main_window = MainFrame()

   File "/home/Python/Prj/View/Main.py", line 52, in __init__

     self.label = LabelBase(text='test')

   File "/home/Python/Prj/View/Main.py", line 16, in __init__

     self.msg = 5

   File "kivy/properties.pyx", line 483, in 

   kivy.properties.Property.__set__

   File "kivy/properties.pyx", line 521, in kivy.properties.Property.set

   File "kivy/properties.pyx", line 512, in kivy.properties.Property.set

   File "kivy/properties.pyx", line 678, in 

   kivy.properties.StringProperty.check

   ValueError: LabelBase.msg accept only str

Renamed the question because it did not correspond to what was happening

I assume you want to do some checks on assignments, like type checking in the kivy case. Your example looks to me very unidiomatic, I'd write it the following way:

class Foo:



    @property

    def var(self):

        return self._var



    @var.setter

    def var(self, value):

        if type(value) == str:

            self._var = value

How does it work? Actually it's a bit complicated, but here is a good ressource. property returns a descriptor which basically means that foo.var = x is translated to foo.var.__set__(x) (which then calls something "equivalent" to Foo.var.setter_function(foo, x)). It just says "instead of storing an assigned value, call this function".

How is the kivy case different? Assume we have the following class:

class Bar(Widget):

    var = StringProperty()

The behavior is very similar to the python code before, but the setter and getter methods are defined by kivy and not here in the class. But if you assign a value to a Bar instance bar.var = x a setter is called bar.var.__set__(x). The setter does not only check for types but also emits events if the value changed.

You can also create properties with getters and setters already provided by implementing a descriptor. You need to implement __set__ and __get__:

class MyStringProperty:



    def __init__(self, default):

        self._default = default



    def __set__(self, instance, value):

        if type(value) == str:

            instance._value = value



    def __get__(self, instance, owner):

        return getattr(instance, "_value", self._default)



class Foo:

    var = MyStringProperty("x")



foo = Foo()

print(foo.var)

foo.var = 3

print(foo.var)

foo.var = "asdf"

print(foo.var)

(The documentation speaks from get, set, delete and set_name, but I think in the common case one can get away with only implementing set and get.)

If you want to check the value, you simply add check code to your setter. For example, if you want var to be limited to int or float, you can modify your setter as:

   @var.setter

   def var(self, value):

      if type(value) not in (int, float):

          raise ValueError('%s.%s accept only int or float (got %r)' % (

              self.__class__.__name__,

              'var', value))

      self._var = value

This code is a slight modification of Kivy check code.