Clash Royale CLAN TAG#URR8PPP

Usage of descriptors

First time trying to use descriptors.

The goal: any integer from 0 to 100 (inclusive) will be a legal value for this descriptor. Non-integers and numbers higher than 100 or lower than 0 should result in an exception being thrown, indicating that the type and/or value is wrong.

My code:

class Percentage():
def __init__(self, initial_value=None):
self.pct_min = 0
self.pct_max = 100
self.value = initial_value

def __get__(self, obj, initial_value):
return self.value

def __set__(self, obj, initial_value):
if self.pct_min <= int(self.value) <= self.pct_max:
return self.value
elif self.value < self.pct_min:
raise ValueError(print("Value to low"))
else:
raise ValueError(print("Value to high"))


class Foo(object):
participation = Percentage()



f1 = Foo()
f1.participation = 30

f2 = Foo()
f2.participation = 70

print(f1.participation) # should print 30
print(f2.participation) # should print 70


For this line: if self.pct_min <= int(self.value) <= self.pct_max:

I get this error:

TypeError: int() argument must be a string, a bytes-like object or a number, not 'NoneType'


I am running it through PythonTutor and it appears I am not passing in the integer, but I do not understand where I am failing.

I am also using this as a guide: https://www.blog.pythonlibrary.org/2016/06/10/python-201-what-are-descriptors/

self.value

None

__init__

Foo

Foo.participation

__get__

__set__

obj

f1

f2

2 Answers
2

This isn't really a problem with descriptors! You've merely set the initial self.value attribute on the descriptor to None:

self.value

None

def __init__(self, initial_value=None):
# ...
self.value = initial_value # this is None by default!


You then try to convert that self.value attribute value, set to None, to an integer:

self.value

None

# self.value starts out as None; int(None) fails.
if self.pct_min <= int(self.value) <= self.pct_max:


You probably want to apply this to the initial_value argument to __set__!

initial_value

__set__

Your descriptor implementation does have several more problems:

__set__

__set__

You are storing the data on the descriptor instance itself. Your class has only a single copy of the descriptor object, so when you use the descriptor between different instances of Foo() you see the exact same data. You want to store the data on the object the descriptor is bound to, so in the __set__ method, on obj, or at the very least, in a place that lets you associate the value with obj. obj is the specific instance of Foo() the descriptor is bound to.

Foo()

__set__

obj

obj

obj

Foo()

The __get__ getter method doesn't take an 'initial value', it takes the object you are being bound to, and the type (class) of that object. When accessed on a class, obj is None and only the type is set.

__get__

obj

None

Don't mix print() and creating an exception instance. ValueError(print(...)) doesn't make much sense, print() writes to the console then returns None. Just use ValueError(...).

print()

ValueError(print(...))

print()

None

ValueError(...)

I'm making the following assumptions:

Then the following code would work:

class Percentage:
def __init__(self, initial_value=None):
self.pct_min = 0
self.pct_max = 100
self.initial_value = initial_value

def __get__(self, obj, owner):
if obj is None:
# accessed on the class, there is no value. Return the
# descriptor itself instead.
return self

# get the value from the bound object, or, if it is missing,
# the initial_value attribute
return getattr(obj, '_percentage_value', self.initial_value)

def __set__(self, obj, new_value):
# validation. Make sure it is an integer and in range
new_value = int(new_value)
if new_value < self.pct_min:
raise ValueError("Value to low")
if new_value > self.pct_max:
raise ValueError("Value to high")

# validation succeeded, set it on the instance
obj._percentage_value = new_value


Demo:

>>> class Foo(object):
... participation = Percentage()
...
>>> f1 = Foo()
>>> f1.participation is None
True
>>> f1.participation = 110
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<string>", line 24, in __set__
ValueError: Value to high
>>> f1.participation = -10
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<string>", line 22, in __set__
ValueError: Value to low
>>> f1.participation = 75
>>> f1.participation
75
>>> vars(f1)
'_percentage_value': 75
>>> Foo.participation # this calls Participation().__get__(None, Foo)
<__main__.Percentage object at 0x105e3a240>
>>> Foo.participation.__get__(f1, Foo) # essentially what f1.participation does
75


Note that the above uses a specific attribute name on the instance to set the value. If you have multiple instances of the descriptor on the same class, that will be an issue!

If you want to avoid that, your options are:

descriptor.__set_name__()

Foo

Foo

'participation'

Note that you should not use the attribute name (participation in your Foo example), at least not directly. Using obj.participation will trigger the descriptor object itself, so Percentage.__get__() or Percencage.__set__(). For a data descriptors (descriptors that implementing either a __set__ or __delete__ method), you could use vars(obj) or obj.__dict__ directly to store attributes, however.

participation

Foo

obj.participation

Percentage.__get__()

Percencage.__set__()

__set__

__delete__

vars(obj)

obj.__dict__

That's not a problem with the descriptor itself. The problem is that, in the __set__, instead of checking the value that you get, you check the existing self.value. Since this is initialized to None, you will always fail on the first assignment.

__set__

self.value

None

There's also an (unrelated) problem with the descriptor itself. The getter and setters are supposed to read/write from the obj.__dict__, not from the descriptor object itself. Otherwise, all the instances will share a common value

obj.__dict__

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