Introduction
Objectives:
- Learn about descriptors
Files Created: descrip.py
Files Modified: validate.py
Objectives:
Files Created: descrip.py
Files Modified: validate.py
Earlier, you created a class Stock
that made use of slots, properties, and other features. All of these features are implemented using the descriptor protocol. See it in action by trying this simple experiment.
First, create a stock object, and try looking up a few attributes:
>>> s = Stock('GOOG', 100, 490.10)
>>> s.name
'GOOG'
>>> s.shares
100
>>>
Now, notice that these attributes are in the class dictionary.
>>> Stock.__dict__.keys()
['sell', '__module__', '__weakref__', 'price', '_price', 'shares', '_shares',
'__slots__', 'cost', '__repr__', '__doc__', '__init__']
>>>
Try these steps which illustrate how descriptors get and set values on an instance:
>>> q = Stock.__dict__['shares']
>>> q.__get__(s)
100
>>> q.__set__(s,75)
>>> s.shares
75
>>> q.__set__(s, '75')
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "stock.py", line 23, in shares
raise TypeError('Expected an integer')
TypeError: Expected an integer
>>>
The execution of __get__()
and __set__()
occurs automatically whenever you access instances.
Define the descriptor class from the notes:
## descrip.py
class Descriptor:
def __init__(self, name):
self.name = name
def __get__(self, instance, cls):
print('%s:__get__' % self.name)
def __set__(self, instance, value):
print('%s:__set__ %s' % (self.name, value))
def __delete__(self, instance):
print('%s:__delete__' % self.name)
Now, try defining a simple class that uses this descriptor:
>>> class Foo:
a = Descriptor('a')
b = Descriptor('b')
c = Descriptor('c')
>>> f = Foo()
>>> f
<__main__.Foo object at 0x38e130> <class __main__.Foo>
>>> f.a
a:__get__
>>> f.b
b:__get__
>>> f.a = 23
a:__set__ 23
>>> del f.a
a:__delete__
>>>
Ponder the fact that you have captured the dot-operator for a specific attribute.
In the previous exercise, you wrote a series of classes that could perform checking. For example:
>>> PositiveInteger.check(10)
10
>>> PositiveInteger.check('10')
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
raise TypeError('Expected %s' % cls.expected_type)
TypeError: expected <class 'int'>
>>> PositiveInteger.check(-10)
You can extend this to descriptors by making a simple change to the Validator
base class. Change it to the following:
## validate.py
class Validator:
def __init__(self, name):
self.name = name
@classmethod
def check(cls, value):
return value
def __set__(self, instance, value):
instance.__dict__[self.name] = self.check(value)
Note: The lack of the __get__()
method in the descriptor means that Python will use its default implementation of attribute lookup. This requires that the supplied name matches the name used in the instance dictionary.
No other changes should be necessary. Now, try modifying the Stock
class to use the validators as descriptors like this:
class Stock:
name = String('name')
shares = PositiveInteger('shares')
price = PositiveFloat('price')
def __init__(self,name,shares,price):
self.name = name
self.shares = shares
self.price = price
You'll find that your class works the same way as before, involves much less code, and gives you all of the desired checking:
>>> s = Stock('GOOG', 100, 490.10)
>>> s.name
'GOOG'
>>> s.shares
100
>>> s.shares = 75
>>> s.shares = '75'
... TypeError ...
>>> s.shares = -50
... ValueError ...
>>>
This is pretty cool. Descriptors have allowed you to greatly simplify the implementation of the Stock
class. This is the real power of descriptors--you get low level control over the dot and can use it to do amazing things.
One annoying thing about descriptors is the redundant name specification. For example:
class Stock:
...
shares = PositiveInteger('shares')
...
We can fix that. Change the top-level Validator
class to include a __set_name__()
method like this:
## validate.py
class Validator:
def __init__(self, name=None):
self.name = name
def __set_name__(self, cls, name):
self.name = name
@classmethod
def check(cls, value):
return value
def __set__(self, instance, value):
instance.__dict__[self.name] = self.check(value)
Now, try rewriting your Stock
class so that it looks like this:
class Stock:
name = String()
shares = PositiveInteger()
price = PositiveFloat()
def __init__(self,name,shares,price):
self.name = name
self.shares = shares
self.price = price
Ah, much nicer. Be aware that this ability to set the name is a Python 3.6 feature however. It won't work on older versions.
Congratulations! You have completed the Learn About Descriptors lab. You can practice more labs in LabEx to improve your skills.