Python Language OOP Attributes and Properties
Internal
Overview
Attributes are variables associated with a class that carry state either for the class itself, or for the instances of that class. Properties are class constructs that behave like attributes, but are not variables. Technically, bona-fide attributes, properties and also the methods are all attributes on a class. Methods are just callable attributes.
Attributes
Attributes are variables associated with a class that hold state either for the instances of that class (instance attributes) or for the class itself (class attributes). An object instance carries its state as attributes. To differentiate attributes from properties, they are sometimes referred to as standard data attributes.
Instance Attributes
Declaring Instance Attributes
All instance attributes must be declared inside the __init__() method.
class A:
def __init__(self):
self.color = 'blue'
If an attribute is first used in a method other than __init__(), static analysis identifies this as a warning "instance attribute defined outside __init__()".
Accessing and Mutating Instance Attributes
Idiomatic Python favors direct attribute access. The instance attributes can be accessed and mutated inside the class definition using self.<attribute-name>. Outside the class definition, they can be accessed and mutated via the variable holding the reference to the class instance: my_instance.<attribute-name>. If an attribute was not explicitly declared inside the instance with self.some_attribute, even just to be assigned to None, an attempt to access the attribute will end up in:
AttributeError: 'MyClass' object has no attribute 'some_attribute'
Deleting Instance Attributes
An attribute can also be deleted, meaning that it will be removed from the instance it was deleted from.
class A:
def __init__(self):
self.color = 'blue'
def delete_attr(self):
del self.color
a = A()
a2 = A()
a2.delete_attr()
print(a.color) # this will display "blue"
print(a2.color) # this will raise AttributeError: 'A' object has no attribute 'color'
Deleting instance attributes has limited usefulness.
Attribute Visibility
Unlike in other languages, all attributes are public in Python. There are naming conventions to designate attributes as protected, or even private, but these conventions depend on others' willingness to abide by them - the interpreter won't prevent access to an attribute conventionally declared "protected" or "private", they're still public.
"Protected" Attributes
Prepending a single underscore (_) to the attribute name provides some support for protecting module variables and functions, as well as class attributes and methods. Linters and IDE static analysis will flag protected member access. PyCharm explicitly shows them as "Protected Attributes":
Also see Read-Only Properties, below, and:
"Private" Attributes
Prepending a double underscore (__) (also known as “dunder”) to an instance variable or method effectively makes the variable or method private to its class, using name mangling. Google Python Style Guide discourages this use as it impacts readability and testability, and isn’t really private. It advises to use a single underscore.
Class Attributes
TODO
Properties
A property is semantically equivalent with an attribute, in that is supposed to give read and write access to some state associated with the class instance. However, it does not do it by simply designating a variable to hold the state. It does it by defining the accessors and mutators (getters and setters) methods instead. Properties are customizable attributes.
An advantage of using properties over direct attribute access is that if the definition of the attribute changes, only the code within the class definition needs to be changed, not the caller code.
There are two ways to declare the accessor, mutator and deleted method for state associated with the class instance: using the property() built-in and using decorators.
Defining Properties with the property() Built-in Function
The property() built-in function defines a class construct that acts like a virtual attribute, or a proxy to an attribute, by defining the accessor (getter) function, and optionally the mutator (setter), and proxying any requests to set or access the attribute through those methods. The property() built-in function also allows to declare an optional deleter function, and a docstring for the property. If the docstring is not provided, it will be copied from the docstring of the first argument, the getter method. The property() is like a constructor for such a proxy, and the proxy is set as a public-facing member for the given attribute.
class SomeClass:
def __init__(self):
self._internal_state = None
def _some_getter(self):
return self._internal_state
def _some_setter(self, value):
self._internal_state = value
def _some_deleter(self):
pass
some_property = property(_some_getter, _some_setter, _some_deleter, "this is docstring")
Use some_property as it was an attribute. Internally, some_property calls the _some_getter() and _some_setter() methods whenever the value of the property is accessed or changed.
sc = SomeClass()
sc.some_property = 'elephant'
assert sc.some_property == 'elephant'
⚠️ Do not use the property name as it would be a method name. sc.some_property('elephant') will raise an exception:
TypeError: 'NoneType' object is not callable
Defining Properties with Decorators
A fully equivalent pattern of declaring a property as a proxy to a virtual attribute uses the @property decorator, to designate the getter method, the @<attribute-name>.setter decorator to designate the setter method, and the <attribute-name>.deleter decorator to designate the deleter method. The setter and the deleter are optional.
class SomeClass:
def __init__(self):
self._internal_state = None
# The corresponding attribute name will be "some_property", the same as the name of the getter method
@property
def some_property(self):
return self._internal_state
# The corresponding attribute name will be "some_property", the same as the decorator prefix AND the name of the setter method
@some_property.setter
def some_property(self, value):
self._internal_state = value
@some_property.deleter
def some_property(self):
pass
Use some_property as it was an attribute. Internally, some_property calls the some_property() and some_property(value) methods whenever the value of the property is accessed or changed.
sc = SomeClass()
sc.some_property = 'elephant'
assert sc.some_property == 'elephant'
⚠️ Do not use the property name as it would be a method name. sc.some_property('elephant') will raise an exception:
TypeError: 'NoneType' object is not callable
The syntax:
@property
def some_property(self):
...
applies the property() function as a decorator, and it is equivalent with the previous syntax:
some_property = property(some_property)
Read-Only Properties
The state associated with a computed value can be prevented from being written by omitting the corresponding setter.
However, if properties are used to proxy access to an internal variable, the pattern is not entirely safe, even if the setter is omitted: the internal variable can still be accessed directly, as all attributes are public in Python. However, marking the internal variable with a leading underscore is an indication that the variable should not be accessed directly.
TO DEPLETE
Properties are class construct that associate getter and setter methods to an attribute. The attribute name can the be used to access and write state inside the class by invoking the associated "property" methods. In other words, the getter and setter methods are "properties" of the attribute with the given name.
class A:
def __init__(self, c):
self.internal_color = c
def get_color(self):
return self.internal_color
def set_color(self, c):
self.internal_color = c
# 'color' is an attribute, though not explicitly declared on self, and get_color() and set_color() are properties of the attribute
color = property(get_color, set_color)
The first argument to property() is the getter method, and the second argument is the setter method.
Internal class instance state can be accesses and written through the attribute, though there is no actual attribute with that name initialized on self:
a = A('red')
assert 'red' == a.color
a.color = 'blue'
assert 'blue' == a.color
TO DEPLETE Defining Properties with Decorators
Another way to define properties is with decorators. The same attribute color, which is not declared on self, can defined by two different property methods, one getter and one setter, preceded by corresponding decorators (annotations):
@property: it annotates the getter method. ⚠️ The name of the method must match the name of the attribute.@<attribute-name>.setter: it annotates the setter method. ⚠️ The<attribute-name>that is part of the annotation must match the name of the attribute and the name of the setter method.
class A:
def __init__(self, c):
self.internal_color = c
@property
def color(self):
return self.internal_color
# not exposing a setter prevents the attribute from being written
@color.setter
def color(self, c):
self.internal_color = c
The interaction with the internal state is done identically as in the case of the property() declaration:
a = A('red')
assert 'red' == a.color
a.color = 'blue'
assert 'blue' == a.color
For more details on decorators see: