abc – Abstract Base Classes¶
Purpose: | Define and use abstract base classes for API checks in your code. |
---|---|
Available In: | 2.6 |
Why use Abstract Base Classes?¶
Abstract base classes are a form of interface checking more strict than individual hasattr() checks for particular methods. By defining an abstract base class, you can define a common API for a set of subclasses. This capability is especially useful in situations where a third-party is going to provide implementations, such as with plugins to an application, but can also aid you when working on a large team or with a large code-base where keeping all classes in your head at the same time is difficult or not possible.
How ABCs Work¶
abc works by marking methods of the base class as abstract, and then registering concrete classes as implementations of the abstract base. If your code requires a particular API, you can use issubclass() or isinstance() to check an object against the abstract class.
Let’s start by defining an abstract base class to represent the API of a set of plugins for saving and loading data.
import abc
class PluginBase(object):
__metaclass__ = abc.ABCMeta
@abc.abstractmethod
def load(self, input):
"""Retrieve data from the input source and return an object."""
return
@abc.abstractmethod
def save(self, output, data):
"""Save the data object to the output."""
return
Registering a Concrete Class¶
There are two ways to indicate that a concrete class implements an abstract: register the class with the abc or subclass directly from the abc.
import abc
from abc_base import PluginBase
class RegisteredImplementation(object):
def load(self, input):
return input.read()
def save(self, output, data):
return output.write(data)
PluginBase.register(RegisteredImplementation)
if __name__ == '__main__':
print 'Subclass:', issubclass(RegisteredImplementation, PluginBase)
print 'Instance:', isinstance(RegisteredImplementation(), PluginBase)
In this example the PluginImplementation is not derived from PluginBase, but is registered as implementing the PluginBase API.
$ python abc_register.py
Subclass: True
Instance: True
Implementation Through Subclassing¶
By subclassing directly from the base, we can avoid the need to register the class explicitly.
import abc
from abc_base import PluginBase
class SubclassImplementation(PluginBase):
def load(self, input):
return input.read()
def save(self, output, data):
return output.write(data)
if __name__ == '__main__':
print 'Subclass:', issubclass(SubclassImplementation, PluginBase)
print 'Instance:', isinstance(SubclassImplementation(), PluginBase)
In this case the normal Python class management is used to recognize PluginImplementation as implementing the abstract PluginBase.
$ python abc_subclass.py
Subclass: True
Instance: True
A side-effect of using direct subclassing is it is possible to find all of the implementations of your plugin by asking the base class for the list of known classes derived from it (this is not an abc feature, all classes can do this).
import abc
from abc_base import PluginBase
import abc_subclass
import abc_register
for sc in PluginBase.__subclasses__():
print sc.__name__
Notice that even though abc_register is imported, RegisteredImplementation is not among the list of subclasses because it is not actually derived from the base.
$ python abc_find_subclasses.py
SubclassImplementation
Dr. André Roberge has described using this capability to discover plugins by importing all of the modules in a directory dynamically and then looking at the subclass list to find the implementation classes.
Incomplete Implementations¶
Another benefit of subclassing directly from your abstract base class is that the subclass cannot be instantiated unless it fully implements the abstract portion of the API. This can keep half-baked implementations from triggering unexpected errors at runtime.
import abc
from abc_base import PluginBase
class IncompleteImplementation(PluginBase):
def save(self, output, data):
return output.write(data)
PluginBase.register(IncompleteImplementation)
if __name__ == '__main__':
print 'Subclass:', issubclass(IncompleteImplementation, PluginBase)
print 'Instance:', isinstance(IncompleteImplementation(), PluginBase)
$ python abc_incomplete.py
Subclass: True
Instance:
Traceback (most recent call last):
File "abc_incomplete.py", line 22, in <module>
print 'Instance:', isinstance(IncompleteImplementation(), PluginBase)
TypeError: Can't instantiate abstract class IncompleteImplementation with abstract methods load
Concrete Methods in ABCs¶
Although a concrete class must provide an implementation of an abstract methods, the abstract base class can also provide an implementation that can be invoked via super(). This lets you re-use common logic by placing it in the base class, but force subclasses to provide an overriding method with (potentially) custom logic.
import abc
from cStringIO import StringIO
class ABCWithConcreteImplementation(object):
__metaclass__ = abc.ABCMeta
@abc.abstractmethod
def retrieve_values(self, input):
print 'base class reading data'
return input.read()
class ConcreteOverride(ABCWithConcreteImplementation):
def retrieve_values(self, input):
base_data = super(ConcreteOverride, self).retrieve_values(input)
print 'subclass sorting data'
response = sorted(base_data.splitlines())
return response
input = StringIO("""line one
line two
line three
""")
reader = ConcreteOverride()
print reader.retrieve_values(input)
print
Since ABCWithConcreteImplementation is an abstract base class, it isn’t possible to instantiate it to use it directly. Subclasses must provide an override for retrieve_values(), and in this case the concrete class massages the data before returning it at all.
$ python abc_concrete_method.py
base class reading data
subclass sorting data
['line one', 'line three', 'line two']
Abstract Properties¶
If your API specification includes attributes in addition to methods, you can require the attributes in concrete classes by defining them with @abstractproperty.
import abc
class Base(object):
__metaclass__ = abc.ABCMeta
@abc.abstractproperty
def value(self):
return 'Should never get here'
class Implementation(Base):
@property
def value(self):
return 'concrete property'
try:
b = Base()
print 'Base.value:', b.value
except Exception, err:
print 'ERROR:', str(err)
i = Implementation()
print 'Implementation.value:', i.value
The Base class in the example cannot be instantiated because it has only an abstract version of the property getter method.
$ python abc_abstractproperty.py
ERROR: Can't instantiate abstract class Base with abstract methods value
Implementation.value: concrete property
You can also define abstract read/write properties.
import abc
class Base(object):
__metaclass__ = abc.ABCMeta
def value_getter(self):
return 'Should never see this'
def value_setter(self, newvalue):
return
value = abc.abstractproperty(value_getter, value_setter)
class PartialImplementation(Base):
@abc.abstractproperty
def value(self):
return 'Read-only'
class Implementation(Base):
_value = 'Default value'
def value_getter(self):
return self._value
def value_setter(self, newvalue):
self._value = newvalue
value = property(value_getter, value_setter)
try:
b = Base()
print 'Base.value:', b.value
except Exception, err:
print 'ERROR:', str(err)
try:
p = PartialImplementation()
print 'PartialImplementation.value:', p.value
except Exception, err:
print 'ERROR:', str(err)
i = Implementation()
print 'Implementation.value:', i.value
i.value = 'New value'
print 'Changed value:', i.value
Notice that the concrete property must be defined the same way as the abstract property. Trying to override a read/write property in PartialImplementation with one that is read-only does not work.
$ python abc_abstractproperty_rw.py
ERROR: Can't instantiate abstract class Base with abstract methods value
ERROR: Can't instantiate abstract class PartialImplementation with abstract methods value
Implementation.value: Default value
Changed value: New value
To use the decorator syntax does with read/write abstract properties, the methods to get and set the value should be named the same.
import abc
class Base(object):
__metaclass__ = abc.ABCMeta
@abc.abstractproperty
def value(self):
return 'Should never see this'
@value.setter
def value(self, newvalue):
return
class Implementation(Base):
_value = 'Default value'
@property
def value(self):
return self._value
@value.setter
def value(self, newvalue):
self._value = newvalue
i = Implementation()
print 'Implementation.value:', i.value
i.value = 'New value'
print 'Changed value:', i.value
Notice that both methods in the Base and Implementation classes are named value(), although they have different signatures.
$ python abc_abstractproperty_rw_deco.py
Implementation.value: Default value
Changed value: New value
Collection Types¶
The collections module defines several abstract base classes related to container (and containable) types.
General container classes:
- Container
- Sized
Iterator and Sequence classes:
- Iterable
- Iterator
- Sequence
- MutableSequence
Unique values:
- Hashable
- Set
- MutableSet
Mappings:
- Mapping
- MutableMapping
- MappingView
- KeysView
- ItemsView
- ValuesView
Miscelaneous:
- Callable
In addition to serving as detailed real-world examples of abstract base classes, Python’s built-in types are automatically registered to these classes when you import collections. This means you can safely use isinstance() to check parameters in your code to ensure that they support the API you need. The base classes can also be used to define your own collection types, since many of them provide concrete implementations of the internals and only need a few methods overridden. Refer to the standard library docs for collections for more details.
See also
- abc
- The standard library documentation for this module.
- PEP 3119
- Introducing Abstract Base Classes
- collections
- The collections module includes abstract base classes for several collection types.
- collections
- The standard library documentation for collections.
- PEP 3141
- A Type Hierarchy for Numbers
- Wikipedia: Strategy Pattern
- Description and examples of the strategy pattern.
- Plugins and monkeypatching
- PyCon 2009 presentation by Dr. André Roberge