[Python-3000] An introduction to ABC's
talin at acm.org
Sat Apr 14 19:57:28 CEST 2007
Part of the reason why I haven't volunteered to write a PEP for ABC's is
because I don't feel that I understand the various proposals and the
background discussion well enough. However, it occurs to me that writing
the rationale section of a PEP can often be the hardest part, and I
think I understand the issues well enough to write that preface. So
here's my contribution:
In the domain of object-oriented programming, the usage patterns for
interacting with an object can be divided into two basic categories,
which are 'invocation' and 'inspection'.
Invocation means interacting with an object by invoking its methods.
Usually this is combined with polymorphism, so that invoking a given
method may run different code depending on the type of an object.
Inspection means the ability for external code (outside of the object's
methods) to examine the type or properties of that object, and make
decisions on how to treat that object based on that information.
Both usage patterns serve the same general end, which is to be able to
support the processing of diverse and potentially novel objects in a
uniform way, but at the same time allowing processing decisions to be
customized for each different type of object.
In classical OOP theory, invocation is the preferred usage pattern, and
inspection is actively discouraged, being considered a relic of an
earlier, procedural programming style. However, in practice this view is
simply too dogmatic and inflexible, and leads to a kind of design
rigidity that is very much at odds with the dynamic nature of a language
In particular, there is often a need to process objects in a way that
wasn't anticipated by the creator of the object class. It is not always
the best solution to build in to every object methods that satisfy the
needs of every possible user of that object. Moreover, there are many
powerful dispatch philosophies that are in direct contrast to the
classic OOP requirement of behavior being strictly encapsulated within
an object, examples being rule or pattern-match driven logic.
On the the other hand, one of the criticisms of inspection by classic
OOP theorists is the lack of formalisms and the ad hoc nature of what is
being inspected. In a language such as Python, in which almost any
aspect of an object can be reflected and directly accessed by external
code, there are many different ways to test whether an object conforms
to a particular protocol or not. For example, if asking 'is this object
a mutable sequence container?', one can look for a base class of 'list',
or one can look for a method named '__getitem__'. But note that although
these tests may seem obvious, neither of them are correct, as one
generates false negatives, and the other false positives.
The generally agreed-upon remedy is to standardize the tests, and group
them into a formal arrangement. This is most easily done by associating
with each class a set of standard testable properties, either via the
inheritance mechanism or some other means. Each test carries with it a
set of promises: it contains a promise about the general behavior of the
class, and a promise as to what other class methods will be available.
This PEP proposes a particular strategy for organizing these tests known
as Abstract Base Classes, or ABC. ABCs are simply Python classes that
are added into an object's inheritance tree to signal certain features
of that object to an external inspector. Tests are done using
isinstance(), and the presence of a particular ABC means that the test
Like all other things in Python, these promises are in the nature of a
gentlemen's agreement - which means that the language does not attempt
to enforce that these promises are kept.
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