About OOP

Author: guibog

docs/writing/structure.rst

@@ -172,6 +172,83 @@ Lastly, a convenient syntax is available for importing deeply nested packages:
 `import very.deep.module as mod` allow to use `mod` in place of the verbose
 repetition of `very.deep.module` in front of each calls to module items.
 
+Object-oriented programming
+---------------------------
+
+Python is sometime described as an object-oriented programming language. This
+can be somewhat misleading and need to be clarified.
+
+In Python, everything is an object, and can be handled as such. This is what is
+meant when we say that, for example, functions are first-class objects.
+Functions, classes, strings, and even types are objects in Python: like any
+objects, they have a type, they can be passed as function arguments, they may
+have methods and properties. In this understanding, Python is an
+object-oriented language.
+
+However, unlike Java, Python do not impose object-oriented programming as the
+main programming paradigm. It is perfectly viable for a Python project to not
+be object-oriented, ie. to use no or very few class definitions, class
+inheritance, and any other mechanism that are specific to object-oriented
+programming.
+
+Moreover, as seen in the modules_ section, the way Python handles modules and
+namespaces gives directly to the developer a natural way to ensure
+encapsulation and separation of abstraction layers, both being the most common
+reasons to use object-orientation. Therefore, Python programmers have more
+latitude to not use object-orientation, when it is not required by the business
+model to be constructed.
+
+There are some reasons to avoid unnecessary object-orientation. Definining
+custom classes is useful when we want to glue together some state and some
+functionality.  The problem, as pointed out by the discussions about functional
+programming, comes from the "state" part of the equation.
+
+In some architectures, typically web applications, instances of Python
+processes are spawned simultaneously to answer to external requests that can
+happen at the same time. In this case, holding some state into instanciated
+objects, which means keeping some static information about the world, is prone
+to concurrency problems or race-conditions: between the initialization of the
+state of an object, usually done with the __init__() method, and the actual use
+of the object state through one of its method, the world may have changed, and
+the retained state may be outdated. For example, a request may load an item in
+memory and mark it as read by a user. If another request requires the deletion
+of this item at the same, it may happen that the deletion actually occur after
+the first process loaded the item, and then we have to mark as read a deleted
+object.
+
+This and other issues led to the idea that using stateless functions is a
+better programming paradigm.
+
+Another way to say the same thing is to propose to use functions and procedures
+with as few implicit context and side-effects as possible.  A function's
+implicit context is decelable when the function body refers to some global
+variables or fetches data from the persistence layer. Side-effects are the
+opposite: if a function body modifies the global context or save or delete data
+on the persistence layer, it is said to have side-effect.
+
+Isolating carefully functions with context and side-effects from functions with
+logic (called pure functions) allow the following benefits:
+
+- Pure functions are more likely to be deterministic: given a fixed input,
+  the output will always be the same.
+
+- Pure functions are much easier to change or replace if they need to
+  be refactored or optimized.
+
+- Pure functions are easier to test with unit-tests: There is less
+  need for complex context setup and data cleaning afterwards.
+
+- Pure functions are easier to manipulate, decorate_, pass-around.
+
+In summary, pure functions, without any context or side-effects, are more
+efficient building blocks than classes and objects for some architectures.
+
+Obviously, object-orientation is useful and even necessary in many cases, for
+example when developing graphical desktop applications or games, where the
+things that are manipulated (windows, buttons, avatars, vehicles) have a
+relatively long life of their own in the computer's memory.
+
+
 Vendorizing Dependencies
 ------------------------