attributes, properties, and accessors -- philosophy
bruno.42.desthuilliers at websiteburo.invalid
Wed Nov 25 10:55:05 CET 2009
Ethan Furman a écrit :
> Let's head towards murkier waters (at least murkier to me -- hopefully
> they can be easily clarified): some of the attributes are read-only,
> such as record count; others are not directly exposed, but still
> settable, such as table version; and still others require a small amount
> of processing... at which point do I switch from simple attribute access
> to method access?
Short answer : you don't !-)
Long answer : well, in fact you do, but the client code doesn't have to
be aware that it's in fact calling an accessor.
Before we go into more details, you have to know that Python has a
pretty good support for computed attributes, with both a simple generic
solution (the property type) and the full monty (custom types
implementing the descriptor protocol). So from the "interface" POV, you
should never have an explicit accessor method for what is semantically
an attribute (wheter the attribute is a plain or a computed one being
part of the implementation).
Let's start with your second point: "not directly exposed but still
settable". I assume you mean "not part of the interface, only supposed
to be accessed (rw) from the methods" - if not, please pardon my
stupidity and provide better explanations !-). If yes: Python doesn't
have "language inforced" access restrictions (private / protected /
etc), but a *very strong* naming convention which is that names starting
with a leading underscore are implementation details, not part of the
official interface, and shouldn't be accessed directly. Kind of a
"warranty voided if unsealed".
So if you have attributes you don't want to "expose" to the outside
world, just add a single leading underscore to their names.
First and third points are solved by using computed attributes - usually
a property. The property type takes a few accessor functions as
arguments - typically, a getter and a setter, and eventually a
"deleter". Used as a class attribute, a property instance will hook up
into the attribute lookup / setup mechanism (__getattribute__ and
__setattr__), and will call resp. it's getter or setter function,
passing it the instance and (for the setter) value.
This directly solves the third point. For the first one, the obvious
solution is to use a property with a setter that raises an exception -
canonically, an AttributeError with a message explaining that the
attribute is read-only.
And for something more hands-on:
def __init__(self, firstname, lastname, birthdate):
self.firstname = firstname
self.lastname = lastnale
self.birthdate = birthdate
self._foo = 42 # implementation only
return "%s %s" % (self.firstname, self.lastname)
def _setfullname(self, value):
raise AttributeError("%s.fullname is read-only" % type(self)
fullname = property(fget=_getfullname, fset=_setfullname)
def _setage(self, value):
raise AttributeError("%s.age is read-only" % type(self)
age = property(fget=_getage, fset=_setage)
For more on computed attributes, you may want to read about the
"descriptor protocol" (google is your friend as usual). This and the
attribute resolution mechanism are fundamental parts of Python's inner
working. Learn how it works if you really want to leverage Python's power.
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