[Help] [Newbie] Require help migrating from Perl to Python 2.7 (namespaces)

Dave Angel d at davea.name
Tue Dec 25 00:09:23 CET 2012


(Part 3 of my dissertation;  I hope it's useful for you in particular)

Up to now in my discussion, it wasn't usually important to know that
everything is a class.  You just know that everything has attributes,
and that you use the dot notation to get at an attribute.   So what if
"%x".format() is a class method of the class str, you can just learn
"this is how you can format text".  But once you learn how to write
classes, it starts to all come together.

class  MyClass (myParentClass):

This line, and all indented lines below it, constitute a class.  The
defs inside are functions, but they're also methods, with special
behavior of the first parameter, called 'self' by convention.  So a def
might look like:
      def myMethod(self, arg1):
             return arg1 * 2

Now, if somebody has an instance of your class, they can call this
method like:
      myinstance.myMethod(42)
and the return value will be 84.  This is remarkably like the notation
we used before inside a module.  So a class *could* be used just to
encode the namespace. But the power comes when there are attributes on
the object, and when self is used to get at those attributes.  In that
situation, the self can refer to all kinds of data.  In a sense you
could think of that data as being like the globals of a module, except
for one very important thing.  There's only one instance of the module,
so those globals are shared between everyone.  But with an instance, you
can have many instances, and each has its own set of attributes.

Simplest way to illustrate this is with a MyFile class.  There's already
a very nice class in the standard library, with a builtin function
open() to create a new instance.  But we can pretend to write such a
class, and see how making it a class is probably better than any other
way to code the functionality.  And in fact, many people have done
something like that, to reference something analogous to a file.

When we open a class, some code somewhere has to keep track of the
system's file handle, the file position, the mode of operation, any
buffers that might be used, etc.  Whatever that data is, if it's kept in
an instance, then it's possible to open multiple files at the same time,
some for writing, some for reading, etc.  So how might we go about doing
that?

class  MyFile(object):
     def __init__(self, filename, mode):
         self.filename = filename    #remember the filename, in case
someone wants it
         self.handle =   someOScall(filename, mode)    #do whatever it
might take to actually open a file
         self.position = 0
         self.opened = True
     def read(self, size):
         data = someOScallToRead(self.handle, self.size)    #do whatever
it takes to read some data
         self.position += size               #probably it's more
complicated than this
         return data

Now __init__() is a special method name.  It's called implicitly when an
object of this class is made.  A newly created dummy object of the right
type is passed to __init__() as self, and we want to stuff that object
with the appropriate attributes (typically a dozen or more).

read() is a simple method, although it could be *much* more complex for
a real file.  It might do buffering, or it might translate characters,
or almost anything.  But by the time it's called, we know the object's
__init__() method has been called, a file is open, the handle
initialized, etc.

So now the user can create two different objects:
     file1 = MyFile("firstfile.txt", 12)
     file2 = MyFile("otherdirectory/secondfile.txt", 49)

and can unambiguously read from whichever one she likes.

This notion that the attributes of the object carries all its necessary
data, and that the user of the class need not know any of the details is
the reason one can readily write code in separate modules that knows
little about the internals.  Just pass the necessary object around, and
if the class was done right, it'll be ready to do all the operations
defined on it.

Hope this helps. It's just barely scratched the surface of what's possible.


-- 

DaveA




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