Using the Class Statement
Did all of the above make sense? If not, don’t worry; now that
we’ve had a quick tour, we’re going to dig a bit deeper
and study the concepts we’ve introduced in more detail. We met
the class
statement in our first examples, but let’s formalize some of
the ideas we introduced. As in C++, the class
statement is Python’s main OOP tool. Unlike in C++,
class
isn’t really a declaration; like
def
, class
is an object
builder, and an implicit assignment—when run, it generates a
class object, and stores a reference to it in the name used in the
header.
General Form
As we saw on our quick tour, class
is a compound
statement with a body of indented statements under it. In the header,
superclasses are listed in parentheses after the class name,
separated by commas. Listing more than one superclass leads to
multiple inheritance (which we’ll say more about later in this
chapter):
class <name>(superclass,...): # assign to name data = value # shared class data def method(self,...): # methods self.member = value # per-instance data
Within the class statement, specially-named methods overload
operators; for instance, a function called
_
_ init
__
is called at
instance object construction time, if defined.
Example
At the start of this chapter, we mentioned that classes are mostly just namespaces— a tool for defining names (called attributes) that export data and logic to clients. So how do you get from the statement to a namespace?
Here’s how. Just as with modules, the statements ...
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