[Tutor] namespaces and the __dict__ function?
kent37 at tds.net
Fri Jan 25 20:15:22 CET 2008
John Gunderman wrote:
> I am new to python and have found both the concept of namespaces and the
> __dict__ function to be rather confusing, and I cant find any good
> explanations on the web. Could any of you give me a good explanation of
> these? And for __dict__, is is the same thing as __str__ except in
> string form, or does it store all current objects of that class in a
First, __dict__ is an attribute of an object whose value is a dict, not
A namespace is a place where names can be defined. The official tutorial
A namespace is a mapping from names to objects. Most namespaces are
currently implemented as Python dictionaries... Examples of namespaces
are: the set of built-in names (functions such as abs(), and built-in
exception names); the global names in a module; and the local names in a
function invocation. In a sense the set of attributes of an object also
form a namespace.
Another way to think of it is, a namespace is a place where names are
looked up. When you use a bare name (not an attribute), it is looked up
in the local namespace, then the global namespace, then the built-in
namespace. For example:
y = 2 # Defines y in the global (module) namespace
x = 1 # Defines x in the local (function) namespace
# This looks up x, finds it in the local namespace
# looks up abs, finds it in the built-in namespace
# This looks up y, finds it in the global namespace
Note that none of the above namespaces have a related __dict__
attribute, the namespace mappings are not stored that way.
Objects also define a sort of namespace, where attributes are defined
and looked up. The dict containing the namespace of an object is itself
stored as an attribute of the object, called __dict__. So __dict__ is an
implementation detail of the way object attributes are stored.
As a beginner, it is important to understand the way bare names are
looked up (local, global, built-in namespace) and a bit about the way
attributes work. You don't have to be concerned with the implementation
details such as __dict__.
(For completeness, I will say that the full attribute lookup model is
much more complicated than I have indicated above. I have simplified to
focus on __dict__.)
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