GNU Info

(python2.1-tut.info)Python Scopes and Name Spaces

---

Next: A First Look at Classes Prev: A Word About Terminology Up: Classes

---

Enter node , (file) or (file)node

Python Scopes and Name Spaces
=============================

Before introducing classes, I first have to tell you something about
Python's scope rules.  Class definitions play some neat tricks with
namespaces, and you need to know how scopes and namespaces work to
fully understand what's going on.  Incidentally, knowledge about this
subject is useful for any advanced Python programmer.

Let's begin with some definitions.

A _namespace_ is a mapping from names to objects.  Most namespaces are
currently implemented as Python dictionaries, but that's normally not
noticeable in any way (except for performance), and it may change in
the future.  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.  The
important thing to know about namespaces is that there is absolutely no
relation between names in different namespaces; for instance, two
different modules may both define a function "maximize" without
confusion -- users of the modules must prefix it with the module name.

By the way, I use the word _attribute_ for any name following a dot --
for example, in the expression `z.real', `real' is an attribute of the
object `z'.  Strictly speaking, references to names in modules are
attribute references: in the expression `modname.funcname', `modname'
is a module object and `funcname' is an attribute of it.  In this case
there happens to be a straightforward mapping between the module's
attributes and the global names defined in the module: they share the
same namespace!  (1)

Attributes may be read-only or writable.  In the latter case,
assignment to attributes is possible.  Module attributes are writable:
you can write `modname.the_answer = 42'.  Writable attributes may also
be deleted with the `del' statement, e.g.  `del modname.the_answer'.

Name spaces are created at different moments and have different
lifetimes.  The namespace containing the built-in names is created when
the Python interpreter starts up, and is never deleted.  The global
namespace for a module is created when the module definition is read
in; normally, module namespaces also last until the interpreter quits.
The statements executed by the top-level invocation of the interpreter,
either read from a script file or interactively, are considered part of
a module called `__main__', so they have their own global namespace.
(The built-in names actually also live in a module; this is called
`__builtin__'.)

The local namespace for a function is created when the function is
called, and deleted when the function returns or raises an exception
that is not handled within the function.  (Actually, forgetting would
be a better way to describe what actually happens.)  Of course,
recursive invocations each have their own local namespace.

A _scope_ is a textual region of a Python program where a namespace is
directly accessible.  "Directly accessible" here means that an
unqualified reference to a name attempts to find the name in the
namespace.

Although scopes are determined statically, they are used dynamically.
At any time during execution, exactly three nested scopes are in use
(i.e., exactly three namespaces are directly accessible): the innermost
scope, which is searched first, contains the local names, the middle
scope, searched next, contains the current module's global names, and
the outermost scope (searched last) is the namespace containing
built-in names.

Usually, the local scope references the local names of the (textually)
current function.  Outside of functions, the local scope references the
same namespace as the global scope: the module's namespace.  Class
definitions place yet another namespace in the local scope.

It is important to realize that scopes are determined textually: the
global scope of a function defined in a module is that module's
namespace, no matter from where or by what alias the function is
called.  On the other hand, the actual search for names is done
dynamically, at run time -- however, the language definition is
evolving towards static name resolution, at "compile" time, so don't
rely on dynamic name resolution!  (In fact, local variables are already
determined statically.)

A special quirk of Python is that assignments always go into the
innermost scope.  Assignments do not copy data -- they just bind names
to objects.  The same is true for deletions: the statement `del x'
removes the binding of `x' from the namespace referenced by the local
scope.  In fact, all operations that introduce new names use the local
scope: in particular, import statements and function definitions bind
the module or function name in the local scope.  (The `global'
statement can be used to indicate that particular variables live in the
global scope.)

---------- Footnotes ----------

(1)  Except for one thing.  Module objects have a secret read-only
attribute called `__dict__' which returns the dictionary used to
implement the module's namespace; the name `__dict__' is an attribute
but not a global name. Obviously, using this violates the abstraction
of namespace implementation, and should be restricted to things like
post-mortem debuggers.