|An Introduction to Python|
by Guido van Rossum and Fred L. Drake, Jr.
Paperback (6"x9"), 124 pages
RRP £12.95 ($19.95)
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9.2 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
an attribute of the object
z. Strictly speaking, references to
names in modules are attribute references: in the expression
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!(4)
Attributes may be read-only or writable. In the latter case,
assignment to attributes is possible. Module attributes are writable:
you can write assignments such as ‘modname.the_answer = 42’. Writable attributes may
also be deleted with the
del statement. For example,
‘del modname.the_answer’ will remove the attribute
the_answer from the object named by
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, there are at least three nested scopes whose namespaces are directly accessible: the innermost scope, which is searched first, contains the local names; the namespaces of any enclosing functions, which are searched starting with the nearest enclosing scope; 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.
If a name is declared global, then all references and assignments go directly to the middle scope containing the module's global names. Otherwise, all variables found outside of the innermost scope are read-only (an attempt to write to such a variable will simply create a new local variable in the innermost scope, leaving the identically named outer variable unchanged).
Usually, the local scope references the local names of the (textually) current function. Outside 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
global statement can be used to indicate that
particular variables live in the global scope.)
|ISBN 0954161769||An Introduction to Python||See the print edition|