When referring to namespaces, always say "namespaces" instead of
"name spaces". Inconsistency noted by Keith Briggs <keith.briggs@bt.com>.
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@ -480,9 +480,9 @@ in \code{cmp_op[\var{opname}]}.
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\begin{opcodedesc}{IMPORT_NAME}{namei}
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Imports the module \code{co_names[\var{namei}]}. The module object is
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pushed onto the stack. The current name space is not affected: for a
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pushed onto the stack. The current namespace is not affected: for a
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proper import statement, a subsequent \code{STORE_FAST} instruction
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modifies the name space.
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modifies the namespace.
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\end{opcodedesc}
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\begin{opcodedesc}{IMPORT_FROM}{namei}
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@ -227,7 +227,7 @@ class instances are callable if they have a \method{__call__()} method.
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The arguments are a file name and two optional dictionaries. The
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file is parsed and evaluated as a sequence of Python statements
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(similarly to a module) using the \var{globals} and \var{locals}
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dictionaries as global and local name space. If the \var{locals}
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dictionaries as global and local namespace. If the \var{locals}
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dictionary is omitted it defaults to the \var{globals} dictionary.
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If both dictionaries are omitted, the expression is executed in the
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environment where \function{execfile()} is called. The return value is
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@ -888,7 +888,7 @@ different object types.
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The implementation adds two special read-only attributes:
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\code{\var{f}.func_code} is a function's \dfn{code
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object}\obindex{code} (see below) and \code{\var{f}.func_globals} is
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the dictionary used as the function's global name space (this is the
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the dictionary used as the function's global namespace (this is the
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same as \code{\var{m}.__dict__} where \var{m} is the module in which
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the function \var{f} was defined).
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@ -317,7 +317,7 @@ commands. This is similar to the \file{.profile} feature of the
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This file is only read in interactive sessions, not when Python reads
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commands from a script, and not when \file{/dev/tty} is given as the
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explicit source of commands (which otherwise behaves like an
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interactive session). It is executed in the same name space where
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interactive session). It is executed in the same namespace where
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interactive commands are executed, so that objects that it defines or
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imports can be used without qualification in the interactive session.
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You can also change the prompts \code{sys.ps1} and \code{sys.ps2} in
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@ -3236,24 +3236,24 @@ Pascal.
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Before introducing classes, I first have to tell you something about
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Python's scope rules. Class definitions play some neat tricks with
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name spaces, and you need to know how scopes and name spaces work to
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namespaces, and you need to know how scopes and namespaces work to
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fully understand what's going on. Incidentally, knowledge about this
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subject is useful for any advanced Python programmer.
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Let's begin with some definitions.
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A \emph{name space} is a mapping from names to objects. Most name
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spaces are currently implemented as Python dictionaries, but that's
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normally not noticeable in any way (except for performance), and it
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may change in the future. Examples of name spaces are: the set of
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built-in names (functions such as \function{abs()}, and built-in exception
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names); the global names in a module; and the local names in a
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function invocation. In a sense the set of attributes of an object
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also form a name space. The important thing to know about name
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spaces is that there is absolutely no relation between names in
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different name spaces; for instance, two different modules may both
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define a function ``maximize'' without confusion --- users of the
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modules must prefix it with the module name.
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A \emph{namespace} is a mapping from names to objects. Most
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namespaces are currently implemented as Python dictionaries, but
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that's normally not noticeable in any way (except for performance),
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and it may change in the future. Examples of namespaces are: the set
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of built-in names (functions such as \function{abs()}, and built-in
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exception names); the global names in a module; and the local names in
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a function invocation. In a sense the set of attributes of an object
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also form a namespace. The important thing to know about namespaces
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is that there is absolutely no relation between names in different
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namespaces; for instance, two different modules may both define a
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function ``maximize'' without confusion --- users of the modules must
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prefix it with the module name.
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By the way, I use the word \emph{attribute} for any name following a
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dot --- for example, in the expression \code{z.real}, \code{real} is
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@ -3262,13 +3262,13 @@ names in modules are attribute references: in the expression
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\code{modname.funcname}, \code{modname} is a module object and
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\code{funcname} is an attribute of it. In this case there happens to
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be a straightforward mapping between the module's attributes and the
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global names defined in the module: they share the same name
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space!\footnote{
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global names defined in the module: they share the same namespace!
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\footnote{
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Except for one thing. Module objects have a secret read-only
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attribute called \code{__dict__} which returns the dictionary
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used to implement the module's name space; the name
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\code{__dict__} is an attribute but not a global name.
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Obviously, using this violates the abstraction of name space
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attribute called \member{__dict__} which returns the dictionary
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used to implement the module's namespace; the name
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\member{__dict__} is an attribute but not a global name.
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Obviously, using this violates the abstraction of namespace
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implementation, and should be restricted to things like
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post-mortem debuggers.
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}
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@ -3280,54 +3280,54 @@ also be deleted with the \keyword{del} statement, e.g.
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\samp{del modname.the_answer}.
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Name spaces are created at different moments and have different
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lifetimes. The name space containing the built-in names is created
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lifetimes. The namespace containing the built-in names is created
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when the Python interpreter starts up, and is never deleted. The
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global name space for a module is created when the module definition
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is read in; normally, module name spaces also last until the
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global namespace for a module is created when the module definition
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is read in; normally, module namespaces also last until the
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interpreter quits. The statements executed by the top-level
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invocation of the interpreter, either read from a script file or
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interactively, are considered part of a module called
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\module{__main__}, so they have their own global name space. (The
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\module{__main__}, so they have their own global namespace. (The
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built-in names actually also live in a module; this is called
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\module{__builtin__}.)
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The local name space for a function is created when the function is
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The local namespace for a function is created when the function is
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called, and deleted when the function returns or raises an exception
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that is not handled within the function. (Actually, forgetting would
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be a better way to describe what actually happens.) Of course,
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recursive invocations each have their own local name space.
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recursive invocations each have their own local namespace.
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A \emph{scope} is a textual region of a Python program where a name space
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is directly accessible. ``Directly accessible'' here means that an
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unqualified reference to a name attempts to find the name in the name
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space.
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A \emph{scope} is a textual region of a Python program where a
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namespace is directly accessible. ``Directly accessible'' here means
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that an unqualified reference to a name attempts to find the name in
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the namespace.
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Although scopes are determined statically, they are used dynamically.
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At any time during execution, exactly three nested scopes are in use
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(i.e., exactly three name spaces are directly accessible): the
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(i.e., exactly three namespaces are directly accessible): the
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innermost scope, which is searched first, contains the local names,
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the middle scope, searched next, contains the current module's global
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names, and the outermost scope (searched last) is the name space
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names, and the outermost scope (searched last) is the namespace
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containing built-in names.
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Usually, the local scope references the local names of the (textually)
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current function. Outside of functions, the local scope references
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the same name space as the global scope: the module's name space.
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Class definitions place yet another name space in the local scope.
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the same namespace as the global scope: the module's namespace.
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Class definitions place yet another namespace in the local scope.
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It is important to realize that scopes are determined textually: the
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global scope of a function defined in a module is that module's name
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space, no matter from where or by what alias the function is called.
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On the other hand, the actual search for names is done dynamically, at
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run time --- however, the language definition is evolving towards
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static name resolution, at ``compile'' time, so don't rely on dynamic
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name resolution! (In fact, local variables are already determined
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statically.)
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global scope of a function defined in a module is that module's
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namespace, no matter from where or by what alias the function is
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called. On the other hand, the actual search for names is done
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dynamically, at run time --- however, the language definition is
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evolving towards static name resolution, at ``compile'' time, so don't
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rely on dynamic name resolution! (In fact, local variables are
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already determined statically.)
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A special quirk of Python is that assignments always go into the
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innermost scope. Assignments do not copy data --- they just
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bind names to objects. The same is true for deletions: the statement
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\samp{del x} removes the binding of \code{x} from the name space
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\samp{del x} removes the binding of \code{x} from the namespace
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referenced by the local scope. In fact, all operations that introduce
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new names use the local scope: in particular, import statements and
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function definitions bind the module or function name in the local
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@ -3366,14 +3366,14 @@ inside a class normally have a peculiar form of argument list,
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dictated by the calling conventions for methods --- again, this is
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explained later.
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When a class definition is entered, a new name space is created, and
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When a class definition is entered, a new namespace is created, and
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used as the local scope --- thus, all assignments to local variables
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go into this new name space. In particular, function definitions bind
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go into this new namespace. In particular, function definitions bind
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the name of the new function here.
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When a class definition is left normally (via the end), a \emph{class
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object} is created. This is basically a wrapper around the contents
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of the name space created by the class definition; we'll learn more
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of the namespace created by the class definition; we'll learn more
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about class objects in the next section. The original local scope
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(the one in effect just before the class definitions was entered) is
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reinstated, and the class object is bound here to the class name given
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@ -3387,7 +3387,7 @@ and instantiation.
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\emph{Attribute references} use the standard syntax used for all
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attribute references in Python: \code{obj.name}. Valid attribute
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names are all the names that were in the class's name space when the
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names are all the names that were in the class's namespace when the
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class object was created. So, if the class definition looked like
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this:
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@ -3766,9 +3766,10 @@ code that is byte-compiled together. The same restriction applies to
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when referencing \code{__dict__} directly.
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Here's an example of a class that implements its own
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\code{__getattr__} and \code{__setattr__} methods and stores all
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attributes in a private variable, in a way that works in Python 1.4 as
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well as in previous versions:
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\method{__getattr__()} and \method{__setattr__()} methods and stores
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all attributes in a private variable, in a way that works in all
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versions of Python, including those available before this feature was
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added:
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\begin{verbatim}
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class VirtualAttributes:
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