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<H1><A NAME="SECTION006100000000000000000"></A><A NAME="naming"></A>
<BR>
4.1 Naming and binding 
</H1>
<a id='l2h-306' xml:id='l2h-306'></a><a id='l2h-314' xml:id='l2h-314'></a>

<P>
<i class="dfn">Names</i><a id='l2h-315' xml:id='l2h-315'></a> refer to objects.  Names are introduced by
name binding operations.  Each occurrence of a name in the program
text refers to the <i class="dfn">binding</i><a id='l2h-307' xml:id='l2h-307'></a> of that name
established in the innermost function block containing the use.

<P>
A <i class="dfn">block</i><a id='l2h-316' xml:id='l2h-316'></a> is a piece of Python program text that is
executed as a unit.  The following are blocks: a module, a function
body, and a class definition.  Each command typed interactively is a
block.  A script file (a file given as standard input to the
interpreter or specified on the interpreter command line the first
argument) is a code block.  A script command (a command specified on
the interpreter command line with the `<strong>-c</strong>' option) is a code
block.  The file read by the built-in function <tt class="function">execfile()</tt>
is a code block.  The string argument passed to the built-in function
<tt class="function">eval()</tt> and to the <tt class="keyword">exec</tt> statement is a code block.
The expression read and evaluated by the built-in function
<tt class="function">input()</tt> is a code block.

<P>
A code block is executed in an <i class="dfn">execution
frame</i><a id='l2h-308' xml:id='l2h-308'></a>.  A frame contains some
administrative information (used for debugging) and determines where
and how execution continues after the code block's execution has
completed.

<P>
A <i class="dfn">scope</i><a id='l2h-317' xml:id='l2h-317'></a> defines the visibility of a name within a
block.  If a local variable is defined in a block, its scope includes
that block.  If the definition occurs in a function block, the scope
extends to any blocks contained within the defining one, unless a
contained block introduces a different binding for the name.  The
scope of names defined in a class block is limited to the class block;
it does not extend to the code blocks of methods.

<P>
When a name is used in a code block, it is resolved using the nearest
enclosing scope.  The set of all such scopes visible to a code block
is called the block's <i class="dfn">environment</i><a id='l2h-318' xml:id='l2h-318'></a>.  

<P>
If a name is bound in a block, it is a local variable of that block.
If a name is bound at the module level, it is a global variable.  (The
variables of the module code block are local and global.)  If a
variable is used in a code block but not defined there, it is a
<i class="dfn">free variable</i><a id='l2h-309' xml:id='l2h-309'></a>.

<P>
When a name is not found at all, a
<tt class="exception">NameError</tt><a id='l2h-311' xml:id='l2h-311'></a> exception is raised.  If the name
refers to a local variable that has not been bound, a
<tt class="exception">UnboundLocalError</tt><a id='l2h-319' xml:id='l2h-319'></a> exception is
raised.  <tt class="exception">UnboundLocalError</tt> is a subclass of
<tt class="exception">NameError</tt>.

<P>
The following constructs bind names: formal parameters to functions,
<tt class="keyword">import</tt> statements, class and function definitions (these
bind the class or function name in the defining block), and targets
that are identifiers if occurring in an assignment, <tt class="keyword">for</tt> loop
header, or in the second position of an <tt class="keyword">except</tt> clause
header.  The <tt class="keyword">import</tt> statement of the form ``"<tt class="samp">from
...import *</tt>"''<a id='l2h-312' xml:id='l2h-312'></a> binds all names defined in the
imported module, except those beginning with an underscore.  This form
may only be used at the module level.

<P>
A target occurring in a <tt class="keyword">del</tt> statement is also considered bound
for this purpose (though the actual semantics are to unbind the
name).  It is illegal to unbind a name that is referenced by an
enclosing scope; the compiler will report a <tt class="exception">SyntaxError</tt>.

<P>
Each assignment or import statement occurs within a block defined by a
class or function definition or at the module level (the top-level
code block).

<P>
If a name binding operation occurs anywhere within a code block, all
uses of the name within the block are treated as references to the
current block.  This can lead to errors when a name is used within a
block before it is bound.
This rule is subtle.  Python lacks declarations and allows
name binding operations to occur anywhere within a code block.  The
local variables of a code block can be determined by scanning the
entire text of the block for name binding operations.

<P>
If the global statement occurs within a block, all uses of the name
specified in the statement refer to the binding of that name in the
top-level namespace.  Names are resolved in the top-level namespace by
searching the global namespace, i.e. the namespace of the module
containing the code block, and the builtin namespace, the namespace of
the module <tt class="module">__builtin__</tt>.  The global namespace is searched
first.  If the name is not found there, the builtin namespace is
searched.  The global statement must precede all uses of the name.

<P>
The built-in namespace associated with the execution of a code block
is actually found by looking up the name <code>__builtins__</code> in its
global namespace; this should be a dictionary or a module (in the
latter case the module's dictionary is used).  Normally, the
<code>__builtins__</code> namespace is the dictionary of the built-in module
<tt class="module">__builtin__</tt> (note: no `s').  If it isn't, restricted
execution<a id='l2h-313' xml:id='l2h-313'></a> mode is in effect.

<P>
The namespace for a module is automatically created the first time a
module is imported.  The main module for a script is always called
<tt class="module">__main__</tt><a id='l2h-320' xml:id='l2h-320'></a>.

<P>
The global statement has the same scope as a name binding operation
in the same block.  If the nearest enclosing scope for a free variable
contains a global statement, the free variable is treated as a global.

<P>
A class definition is an executable statement that may use and define
names.  These references follow the normal rules for name resolution.
The namespace of the class definition becomes the attribute dictionary
of the class.  Names defined at the class scope are not visible in
methods. 

<P>

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	54
	55	<H1><A NAME="SECTION006100000000000000000"></A><A NAME="naming"></A>
	56	<BR>
	57	4.1 Naming and binding
	58	</H1>
	59	<a id='l2h-306' xml:id='l2h-306'></a><a id='l2h-314' xml:id='l2h-314'></a>
	60
	61	<P>
	62	<i class="dfn">Names</i><a id='l2h-315' xml:id='l2h-315'></a> refer to objects. Names are introduced by
	63	name binding operations. Each occurrence of a name in the program
	64	text refers to the <i class="dfn">binding</i><a id='l2h-307' xml:id='l2h-307'></a> of that name
65	established in the innermost function block containing the use.
66
67	<P>
68	A <i class="dfn">block</i><a id='l2h-316' xml:id='l2h-316'></a> is a piece of Python program text that is
69	executed as a unit. The following are blocks: a module, a function
70	body, and a class definition. Each command typed interactively is a
71	block. A script file (a file given as standard input to the
72	interpreter or specified on the interpreter command line the first
73	argument) is a code block. A script command (a command specified on
74	the interpreter command line with the `<strong>-c</strong>' option) is a code
75	block. The file read by the built-in function <tt class="function">execfile()</tt>
76	is a code block. The string argument passed to the built-in function
77	<tt class="function">eval()</tt> and to the <tt class="keyword">exec</tt> statement is a code block.
78	The expression read and evaluated by the built-in function
79	<tt class="function">input()</tt> is a code block.
80
81	<P>
82	A code block is executed in an <i class="dfn">execution
83	frame</i><a id='l2h-308' xml:id='l2h-308'></a>. A frame contains some
84	administrative information (used for debugging) and determines where
85	and how execution continues after the code block's execution has
86	completed.
87
88	<P>
89	A <i class="dfn">scope</i><a id='l2h-317' xml:id='l2h-317'></a> defines the visibility of a name within a
90	block. If a local variable is defined in a block, its scope includes
91	that block. If the definition occurs in a function block, the scope
92	extends to any blocks contained within the defining one, unless a
93	contained block introduces a different binding for the name. The
94	scope of names defined in a class block is limited to the class block;
95	it does not extend to the code blocks of methods.
96
97	<P>
98	When a name is used in a code block, it is resolved using the nearest
99	enclosing scope. The set of all such scopes visible to a code block
100	is called the block's <i class="dfn">environment</i><a id='l2h-318' xml:id='l2h-318'></a>.
101
102	<P>
103	If a name is bound in a block, it is a local variable of that block.
104	If a name is bound at the module level, it is a global variable. (The
105	variables of the module code block are local and global.) If a
106	variable is used in a code block but not defined there, it is a
107	<i class="dfn">free variable</i><a id='l2h-309' xml:id='l2h-309'></a>.
108
109	<P>
110	When a name is not found at all, a
111	<tt class="exception">NameError</tt><a id='l2h-311' xml:id='l2h-311'></a> exception is raised. If the name
112	refers to a local variable that has not been bound, a
113	<tt class="exception">UnboundLocalError</tt><a id='l2h-319' xml:id='l2h-319'></a> exception is
114	raised. <tt class="exception">UnboundLocalError</tt> is a subclass of
115	<tt class="exception">NameError</tt>.
116
117	<P>
118	The following constructs bind names: formal parameters to functions,
119	<tt class="keyword">import</tt> statements, class and function definitions (these
120	bind the class or function name in the defining block), and targets
121	that are identifiers if occurring in an assignment, <tt class="keyword">for</tt> loop
122	header, or in the second position of an <tt class="keyword">except</tt> clause
123	header. The <tt class="keyword">import</tt> statement of the form ``"<tt class="samp">from
124	...import *</tt>"''<a id='l2h-312' xml:id='l2h-312'></a> binds all names defined in the
125	imported module, except those beginning with an underscore. This form
126	may only be used at the module level.
127
128	<P>
129	A target occurring in a <tt class="keyword">del</tt> statement is also considered bound
130	for this purpose (though the actual semantics are to unbind the
131	name). It is illegal to unbind a name that is referenced by an
132	enclosing scope; the compiler will report a <tt class="exception">SyntaxError</tt>.
133
134	<P>
135	Each assignment or import statement occurs within a block defined by a
136	class or function definition or at the module level (the top-level
137	code block).
138
139	<P>
140	If a name binding operation occurs anywhere within a code block, all
141	uses of the name within the block are treated as references to the
142	current block. This can lead to errors when a name is used within a
143	block before it is bound.
144	This rule is subtle. Python lacks declarations and allows
145	name binding operations to occur anywhere within a code block. The
146	local variables of a code block can be determined by scanning the
147	entire text of the block for name binding operations.
148
149	<P>
150	If the global statement occurs within a block, all uses of the name
151	specified in the statement refer to the binding of that name in the
152	top-level namespace. Names are resolved in the top-level namespace by
153	searching the global namespace, i.e. the namespace of the module
154	containing the code block, and the builtin namespace, the namespace of
155	the module <tt class="module">__builtin__</tt>. The global namespace is searched
156	first. If the name is not found there, the builtin namespace is
157	searched. The global statement must precede all uses of the name.
158
159	<P>
160	The built-in namespace associated with the execution of a code block
161	is actually found by looking up the name <code>__builtins__</code> in its
162	global namespace; this should be a dictionary or a module (in the
163	latter case the module's dictionary is used). Normally, the
164	<code>__builtins__</code> namespace is the dictionary of the built-in module
165	<tt class="module">__builtin__</tt> (note: no `s'). If it isn't, restricted
166	execution<a id='l2h-313' xml:id='l2h-313'></a> mode is in effect.
167
168	<P>
169	The namespace for a module is automatically created the first time a
170	module is imported. The main module for a script is always called
171	<tt class="module">__main__</tt><a id='l2h-320' xml:id='l2h-320'></a>.
172
173	<P>
174	The global statement has the same scope as a name binding operation
175	in the same block. If the nearest enclosing scope for a free variable
176	contains a global statement, the free variable is treated as a global.
177
178	<P>
179	A class definition is an executable statement that may use and define
180	names. These references follow the normal rules for name resolution.
181	The namespace of the class definition becomes the attribute dictionary
182	of the class. Names defined at the class scope are not visible in
183	methods.
184
185	<P>
186
187	<p><br /></p><hr class='online-navigation' />
188	<div class='online-navigation'>
189	<!--Table of Child-Links-->
190	<A NAME="CHILD_LINKS"><STRONG>Subsections</STRONG></a>
191
192	<UL CLASS="ChildLinks">
193	<LI><A href="dynamic-features.html">4.1.1 Interaction with dynamic features</a>
194	</ul>
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