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<H2><A NAME="SECTION007340000000000000000"></A><A NAME="calls"></A>
<a id='l2h-386' xml:id='l2h-386'></a>
<BR>
5.3.4 Calls
</H2>

<P>
A call calls a callable object (e.g., a function) with a possibly empty
series of arguments:
<a id='l2h-387' xml:id='l2h-387'></a>
<P>
<dl><dd class="grammar">
<div class="productions">
<table>
<tr>
    <td><a id='tok-call' xml:id='tok-call'>call</a></td>
    <td>::=</td>
    <td><a class='grammartoken' href="primaries.html#tok-primary">primary</a> "(" [<a class='grammartoken' href="calls.html#tok-argument_list">argument_list</a> [","]] ")"</td></tr>
             <a class='grammartoken' href="primaries.html#tok-primary">primary</a> "(" [<a class='grammartoken' href="calls.html#tok-argument_list">argument_list</a> [","] |
	      <a class='grammartoken' href="lists.html#tok-test">test</a> <a class='grammartoken' href="genexpr.html#tok-genexpr_for">genexpr_for</a> ] ")"                                                   
  <tr>
    <td><a id='tok-argument_list' xml:id='tok-argument_list'>argument_list</a></td>
    <td>::=</td>
    <td><a class='grammartoken' href="calls.html#tok-positional_arguments">positional_arguments</a> ["," <a class='grammartoken' href="calls.html#tok-keyword_arguments">keyword_arguments</a>]</td></tr>
  <tr>
    <td></td>
    <td></td>
    <td><code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;["," "*" <a class='grammartoken' href="Booleans.html#tok-expression">expression</a>]</code></td></tr>
  <tr>
    <td></td>
    <td></td>
    <td><code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;["," "**" <a class='grammartoken' href="Booleans.html#tok-expression">expression</a>]</code></td></tr>
  <tr>
    <td></td>
    <td></td>
    <td><code>| <a class='grammartoken' href="calls.html#tok-keyword_arguments">keyword_arguments</a> ["," "*" <a class='grammartoken' href="Booleans.html#tok-expression">expression</a>]</code></td></tr>
  <tr>
    <td></td>
    <td></td>
    <td><code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;["," "**" <a class='grammartoken' href="Booleans.html#tok-expression">expression</a>]</code></td></tr>
  <tr>
    <td></td>
    <td></td>
    <td><code>| "*" <a class='grammartoken' href="Booleans.html#tok-expression">expression</a> ["," "**" <a class='grammartoken' href="Booleans.html#tok-expression">expression</a>]</code></td></tr>
  <tr>
    <td></td>
    <td></td>
    <td><code>| "**" <a class='grammartoken' href="Booleans.html#tok-expression">expression</a></code></td></tr>
  <tr>
    <td><a id='tok-positional_arguments' xml:id='tok-positional_arguments'>positional_arguments</a></td>
    <td>::=</td>
    <td><a class='grammartoken' href="Booleans.html#tok-expression">expression</a> ("," <a class='grammartoken' href="Booleans.html#tok-expression">expression</a>)*</td></tr>
  <tr>
    <td><a id='tok-keyword_arguments' xml:id='tok-keyword_arguments'>keyword_arguments</a></td>
    <td>::=</td>
    <td><a class='grammartoken' href="calls.html#tok-keyword_item">keyword_item</a> ("," <a class='grammartoken' href="calls.html#tok-keyword_item">keyword_item</a>)*</td></tr>
  <tr>
    <td><a id='tok-keyword_item' xml:id='tok-keyword_item'>keyword_item</a></td>
    <td>::=</td>
    <td><a class='grammartoken' href="identifiers.html#tok-identifier">identifier</a> "=" <a class='grammartoken' href="Booleans.html#tok-expression">expression</a></td></tr>
</table>
</div>
<a class="grammar-footer"
  href="grammar.txt" type="text/plain"
  >Download entire grammar as text.</a>
</dd></dl>

<P>
A trailing comma may be present after the positional and keyword
arguments but does not affect the semantics.

<P>
The primary must evaluate to a callable object (user-defined
functions, built-in functions, methods of built-in objects, class
objects, methods of class instances, and certain class instances
themselves are callable; extensions may define additional callable
object types).  All argument expressions are evaluated before the call
is attempted.  Please refer to section <A href="function.html#function">7.5</A> for the syntax
of formal parameter lists.

<P>
If keyword arguments are present, they are first converted to
positional arguments, as follows.  First, a list of unfilled slots is
created for the formal parameters.  If there are N positional
arguments, they are placed in the first N slots.  Next, for each
keyword argument, the identifier is used to determine the
corresponding slot (if the identifier is the same as the first formal
parameter name, the first slot is used, and so on).  If the slot is
already filled, a <tt class="exception">TypeError</tt> exception is raised.
Otherwise, the value of the argument is placed in the slot, filling it
(even if the expression is <code>None</code>, it fills the slot).  When all
arguments have been processed, the slots that are still unfilled are
filled with the corresponding default value from the function
definition.  (Default values are calculated, once, when the function
is defined; thus, a mutable object such as a list or dictionary used
as default value will be shared by all calls that don't specify an
argument value for the corresponding slot; this should usually be
avoided.)  If there are any unfilled slots for which no default value
is specified, a <tt class="exception">TypeError</tt> exception is raised.  Otherwise,
the list of filled slots is used as the argument list for the call.

<P>
If there are more positional arguments than there are formal parameter
slots, a <tt class="exception">TypeError</tt> exception is raised, unless a formal
parameter using the syntax "<tt class="samp">*identifier</tt>" is present; in this
case, that formal parameter receives a tuple containing the excess
positional arguments (or an empty tuple if there were no excess
positional arguments).

<P>
If any keyword argument does not correspond to a formal parameter
name, a <tt class="exception">TypeError</tt> exception is raised, unless a formal
parameter using the syntax "<tt class="samp">**identifier</tt>" is present; in this
case, that formal parameter receives a dictionary containing the
excess keyword arguments (using the keywords as keys and the argument
values as corresponding values), or a (new) empty dictionary if there
were no excess keyword arguments.

<P>
If the syntax "<tt class="samp">*expression</tt>" appears in the function call,
"<tt class="samp">expression</tt>" must evaluate to a sequence.  Elements from this
sequence are treated as if they were additional positional arguments;
if there are postional arguments <var>x1</var>,...,<var>xN</var> , and
"<tt class="samp">expression</tt>" evaluates to a sequence <var>y1</var>,...,<var>yM</var>, this
is equivalent to a call with M+N positional arguments
<var>x1</var>,...,<var>xN</var>,<var>y1</var>,...,<var>yM</var>.

<P>
A consequence of this is that although the "<tt class="samp">*expression</tt>" syntax
appears <em>after</em> any keyword arguments, it is processed
<em>before</em> the keyword arguments (and the
"<tt class="samp">**expression</tt>" argument, if any - see below).  So:

<P>
<div class="verbatim"><pre>
&gt;&gt;&gt; def f(a, b):
...  print a, b
...
&gt;&gt;&gt; f(b=1, *(2,))
2 1
&gt;&gt;&gt; f(a=1, *(2,))
Traceback (most recent call last):
  File "&lt;stdin&gt;", line 1, in ?
TypeError: f() got multiple values for keyword argument 'a'
&gt;&gt;&gt; f(1, *(2,))
1 2
</pre></div>

<P>
It is unusual for both keyword arguments and the
"<tt class="samp">*expression</tt>" syntax to be used in the same call, so in practice
this confusion does not arise.

<P>
If the syntax "<tt class="samp">**expression</tt>" appears in the function call,
"<tt class="samp">expression</tt>" must evaluate to a (subclass of) dictionary, the
contents of which are treated as additional keyword arguments.  In the
case of a keyword appearing in both "<tt class="samp">expression</tt>" and as an
explicit keyword argument, a <tt class="exception">TypeError</tt> exception is
raised.

<P>
Formal parameters using the syntax "<tt class="samp">*identifier</tt>" or
"<tt class="samp">**identifier</tt>" cannot be used as positional argument slots or
as keyword argument names.  Formal parameters using the syntax
"<tt class="samp">(sublist)</tt>" cannot be used as keyword argument names; the
outermost sublist corresponds to a single unnamed argument slot, and
the argument value is assigned to the sublist using the usual tuple
assignment rules after all other parameter processing is done.

<P>
A call always returns some value, possibly <code>None</code>, unless it
raises an exception.  How this value is computed depends on the type
of the callable object.

<P>
If it is--

<P>
<DL>
<DT><STRONG>a user-defined function:</STRONG></DT>
<DD>The code block for the function is
executed, passing it the argument list.  The first thing the code
block will do is bind the formal parameters to the arguments; this is
described in section <A href="function.html#function">7.5</A>.  When the code block executes a
<tt class="keyword">return</tt> statement, this specifies the return value of the
function call.
<a id='l2h-388' xml:id='l2h-388'></a><a id='l2h-389' xml:id='l2h-389'></a><a id='l2h-390' xml:id='l2h-390'></a><a id='l2h-391' xml:id='l2h-391'></a>
<P>
</DD>
<DT><STRONG>a built-in function or method:</STRONG></DT>
<DD>The result is up to the
interpreter; see the <em class="citetitle"><a
 href="../lib/built-in-funcs.html"
 title="Python
Library Reference"
 >Python
Library Reference</a></em> for the descriptions of built-in functions and
methods.
<a id='l2h-392' xml:id='l2h-392'></a><a id='l2h-393' xml:id='l2h-393'></a><a id='l2h-394' xml:id='l2h-394'></a><a id='l2h-395' xml:id='l2h-395'></a><a id='l2h-396' xml:id='l2h-396'></a><a id='l2h-397' xml:id='l2h-397'></a><a id='l2h-398' xml:id='l2h-398'></a><a id='l2h-399' xml:id='l2h-399'></a>
<P>
</DD>
<DT><STRONG>a class object:</STRONG></DT>
<DD>A new instance of that class is returned.
<a id='l2h-400' xml:id='l2h-400'></a><a id='l2h-401' xml:id='l2h-401'></a>
<P>
</DD>
<DT><STRONG>a class instance method:</STRONG></DT>
<DD>The corresponding user-defined
function is called, with an argument list that is one longer than the
argument list of the call: the instance becomes the first argument.
<a id='l2h-402' xml:id='l2h-402'></a><a id='l2h-403' xml:id='l2h-403'></a><a id='l2h-404' xml:id='l2h-404'></a>
<P>
</DD>
<DT><STRONG>a class instance:</STRONG></DT>
<DD>The class must define a <tt class="method">__call__()</tt>
method; the effect is then the same as if that method was called.
<a id='l2h-405' xml:id='l2h-405'></a><a id='l2h-407' xml:id='l2h-407'></a>
<P>
</DD>
</DL>

<P>

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	53
	54	<H2><A NAME="SECTION007340000000000000000"></A><A NAME="calls"></A>
	55	<a id='l2h-386' xml:id='l2h-386'></a>
	56	<BR>
	57	5.3.4 Calls
	58	</H2>
	59
	60	<P>
	61	A call calls a callable object (e.g., a function) with a possibly empty
	62	series of arguments:
	63	<a id='l2h-387' xml:id='l2h-387'></a>
	64	<P>
65	<dl><dd class="grammar">
66	<div class="productions">
67	<table>
68	<tr>
69	<td><a id='tok-call' xml:id='tok-call'>call</a></td>
70	<td>::=</td>
71	<td><a class='grammartoken' href="primaries.html#tok-primary">primary</a> "(" [<a class='grammartoken' href="calls.html#tok-argument_list">argument_list</a> [","]] ")"</td></tr>
72	<a class='grammartoken' href="primaries.html#tok-primary">primary</a> "(" [<a class='grammartoken' href="calls.html#tok-argument_list">argument_list</a> [","] \|
73	<a class='grammartoken' href="lists.html#tok-test">test</a> <a class='grammartoken' href="genexpr.html#tok-genexpr_for">genexpr_for</a> ] ")"
74	<tr>
75	<td><a id='tok-argument_list' xml:id='tok-argument_list'>argument_list</a></td>
76	<td>::=</td>
77	<td><a class='grammartoken' href="calls.html#tok-positional_arguments">positional_arguments</a> ["," <a class='grammartoken' href="calls.html#tok-keyword_arguments">keyword_arguments</a>]</td></tr>
78	<tr>
79	<td></td>
80	<td></td>
81	<td><code>                     ["," "*" <a class='grammartoken' href="Booleans.html#tok-expression">expression</a>]</code></td></tr>
82	<tr>
83	<td></td>
84	<td></td>
85	<td><code>                     ["," "**" <a class='grammartoken' href="Booleans.html#tok-expression">expression</a>]</code></td></tr>
86	<tr>
87	<td></td>
88	<td></td>
89	<td><code>\| <a class='grammartoken' href="calls.html#tok-keyword_arguments">keyword_arguments</a> ["," "*" <a class='grammartoken' href="Booleans.html#tok-expression">expression</a>]</code></td></tr>
90	<tr>
91	<td></td>
92	<td></td>
93	<td><code>                    ["," "**" <a class='grammartoken' href="Booleans.html#tok-expression">expression</a>]</code></td></tr>
94	<tr>
95	<td></td>
96	<td></td>
97	<td><code>\| "" <a class='grammartoken' href="Booleans.html#tok-expression">expression</a> ["," "*" <a class='grammartoken' href="Booleans.html#tok-expression">expression</a>]</code></td></tr>
98	<tr>
99	<td></td>
100	<td></td>
101	<td><code>\| "**" <a class='grammartoken' href="Booleans.html#tok-expression">expression</a></code></td></tr>
102	<tr>
103	<td><a id='tok-positional_arguments' xml:id='tok-positional_arguments'>positional_arguments</a></td>
104	<td>::=</td>
105	<td><a class='grammartoken' href="Booleans.html#tok-expression">expression</a> ("," <a class='grammartoken' href="Booleans.html#tok-expression">expression</a>)*</td></tr>
106	<tr>
107	<td><a id='tok-keyword_arguments' xml:id='tok-keyword_arguments'>keyword_arguments</a></td>
108	<td>::=</td>
109	<td><a class='grammartoken' href="calls.html#tok-keyword_item">keyword_item</a> ("," <a class='grammartoken' href="calls.html#tok-keyword_item">keyword_item</a>)*</td></tr>
110	<tr>
111	<td><a id='tok-keyword_item' xml:id='tok-keyword_item'>keyword_item</a></td>
112	<td>::=</td>
113	<td><a class='grammartoken' href="identifiers.html#tok-identifier">identifier</a> "=" <a class='grammartoken' href="Booleans.html#tok-expression">expression</a></td></tr>
114	</table>
115	</div>
116	<a class="grammar-footer"
117	href="grammar.txt" type="text/plain"
118	>Download entire grammar as text.</a>
119	</dd></dl>
120
121	<P>
122	A trailing comma may be present after the positional and keyword
123	arguments but does not affect the semantics.
124
125	<P>
126	The primary must evaluate to a callable object (user-defined
127	functions, built-in functions, methods of built-in objects, class
128	objects, methods of class instances, and certain class instances
129	themselves are callable; extensions may define additional callable
130	object types). All argument expressions are evaluated before the call
131	is attempted. Please refer to section <A href="function.html#function">7.5</A> for the syntax
132	of formal parameter lists.
133
134	<P>
135	If keyword arguments are present, they are first converted to
136	positional arguments, as follows. First, a list of unfilled slots is
137	created for the formal parameters. If there are N positional
138	arguments, they are placed in the first N slots. Next, for each
139	keyword argument, the identifier is used to determine the
140	corresponding slot (if the identifier is the same as the first formal
141	parameter name, the first slot is used, and so on). If the slot is
142	already filled, a <tt class="exception">TypeError</tt> exception is raised.
143	Otherwise, the value of the argument is placed in the slot, filling it
144	(even if the expression is <code>None</code>, it fills the slot). When all
145	arguments have been processed, the slots that are still unfilled are
146	filled with the corresponding default value from the function
147	definition. (Default values are calculated, once, when the function
148	is defined; thus, a mutable object such as a list or dictionary used
149	as default value will be shared by all calls that don't specify an
150	argument value for the corresponding slot; this should usually be
151	avoided.) If there are any unfilled slots for which no default value
152	is specified, a <tt class="exception">TypeError</tt> exception is raised. Otherwise,
153	the list of filled slots is used as the argument list for the call.
154
155	<P>
156	If there are more positional arguments than there are formal parameter
157	slots, a <tt class="exception">TypeError</tt> exception is raised, unless a formal
158	parameter using the syntax "<tt class="samp">*identifier</tt>" is present; in this
159	case, that formal parameter receives a tuple containing the excess
160	positional arguments (or an empty tuple if there were no excess
161	positional arguments).
162
163	<P>
164	If any keyword argument does not correspond to a formal parameter
165	name, a <tt class="exception">TypeError</tt> exception is raised, unless a formal
166	parameter using the syntax "<tt class="samp">**identifier</tt>" is present; in this
167	case, that formal parameter receives a dictionary containing the
168	excess keyword arguments (using the keywords as keys and the argument
169	values as corresponding values), or a (new) empty dictionary if there
170	were no excess keyword arguments.
171
172	<P>
173	If the syntax "<tt class="samp">*expression</tt>" appears in the function call,
174	"<tt class="samp">expression</tt>" must evaluate to a sequence. Elements from this
175	sequence are treated as if they were additional positional arguments;
176	if there are postional arguments <var>x1</var>,...,<var>xN</var> , and
177	"<tt class="samp">expression</tt>" evaluates to a sequence <var>y1</var>,...,<var>yM</var>, this
178	is equivalent to a call with M+N positional arguments
179	<var>x1</var>,...,<var>xN</var>,<var>y1</var>,...,<var>yM</var>.
180
181	<P>
182	A consequence of this is that although the "<tt class="samp">*expression</tt>" syntax
183	appears <em>after</em> any keyword arguments, it is processed
184	<em>before</em> the keyword arguments (and the
185	"<tt class="samp">**expression</tt>" argument, if any - see below). So:
186
187	<P>
188	<div class="verbatim"><pre>
189	>>> def f(a, b):
190	... print a, b
191	...
192	>>> f(b=1, *(2,))
193	2 1
194	>>> f(a=1, *(2,))
195	Traceback (most recent call last):
196	File "<stdin>", line 1, in ?
197	TypeError: f() got multiple values for keyword argument 'a'
198	>>> f(1, *(2,))
199	1 2
200	</pre></div>
201
202	<P>
203	It is unusual for both keyword arguments and the
204	"<tt class="samp">*expression</tt>" syntax to be used in the same call, so in practice
205	this confusion does not arise.
206
207	<P>
208	If the syntax "<tt class="samp">**expression</tt>" appears in the function call,
209	"<tt class="samp">expression</tt>" must evaluate to a (subclass of) dictionary, the
210	contents of which are treated as additional keyword arguments. In the
211	case of a keyword appearing in both "<tt class="samp">expression</tt>" and as an
212	explicit keyword argument, a <tt class="exception">TypeError</tt> exception is
213	raised.
214
215	<P>
216	Formal parameters using the syntax "<tt class="samp">*identifier</tt>" or
217	"<tt class="samp">**identifier</tt>" cannot be used as positional argument slots or
218	as keyword argument names. Formal parameters using the syntax
219	"<tt class="samp">(sublist)</tt>" cannot be used as keyword argument names; the
220	outermost sublist corresponds to a single unnamed argument slot, and
221	the argument value is assigned to the sublist using the usual tuple
222	assignment rules after all other parameter processing is done.
223
224	<P>
225	A call always returns some value, possibly <code>None</code>, unless it
226	raises an exception. How this value is computed depends on the type
227	of the callable object.
228
229	<P>
230	If it is--
231
232	<P>
233	<DL>
234	<DT><STRONG>a user-defined function:</STRONG></DT>
235	<DD>The code block for the function is
236	executed, passing it the argument list. The first thing the code
237	block will do is bind the formal parameters to the arguments; this is
238	described in section <A href="function.html#function">7.5</A>. When the code block executes a
239	<tt class="keyword">return</tt> statement, this specifies the return value of the
240	function call.
241	<a id='l2h-388' xml:id='l2h-388'></a><a id='l2h-389' xml:id='l2h-389'></a><a id='l2h-390' xml:id='l2h-390'></a><a id='l2h-391' xml:id='l2h-391'></a>
242	<P>
243	</DD>
244	<DT><STRONG>a built-in function or method:</STRONG></DT>
245	<DD>The result is up to the
246	interpreter; see the <em class="citetitle"><a
247	href="../lib/built-in-funcs.html"
248	title="Python
249	Library Reference"
250	>Python
251	Library Reference</a></em> for the descriptions of built-in functions and
252	methods.
253	<a id='l2h-392' xml:id='l2h-392'></a><a id='l2h-393' xml:id='l2h-393'></a><a id='l2h-394' xml:id='l2h-394'></a><a id='l2h-395' xml:id='l2h-395'></a><a id='l2h-396' xml:id='l2h-396'></a><a id='l2h-397' xml:id='l2h-397'></a><a id='l2h-398' xml:id='l2h-398'></a><a id='l2h-399' xml:id='l2h-399'></a>
254	<P>
255	</DD>
256	<DT><STRONG>a class object:</STRONG></DT>
257	<DD>A new instance of that class is returned.
258	<a id='l2h-400' xml:id='l2h-400'></a><a id='l2h-401' xml:id='l2h-401'></a>
259	<P>
260	</DD>
261	<DT><STRONG>a class instance method:</STRONG></DT>
262	<DD>The corresponding user-defined
263	function is called, with an argument list that is one longer than the
264	argument list of the call: the instance becomes the first argument.
265	<a id='l2h-402' xml:id='l2h-402'></a><a id='l2h-403' xml:id='l2h-403'></a><a id='l2h-404' xml:id='l2h-404'></a>
266	<P>
267	</DD>
268	<DT><STRONG>a class instance:</STRONG></DT>
269	<DD>The class must define a <tt class="method">__call__()</tt>
270	method; the effect is then the same as if that method was called.
271	<a id='l2h-405' xml:id='l2h-405'></a><a id='l2h-407' xml:id='l2h-407'></a>
272	<P>
273	</DD>
274	</DL>
275
276	<P>
277
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