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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> | |
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312 | <span class="release-info">Release 2.4.2, documentation updated on 28 September 2005.</span> | |
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