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<title>5.3.4 Calls
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<H2><A NAME=
"SECTION007340000000000000000"></A><A NAME=
"calls"></A>
<a id='l2h-
386' xml:id='l2h-
386'
></a>
A call calls a callable object (e.g., a function) with a possibly empty
<a id='l2h-
387' xml:id='l2h-
387'
></a>
<div class=
"productions">
<td><a id='tok-call' xml:id='tok-call'
>call
</a></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> ]
")"
<td><a id='tok-argument_list' xml:id='tok-argument_list'
>argument_list
</a></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>
<td><code> [
"," "*" <a class='grammartoken'
href=
"Booleans.html#tok-expression">expression
</a>]
</code></td></tr>
<td><code> [
"," "**" <a class='grammartoken'
href=
"Booleans.html#tok-expression">expression
</a>]
</code></td></tr>
<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>
<td><code> [
"," "**" <a class='grammartoken'
href=
"Booleans.html#tok-expression">expression
</a>]
</code></td></tr>
<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>
<td><code>|
"**" <a class='grammartoken'
href=
"Booleans.html#tok-expression">expression
</a></code></td></tr>
<td><a id='tok-positional_arguments' xml:id='tok-positional_arguments'
>positional_arguments
</a></td>
<td><a class='grammartoken'
href=
"Booleans.html#tok-expression">expression
</a> (
"," <a class='grammartoken'
href=
"Booleans.html#tok-expression">expression
</a>)*
</td></tr>
<td><a id='tok-keyword_arguments' xml:id='tok-keyword_arguments'
>keyword_arguments
</a></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>
<td><a id='tok-keyword_item' xml:id='tok-keyword_item'
>keyword_item
</a></td>
<td><a class='grammartoken'
href=
"identifiers.html#tok-identifier">identifier
</a> "=" <a class='grammartoken'
href=
"Booleans.html#tok-expression">expression
</a></td></tr>
<a class=
"grammar-footer"
href=
"grammar.txt" type=
"text/plain"
>Download entire grammar as text.
</a>
A trailing comma may be present after the positional and keyword
arguments but does not affect the semantics.
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.
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.
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
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.
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>.
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:
<div class=
"verbatim"><pre>
>>> def f(a, b):
>>> f(b=
1, *(
2,))
>>> f(a=
1, *(
2,))
Traceback (most recent call last):
File
"<stdin>", line
1, in ?
TypeError: f() got multiple values for keyword argument 'a'
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.
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
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.
A call always returns some value, possibly
<code>None
</code>, unless it
raises an exception. How this value is computed depends on the type
<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
<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>
<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"
Library Reference
</a></em> for the descriptions of built-in functions and
<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>
<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>
<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>
<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>
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<span class=
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2.4.2, documentation updated on
28 September
2005.
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