5.3.4 Calls

A call calls a callable object (e.g., a function) with a possibly empty series of arguments:

primary "(" [argument_list [","] | test genexpr_for ] ")"

call	::=	primary "(" [argument_list [","]] ")"
argument_list	::=	positional_arguments ["," keyword_arguments]
		`["," "*" expression]`
		`["," "**" expression]`
		`\| keyword_arguments ["," "*" expression]`
		`["," "**" expression]`
		`\| "" expression ["," "*" expression]`
		`\| "**" expression`
positional_arguments	::=	expression ("," expression)*
keyword_arguments	::=	keyword_item ("," keyword_item)*
keyword_item	::=	identifier "=" expression

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 7.6 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 TypeError exception is raised. Otherwise, the value of the argument is placed in the slot, filling it (even if the expression is None, 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 TypeError 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 TypeError exception is raised, unless a formal parameter using the syntax "*identifier" 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).

If any keyword argument does not correspond to a formal parameter name, a TypeError exception is raised, unless a formal parameter using the syntax "**identifier" 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 "*expression" appears in the function call, "expression" must evaluate to a sequence. Elements from this sequence are treated as if they were additional positional arguments; if there are postional arguments x1,...,xN , and "expression" evaluates to a sequence y1,...,yM, this is equivalent to a call with M+N positional arguments x1,...,xN,y1,...,yM.

A consequence of this is that although the "*expression" syntax appears after any keyword arguments, it is processed before the keyword arguments (and the "**expression" argument, if any - see below). So:

>>> def f(a, b):
...  print a, b
...
>>> f(b=1, *(2,))
2 1
>>> f(a=1, *(2,))
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
TypeError: f() got multiple values for keyword argument 'a'
>>> f(1, *(2,))
1 2

It is unusual for both keyword arguments and the "*expression" syntax to be used in the same call, so in practice this confusion does not arise.

If the syntax "**expression" appears in the function call, "expression" 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 "expression" and as an explicit keyword argument, a TypeError exception is raised.

Formal parameters using the syntax "*identifier" or "**identifier" cannot be used as positional argument slots or as keyword argument names. Formal parameters using the syntax "(sublist)" 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 None, unless it raises an exception. How this value is computed depends on the type of the callable object.

If it is--

a user-defined function:: 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 7.6. When the code block executes a return statement, this specifies the return value of the function call.
a built-in function or method:: The result is up to the interpreter; see the Python Library Reference for the descriptions of built-in functions and methods.
a class object:: A new instance of that class is returned.
a class instance method:: 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 class instance:: The class must define a __call__() method; the effect is then the same as if that method was called.

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