A trailing comma may be present after an argument list 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 7.5 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
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:
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
If it is--
See About this document... for information on suggesting changes.