function call with named/unnamed and variable arguments in python
python
Solution
The catch-all `*args` parameter must come after any explicit arguments:
def foo(func, named_arg=None, *args):
If you also add the catch-all `**kw` keywords parameter to a definition, then that has to come after the `*args` parameter:
def foo(func, named_arg=None, *args, **kw):
Mixing explicit keyword arguments and the catch-all `*args` argument does lead to unexpected behaviour; you cannot both use arbitrary positional arguments and explicitly name the keyword arguments you listed at the same time.
Any extra positionals beyond `func` are first used for `named_arg` which can also act as a positional argument:
>>> def foo(func, named_arg = None, *args):
... print func, named_arg, args
...
>>> foo(1, 2)
1 2 ()
>>> foo(1, named_arg=2)
1 2 ()
>>> foo(1, 3, named_arg=2)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: foo() got multiple values for keyword argument 'named_arg'
>>> foo(1, 2, 3)
1 2 (3,)
This is because the any second positional argument to `foo()` will always be used for `named_arg`.
In Python 3, the `*args` parameter can be placed before the keyword arguments, but that has a new meaning. Normally, keyword parameters can be specified in the call signature as positional arguments (e.g. call your function as `foo(somefunc, 'argument')` would have `'argument'` assigned to `named_arg`). By placing `*args` or a plain `*` in between the positional and the named arguments you exclude the named arguments from being used as positionals; calling `foo(somefunc, 'argument')` would raise an exception instead.
Problem
I have the following code: ``` def foo(func, *args, named_arg = None): return func(*args) ``` returning a `SyntaxError`: ``` File "tester3.py", line 3 def foo(func, *args, named_arg = None): ^ ``` Why is that? And is it possible to define somewhat a function in that way, which takes one argument (`func`), then a list of variable arguments `args` before named arguments? If not, what are my possibilities?