trapping errors in function call syntax

[Avi Kak]

Hello:

Suppose I write a function that I want to be called
with ONLY keyword argumnts, how do I raise an
exception should the function get called with
what look like position-specfic arguments?

Any help would be appreciated.

Avi Kak

 

[Carsten Haese]

Hello:

Suppose I write a function that I want to be called
with ONLY keyword argumnts, how do I raise an
exception should the function get called with
what look like position-specfic arguments?

Any help would be appreciated.

Something like this should do the trick:

def f(*args, **kwargs):
if args: raise TypeError, "Please use keyword arguments."
...

-Carsten

 

[Fredrik Lundh]

Suppose I write a function that I want to be called
with ONLY keyword argumnts, how do I raise an
exception should the function get called with
what look like position-specfic arguments?

here's one way to do it:
.... def myrealfunc(a=1, b=2, c=3):
.... print a, b, c
.... if args:
.... raise TypeError("invalid call")
.... return myrealfunc(**kw)
....Traceback (most recent call last):
File "<stdin>", line 1, in ?
1 2 3

here's another one:
.... if dummy is not None:
.... raise TypeError("invalid call")
.... print a, b, c

(but this is easier to trick).

hope this helps!

</F>

 

[Andrew Gwozdziewycz]

While we're at it... Can someone point me to either an old post, or
documentation about tuple expansion with * ? I recently saw it used
and was shocked as i had no clue what it really did. I didn't know it
could be used outside of function definitions.

 

[Tim Hochberg]

Hello:

Suppose I write a function that I want to be called
with ONLY keyword argumnts, how do I raise an
exception should the function get called with
what look like position-specfic arguments?

Any help would be appreciated.

Say you want the signature to be foo(a, b, c) with no positional
arguments allowed. You can do this:

def foo(*args, **kwargs):
a = kwargs.pop('a')
b = kwargs.pop('b')
c = kwargs.pop('c')
if args or kwargs:
raise TypeError("foo takes only keyword arguments: foo(a,b,c)")
# ...


Or something similar.

-tim

 

[Fredrik Lundh]

While we're at it... Can someone point me to either an old post, or
documentation about tuple expansion with * ? I recently saw it used
and was shocked as i had no clue what it really did. I didn't know it
could be used outside of function definitions.

the reference page is here:

http://docs.python.org/ref/calls.html

</F>

 

[Duncan Booth]

here's one way to do it:

... def myrealfunc(a=1, b=2, c=3):
... print a, b, c
... if args:
... raise TypeError("invalid call")
... return myrealfunc(**kw)
...

If you aren't particular about the message then you can do without the args
argument and just define it with **kw:
def myrealfunc(a=1, b=2, c=3):
print a, b, c
return myrealfunc(**kw)


Traceback (most recent call last):
File "<pyshell#14>", line 1, in -toplevel-
func(1)
TypeError: func() takes exactly 0 arguments (1 given)
and if you are particular about the message perhaps submitting a patch to
generate a more appropriate message would be a good idea.