[Python-Dev] Very Strange Argument Handling Behavior

[Mark Dickinson]

On Fri, Apr 16, 2010 at 9:04 AM, Hagen Fürstenau <hagen at zhuliguan.net> wrote:
>> This behavior seems pretty strange to me, indeed PyPy gives the
>> TypeError for both attempts.  I just wanted to confirm that it was in
>> fact intentional.
>
> Oleg already answered why f(**{1:3}) raises a TypeError. But your
> question seems to be rather why dict(**{1:3}) doesn't.
>
> For functions implemented in Python, non-string arguments are always
> rejected, but C functions (like the dict constructor) don't have to
> reject them. I don't see any benefit in allowing them, but it's probably
> not worth breaking code by disallowing them either.

"dict(x, **y)" as an expression version of x.update(y) seems to be
fairly well known[1], so disallowing non-string keyword arguments
seems likely to break existing code, as well as (probably?) harming
performance.  So I can't see CPython changing here.  I'm not sure
whether other implementations should be required to follow suit,
though---maybe this should be regarded as an implementation-defined
detail?

Mark

[1] http://stackoverflow.com/questions/38987/how-can-i-merge-two-python-dictionaries-as-a-single-expression
)