[Python-Dev] Very Strange Argument Handling Behavior (original) (raw)

Mark Dickinson dickinsm at gmail.com
Fri Apr 16 10:29:21 CEST 2010

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 )

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