Duncan Booth wrote:
Tim Chase <python.l...@tim.thechases.com> wrote:
There _are_ cases where it's a useful behavior, but they're rare,
so I don't advocate getting rid of it. But it is enough of a
beginner gotcha that it really should be in the Python FAQ at
www.python.org/doc/faq/general/
That's an excellent idea!
So excellent in fact that it already is:
http://www.python.org/doc/faq/general/#why-are-default-values-shared-between-objects
A couple of minor quibbles:
* This writeup, and the virtually identical one at effbot.org that Diez
referenced, address the *what* of default arguments, but don't really
address the *why*, beyond the statement that "Default values are created
exactly once, when the function is defined (by executing the *def*
<http://effbot.org/pyref/def.htm> statement)". After all, if parameter
values are supplied to a function when it is called, a newly minted set
of default values could be supplied at that time, also. So *why* was
Python designed one way instead of the other?
* I'd change this sentence near the beginning of the writeup, because
it's just ambiguous enough in the exact area that the writeup is
attempting to clarify:
from:
The first time you call this function, D contains a single item.
The second time, D contains two items ...
to:
After the first call to this function, D contains a single item.
After the second call, D contains two items ...
-John
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