function with list argument defaulting to [] - what's going on here???

Paddy paddy3118 at googlemail.com
Sun Apr 15 05:20:42 CEST 2007


On Apr 15, 3:58 am, Steven D'Aprano
<s... at REMOVE.THIS.cybersource.com.au> wrote:
> On Sat, 14 Apr 2007 17:33:11 -0800, Troy Melhase wrote:
> > On 4/14/07, Mike <m... at woh.rr.com> wrote:
> >> While trying to write a recursive function involving lists, I came
> >> across some (to me) odd behavior which I don't quite understand. Here's
> >> a trivial function showing the problem.
>
> > fromhttp://docs.python.org/ref/function.html:
>
> > Default parameter values are evaluated when the function definition is
> > executed. This means that the expression is evaluated once, when the
> > function is defined, and that that same ``pre-computed'' value is used
> > for each call. This is especially important to understand when a
> > default parameter is a mutable object, such as a list or a dictionary:
> > if the function modifies the object (e.g. by appending an item to a
> > list), the default value is in effect modified.
>
> This comes up so often that I wonder whether Python should issue a warning
> when it sees [] or {} as a default argument.
>
> What do people think? A misuse or good use of warnings?
>
> --
> Steven.

I wonder if it is a check done by Pylint or PyChecker?

- Paddy.




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