On Thu, May 28, 2020 at 12:38 PM Greg Ewing <greg.ew...@canterbury.ac.nz>
wrote:
> On 28/05/20 8:57 pm, Steven D'Aprano wrote:
> > The default value used by a parameter is certainly important, and one of
> > the most common reasons I call `help(func)` is to see what the default
> > values are. They should be in the signature, and it is an annoyance when
> > all the signature shows is that it is None, and then I have to trawl
> > through screenfuls of docs, or search the web, to find out what the
> > actual default is.
>
> I think we need a real example to be able to talk about this
> meaningfully.
>
> But I'm having trouble thinking of one. I can't remember ever
> writing a function with a default argument value that *has* to
> be mutable and *has* to have a new one created on each call
> *unless* the caller provided one.
>
> Anyone else have one?
>
I think the most common cases are where the default is an empty list or
dict. Building on my ChainMap example from before, consider this code:
```
from collections import ChainMap
c1 = ChainMap().new_child()
c2 = ChainMap().new_child()
c1[1] = 2
print(c1[1])
c2[1] = 3
print(c1[1])
```
Here c1 and c2 have separate new children, so modifying c2 doesn't affect
c1, so '2' is printed twice. We can simulate what would happen if a new
default wasn't created each time by passing the same dict to both:
```
from collections import ChainMap
m = {}
c1 = ChainMap().new_child(m)
c2 = ChainMap().new_child(m)
c1[1] = 2
print(c1[1])
c2[1] = 3
print(c1[1])
```
Now the second print shows '3', even though we expect that value to be in
c2 but not c1.
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