#23675: Default value for optional parameter "default" in the model field
reference
is not specified
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Reporter: paulcdejean | Owner: nobody
Type: Cleanup/optimization | Status: closed
Component: Documentation | Version: master
Severity: Normal | Resolution: wontfix
Keywords: | Triage Stage: Accepted
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 1 | UI/UX: 0
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Changes (by aaugustin):
* status: new => closed
* resolution: => wontfix
Comment:
I beg to differ.
There are three strategies for defining functions with optional keyword
arguments:
1. `arg=None`: this works well when `None` doesn't make sense as an
argument. The target function can assume that the argument wasn't passed
if it's `None`.
2. `arg=GUARD` where `GUARD` is a private singleton: this variant is
required when the target function needs to be able to tell apart "arg set
to `None`" and "arg not passed", which is the case here.
3. `**kwargs`: this is an alternative to 2. The target function can check
if `'arg' in kwargs` to check if arg was passed.
Here, Django uses a different code path when the `default` argument is
provided and when it isn't provided. As a consequence, Django doesn't use
its value when it isn't provided. The argument doesn't have a default
value; it simply isn't used at all. Implying there's a default value would
be misleading and confuse users.
It happens that this particular piece of code is implemented with pattern
2 instead of 3, which makes it look like there's a default value, but that
isn't something the users should be aware of. Worse, documenting
`NOT_PROVIDED` would ruin pattern 2, because it only works as long as
users don't know the guard value!
If you aren't convinced yet, I'll simply say that this bug report is based
upon the idea that "optional paramaters in python always have a default
value", which is wrong. Any function that takes `**kwargs` has optional
parameters without a default value -- see `django.shortcuts.render` for a
good example.
--
Ticket URL: <https://code.djangoproject.com/ticket/23675#comment:2>
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