If I ran into this (never have), my first inclination would be to write a
decorator.
The decorator would capture the relevant arguments (either just all of
them, or perhaps using the argument names supplied to the decorator when it
is called, or perhaps using a custom type hint applied to the method
signature letting it know which args to modify) and update their values
with the state of the object members in the desired manner.
Would look something like:
@object_state_args('a')
def func(self, a):
# value of a has already been updated to self.a, if needed
a+=1
return a
The opinions regarding the details of what this decorator ought to do would
probably vary by use case and by individual preference and I doubt it would
be very profitable to add to the std lib, personally. Seems like the type
of thing better put in your own toolkit.
On Wed, Dec 16, 2020, 8:12 AM Local-State <ain-s...@live.com> wrote:
> Hello guys,
> First time for me to submit a feature request for python.
>
> A lot of time we try to use a member variable as the default value of a
> member function argument. So we write the code like this:
> ```python
> class A:
> def __init__(self, a: int = 3):
> self.a = a
>
> def func(self, a: int = None):
> if a is None:
> a = self.a
> a += 1
> return a
> ```
> This seems okay, but when there are more than 10 arguments need to set
> their default values as correlated member variables (with the same name at
> most times), it'll be very painful and the codes turn out to be quite ugly.
> I wish a clean coding style, so I write something like this
> ```python
> def func(self, a: int = 'self'):
> for k, v in process(a=a, b=b):
> eval(f'{k} = v')
> a += 1
> return a
>
> def process(self, **kwargs):
> return {k: getattr(self, k) if (v == 'self' and k in
> self.__dict__.keys()) else v for k, v in kwargs.items()}
> ```
> That seems absolutely much worse for sure!
>
> I dream I can just write
> ```python
> def func(self, a: int = 'self'):
> a += 1
> return a
> ```
> where I know the default value cannot be 'self', but a special flag which
> won't get conflict with any other valid argument values. I don't know what
> it should be though.
>
> Some related questions:
> python:
>
> https://stackoverflow.com/questions/39928345/python-member-variable-as-parameter
> C++:
>
> https://stackoverflow.com/questions/32399730/default-arguments-as-non-static-member-variables
>
> Let me know your ideas!
>
> Ren Pang
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