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Can you propose a format for it?
I'm not sure that including all arguments and their reprs is a good idea, as it will make BA's repr too long.

Something like <BoundArguments: foo=bar, boo=baz>
I have no problem with truncation when it gets too long.

How about we just list bound arguments names, without values?

Em, that kind of defeats the purpose of describing the object, doesn't it?

Yes and no ;)
You can have partially bound args, you can bind just one argument and use defaults for the rest, etc. I agree that it's not an ideal solution, but it is a sane compromise.

I think the point of `__init__` is to know what the object is, and if you're hiding the values then someone has to access the attributes to find out the values and that sucks.

Context: inspect.signature returns an inspect.Signature instance. Its .bind and .bind_partial methods return ab inspect.BoundArguments instance with a standard <...@ 0x...> representation.
>>> import inspect
>>> def foo(a, b=10): pass
>>> ba = inspect.signature(foo).bind(5)
>>> ba
<inspect.BoundArguments object at 0x0000000002C94080>
BAs already have 3 data access methods
>>> ba.arguments # I believe this describes the object pretty fully
OrderedDict([('a', 5)])
>>> ba.args
(5,)
>>> ba.kwargs
{}
A possible proposal for this case would be <BoundArguments: a=5>. However, this contains the same info as ba.arguments but in a less accessible form.
Listing all parameters (and their default values if any) would be wrong as the additional info is not part of the object. The doc says to use Signature.parameters for full info, and it shows how to do that. So I am negative on the proposal.

Another thing I proposed in python-ideas is to have `__getitem__` delegate to `.arguments`, so this proposal is similar in spirit, because I want to have `__repr__` show information from `.arguments`.
To be honest I don't see the point in having to access `.arguments` manually for anything. `BoundArguments` is all about arguments. I don't see a reason to waste anyone's time with accessing an attribute on it to get to the arguments. There isn't much more to get on the thing. So this applies both to __repr__ and __getitem__.

> Another thing I proposed in python-ideas is to have `__getitem__` delegate to `.arguments`, so this proposal is similar in spirit, because I want to have `__repr__` show information from `.arguments`.
Big -1 on __getitem__
> To be honest I don't see the point in having to access `.arguments` manually for anything. `BoundArguments` is all about arguments.
I agree. I'll take a look.

.arguments returns a mutable (ordered) dict that can be modified. See the example in the doc
https://docs.python.org/3.4/library/inspect.html#inspect.BoundArguments 

New changeset a444464a2e87 by Yury Selivanov in branch 'default':
Issue 22547: Implement informative __repr__ for inspect.BoundArguments
https://hg.python.org/cpython/rev/a444464a2e87 

May be omit names for positionalarguments?
>>> def foo(a, *args, b=10, **kwargs): pass
... 
>>> inspect.signature(foo).bind(1, 2, 3, b=4, c=5)
<BoundArguments at 0xb6eee9ec (a=1, args=(2, 3), b=4, kwargs={'c': 5})>
I think it would look better as:
<BoundArguments (1, 2, 3, b=4, c=5)>

Here is an implementation.

Actually, I like the current repr (minus the object ID, I'll update the code).
The thing about BoundArguments is that '.arguments' is the main property. You use it to add more stuff to *args & **kwargs, or to add/remove positional/keyword arguments. Seeing a flattened call signature in the repr won't help you with debugging.
Perhaps, we can implement __str__ using your approach.