feat: add defaults argument

[nh13]

The following (incomplete) pull request adds a new argument to the run command to specify a mapping of defaults for function arguments. It may be useful to have the defaults be set at runtime, for example if we want to read them from a YAML or other configuration file. The below is a toy example of how I'd use the argument.

If you like the path this PR is going, the next steps would be:

1. solve how to pass defaults to nested commands
2. add tests
3. add to docs and examples

import defopt

def foo(*, bar: int) -> None:
    print(bar)

def foo2(*, bar: int) -> None:
    print(bar)


if __name__ == '__main__':
    defaults = {
        "foo": {
            "bar": 5
        },
        "sub": {
            "foo2": {
                "bar": 6
            }
        }
    }

    defopt.run({"foo": foo, "sub": [foo2]}, defaults=defaults)

[nh13]

question: This makes the function argument take precedence. Perhaps it should be the other way around?

[anntzer]

It would seem that if you pass an explicit default, that one should be preferred?

[nh13]

chore: fixme, this isn't true anymore

[anntzer]

I don't know how exactly does your want your yaml-loading works (e.g. how do you specify the funcs arg?) but I suspect I'd rather add proper support for functools.partial so that you can write

import defopt
from functools import partial

def foo(*, bar: int) -> None:
    print(bar)

def foo2(*, bar: int) -> None:
    print(bar)

if __name__ == '__main__':
    defopt.run({"foo": partial(foo, bar=5), "sub": [partial(foo2, bar=6)]})

Thoughts?

[nh13]

proper support for functools.partial Would be great.

[anntzer]

From a quick test the following patch seems enough? Still needs tests and docs of course.

diff --git i/src/defopt.py w/src/defopt.py
index 8855c05..df9e386 100644
--- i/src/defopt.py
+++ w/src/defopt.py
@@ -330,7 +330,8 @@ def _recurse_functions(funcs, subparsers):
         # If this iterable is not a mapping, then convert it to one using the
         # function name itself as the key, but replacing _ with -.
         try:
-            funcs = {func.__name__.replace('_', '-'): func for func in funcs}
+            funcs = {_unwrap_partial(func).__name__.replace('_', '-'): func
+                     for func in funcs}
         except AttributeError as exc:
             # Do not allow a mapping inside of a list
             raise ValueError(
@@ -468,12 +469,16 @@ def signature(func: Union[Callable, str]):
         inspect_sig = _preprocess_inspect_signature(
             func, inspect.signature(func))
         doc_sig = _preprocess_doc_signature(
-            func, signature(inspect.getdoc(func)))
+            func, signature(inspect.getdoc(_unwrap_partial(func))))
         return _merge_signatures(inspect_sig, doc_sig)
 
 
+def _unwrap_partial(func):
+    return func.func if isinstance(func, functools.partial) else func
+
+
 def _preprocess_inspect_signature(func, sig):
-    hints = typing.get_type_hints(func)
+    hints = typing.get_type_hints(_unwrap_partial(func))
     parameters = []
     for name, param in sig.parameters.items():
         if param.name.startswith('_'):

[nh13]

Closing in favor of #130