Maybe loses error information

[hynek]

One example says more than 1000 words:

n [1]: import voluptuous as v

In [2]: v.Schema(v.Maybe(v.Range(1, 2)))
Out[2]: <Schema(Any(None, Range(min=1, max=2, min_included=True, max_included=True, msg=None), msg=None), extra=PREVENT_EXTRA, required=False) object at 0x10c257828>

In [3]: v.Schema(v.Maybe(v.Range(1, 2)))(1)
Out[3]: 1

In [4]: v.Schema(v.Maybe(v.Range(1, 2)))(None)

In [5]: v.Schema(v.Maybe(v.Range(1, 2)))(3)
---------------------------------------------------------------------------
ScalarInvalid                             Traceback (most recent call last)
~/.virtualenvs/cust-ws/lib/python3.7/site-packages/voluptuous/schema_builder.py in __call__(self, data)
    266         try:
--> 267             return self._compiled([], data)
    268         except er.MultipleInvalid:

~/.virtualenvs/cust-ws/lib/python3.7/site-packages/voluptuous/validators.py in _run(self, path, value)
    203     def _run(self, path, value):
--> 204         return self._exec(self._compiled, value, path)
    205

~/.virtualenvs/cust-ws/lib/python3.7/site-packages/voluptuous/validators.py in _exec(self, funcs, v, path)
    255                 raise error if self.msg is None else AnyInvalid(
--> 256                     self.msg, path=path)
    257             raise AnyInvalid(self.msg or 'no valid value found',

~/.virtualenvs/cust-ws/lib/python3.7/site-packages/voluptuous/validators.py in _exec(self, funcs, v, path)
    248                 else:
--> 249                     return func(path, v)
    250             except Invalid as e:

~/.virtualenvs/cust-ws/lib/python3.7/site-packages/voluptuous/schema_builder.py in validate_value(path, data)
    821         if data != schema:
--> 822             raise er.ScalarInvalid('not a valid value', path)
    823         return data

ScalarInvalid: not a valid value

During handling of the above exception, another exception occurred:

MultipleInvalid                           Traceback (most recent call last)
<ipython-input-5-cbbab77861f1> in <module>
----> 1 v.Schema(v.Maybe(v.Range(1, 2)))(3)

~/.virtualenvs/cust-ws/lib/python3.7/site-packages/voluptuous/schema_builder.py in __call__(self, data)
    269             raise
    270         except er.Invalid as e:
--> 271             raise er.MultipleInvalid([e])
    272             # return self.validate([], self.schema, data)
    273

MultipleInvalid: not a valid value

I would like to supply a good error message to the user but setting msg on Maybe doesn't work and Range's gets ignored.

[svisser]

@hynek thanks for reporting this! There is now the ability to set msg on Maybe in the master branch: 43ea94b. I don't know when this will be available in a released version of voluptuous though.

Do you think this change is sufficient for resolving this issue or should we explore further functionality that uses any msg set on the validator that's passed to Maybe (not sure how easy this is though)?

[hynek]

Is there a precedent of such composites bubbling up messages? I think I could live with just setting msg on the Maybe myself.

[svisser]

@hynek just a quick message to say that a new version of voluptuous was released recently with the above change.

[hynek]

Thanks for letting me know and for keeping this great library alive!