Add option to load traitlets values from environement.

[Carreau]

For example this makes the following works:

With the env variables:

 MYAPP__Foo__j=42
 MYAPP__Bar__mylist='[4,5,3]'
 MYAPP__Foo__SubConfigurable__subvalue=-1

$ python examples/myapp.py
app.config:
{'Foo': {'j': DeferredConfigString('42'), 'SubConfigurable': {'subvalue': DeferredConfigString('-1')}}, 'Bar': {'mylist': DeferredConfigString('[4,5,3]')}}
I am MyApp with myapp False and 2 sub configurables Foo and bar:
I am Foo with:
    i    = 0
    j    = 42
    name = Brian
    mode = on
I am SubConfigurable with:
    subvalue = -1
I am Bar with:
    enabled =  True
    mylist  =  [4, 5, 3]
I am SubConfigurable with:
    subvalue = 0
try running with --help-all to see all available flags
[MyApp] WARNING | Warning Message
[MyApp] CRITICAL | Critical Message

We use app.name uppercased as a prefix, as usually env variables are uppercase.

We use __ instead of . as a separator as:

• . is not valid in env variables,
• _ can be present in class names, so we can't use it.

I'm not going the full way of a config loader, as I don't think it is necessary.

I'm wondering if we should put env loading by default.

This also extend one of the examples to load from env, show the parent=self and describe the status of the various traits at run time instead of just the configuration.

[Carreau]

@ndmlny-qs FYI.

[Carreau]

We do not handle errors here, I'm not sure if we should keep going on invalid values.

[blink1073]

I think it should error out if an env variable is set to an invalid value.

[Carreau]

I tested and we do actually already error in the same way than when we pass an invalid CLI value (MYAPP__Foo__i=a42 or --Foo.i=a42)

TB when invalid value

Traceback (most recent call last):
  File "/Users/bussonniermatthias/dev/traitlets/examples/myapp.py", line 155, in <module>
    main()
  File "/Users/bussonniermatthias/dev/traitlets/examples/myapp.py", line 150, in main
    app.initialize()
  File "/Users/bussonniermatthias/dev/traitlets/examples/myapp.py", line 129, in initialize
    self.init_foo()
  File "/Users/bussonniermatthias/dev/traitlets/examples/myapp.py", line 118, in init_foo
    self.foo = Foo(parent=self)
  File "/Users/bussonniermatthias/dev/traitlets/examples/myapp.py", line 56, in __init__
    super().__init__(**kwargs)
  File "/Users/bussonniermatthias/dev/traitlets/traitlets/config/configurable.py", line 108, in __init__
    self.config = config
  File "/Users/bussonniermatthias/dev/traitlets/traitlets/traitlets.py", line 688, in __set__
    self.set(obj, value)
  File "/Users/bussonniermatthias/dev/traitlets/traitlets/traitlets.py", line 677, in set
    obj._notify_trait(self.name, old_value, new_value)
  File "/Users/bussonniermatthias/dev/traitlets/traitlets/traitlets.py", line 1459, in _notify_trait
    self.notify_change(
  File "/Users/bussonniermatthias/dev/traitlets/traitlets/traitlets.py", line 1471, in notify_change
    return self._notify_observers(change)
  File "/Users/bussonniermatthias/dev/traitlets/traitlets/traitlets.py", line 1518, in _notify_observers
    c(event)
  File "/Users/bussonniermatthias/dev/traitlets/traitlets/traitlets.py", line 1100, in compatible_observer
    return func(self, change)
  File "/Users/bussonniermatthias/dev/traitlets/traitlets/config/configurable.py", line 218, in _config_changed
    self._load_config(change.new, traits=traits, section_names=section_names)
  File "/Users/bussonniermatthias/dev/traitlets/traitlets/config/configurable.py", line 180, in _load_config
    setattr(self, name, deepcopy(config_value))
  File "/Users/bussonniermatthias/dev/traitlets/traitlets/traitlets.py", line 688, in __set__
    self.set(obj, value)
  File "/Users/bussonniermatthias/dev/traitlets/traitlets/traitlets.py", line 662, in set
    new_value = self._validate(obj, value)
  File "/Users/bussonniermatthias/dev/traitlets/traitlets/traitlets.py", line 694, in _validate
    value = self.validate(obj, value)
  File "/Users/bussonniermatthias/dev/traitlets/traitlets/traitlets.py", line 2341, in validate
    self.error(obj, value)
  File "/Users/bussonniermatthias/dev/traitlets/traitlets/traitlets.py", line 798, in error
    raise TraitError(e)
traitlets.traitlets.TraitError: The 'i' trait of a Foo instance expected an int, not the str 'a42'.

[blink1073]

Yeah, you don't want to pass some security-related option and have it ignored.

[blink1073]

Thanks!