Add option to load traitlets values from environement. (#856)

examples/myapp.py

@@ -36,6 +36,14 @@
 from traitlets.config.configurable import Configurable
 
 
+class SubConfigurable(Configurable):
+    subvalue = Int(0, help="The integer subvalue.").tag(config=True)
+
+    def describe(self):
+        print("I am SubConfigurable with:")
+        print("    subvalue =", self.subvalue)
+
+
 class Foo(Configurable):
     """A class that has configurable, typed attributes."""
 
@@ -44,9 +52,39 @@ class Foo(Configurable):
     name = Unicode("Brian", help="First name.").tag(config=True, shortname="B")
     mode = Enum(values=["on", "off", "other"], default_value="on").tag(config=True)
 
+    def __init__(self, **kwargs):
+        super().__init__(**kwargs)
+        # using parent=self allows configuration in the form c.Foo.SubConfigurable.subvalue=1
+        # while c.SubConfigurable.subvalue=1 will still work, this allow to
+        # target specific instances of SubConfigurables
+        self.subconf = SubConfigurable(parent=self)
+
+    def describe(self):
+        print("I am Foo with:")
+        print("    i    =", self.i)
+        print("    j    =", self.j)
+        print("    name =", self.name)
+        print("    mode =", self.mode)
+        self.subconf.describe()
+
 
 class Bar(Configurable):
     enabled = Bool(True, help="Enable bar.").tag(config=True)
+    mylist = List([1, 2, 3], help="Just a list.").tag(config=True)
+
+    def describe(self):
+        print("I am Bar with:")
+        print("    enabled = ", self.enabled)
+        print("    mylist  = ", self.mylist)
+        self.subconf.describe()
+
+    def __init__(self, **kwargs):
+        super().__init__(**kwargs)
+        # here we do not use parent=self, so configuration in the form
+        # c.Bar.SubConfigurable.subvalue=1 will not work. Only
+        # c.SubConfigurable.subvalue=1 will work and affect all instances of
+        # SubConfigurable
+        self.subconf = SubConfigurable(config=self.config)
 
 
 class MyApp(Application):
@@ -76,8 +114,8 @@ class MyApp(Application):
     )
 
     def init_foo(self):
-        # Pass config to other classes for them to inherit the config.
-        self.foo = Foo(config=self.config)
+        # You can pass self as parent to automatically propagate config.
+        self.foo = Foo(parent=self)
 
     def init_bar(self):
         # Pass config to other classes for them to inherit the config.
@@ -87,19 +125,26 @@ def initialize(self, argv=None):
         self.parse_command_line(argv)
         if self.config_file:
             self.load_config_file(self.config_file)
+        self.load_config_environ()
         self.init_foo()
         self.init_bar()
 
     def start(self):
         print("app.config:")
         print(self.config)
+        self.describe()
         print("try running with --help-all to see all available flags")
         assert self.log is not None
         self.log.debug("Debug Message")
         self.log.info("Info Message")
         self.log.warning("Warning Message")
         self.log.critical("Critical Message")
 
+    def describe(self):
+        print("I am MyApp with", self.name, self.running, "and 2 sub configurables Foo and bar:")
+        self.foo.describe()
+        self.bar.describe()
+
 
 def main():
     app = MyApp()

traitlets/config/application.py

@@ -23,6 +23,7 @@
     ArgumentError,
     Config,
     ConfigFileNotFound,
+    DeferredConfigString,
     JSONFileConfigLoader,
     KVArgParseConfigLoader,
     PyFileConfigLoader,
@@ -970,6 +971,29 @@ def load_config_file(
         new_config.merge(self.cli_config)
         self.update_config(new_config)
 
+    @catch_config_error
+    def load_config_environ(self) -> None:
+        """Load config files by environment."""
+
+        PREFIX = self.name.upper()
+        new_config = Config()
+
+        self.log.debug('Looping through config variables with prefix "%s"', PREFIX)
+
+        for k, v in os.environ.items():
+            if k.startswith(PREFIX):
+                self.log.debug('Seeing environ "%s"="%s"', k, v)
+                # use __ instead of . as separator in env variable.
+                # Warning, case sensitive !
+                _, *path, key = k.split("__")
+                section = new_config
+                for p in path:
+                    section = section[p]
+                setattr(section, key, DeferredConfigString(v))
+
+        new_config.merge(self.cli_config)
+        self.update_config(new_config)
+
     def _classes_with_config_traits(
         self, classes: ClassesType | None = None
     ) -> t.Generator[type[Configurable], None, None]: