Configuration reference interpolation

Constants are defined in a `[constants]` section, and unused constants will
only warn instead of erroring.

My intended usage looks like this:
```
[constants]
run_current_ab: 1.9

[tmc5160 stepper_x]
run_current: ${constants.run_current_ab}
[tmc5160 stepper_y]
run_current: ${constants.run_current_ab}
```

`${option}` references the current section
`${section.option}` looks up anywhere

interpolation occurs at most 10 times right now to avoid infinite loops

docs/Config_Reference.md

@@ -145,6 +145,30 @@ A collection of DangerKlipper-specific system options
 #log_webhook_method_register_messages: False
 ```
 
+## ⚠️ Configuration references
+
+In your configuration, you can reference other values to share
+configuration between multiple sections. References take the form of
+`${option}` to copy a value in the current section, or
+`${section.option}` to look up a value elsewhere in your configuration.
+
+Optionally, a `[constants]` section may be used specifically to store
+these values. Unlike the rest of your configuration, unused constants
+will show a warning instead of causing an error.
+
+```
+[constants]
+run_current_ab:  1.0
+i_am_not_used: True  # Will show "Constant 'i_am_not_used' is unused"
+
+[tmc5160 stepper_x]
+run_current: ${constants.run_current_ab}
+
+[tmc5160 stepper_y]
+run_current: ${tmc5160 stepper_x.run_current}
+#   Nested references work, but are not advised
+```
+
 ## Common kinematic settings
 
 ### [printer]

klippy/configfile.py

@@ -13,6 +13,38 @@ class sentinel:
     pass
 
 
+class SectionInterpolation(configparser.Interpolation):
+    """
+    variable interpolation replacing ${[section.]option}
+    """
+
+    _KEYCRE = re.compile(
+        r"\$\{(?:(?P<section>[^.:}]+)[.:])?(?P<option>[^}]+)\}"
+    )
+
+    def __init__(self, access_tracking):
+        self.access_tracking = access_tracking
+
+    def before_get(self, parser, section, option, value, defaults):
+        depth = configparser.MAX_INTERPOLATION_DEPTH
+        while depth:
+            depth -= 1
+
+            match = self._KEYCRE.search(value)
+            if not match:
+                break
+
+            sect = match.group("section") or section
+            opt = match.group("option")
+
+            const = parser.get(sect, opt)
+            self.access_tracking.setdefault((sect, opt), const)
+
+            value = value[: match.start()] + const + value[match.end() :]
+
+        return value
+
+
 class ConfigWrapper:
     error = configparser.Error
 
@@ -412,14 +444,17 @@ def _parse_config(self, data, filename, fileconfig, visited):
         visited.remove(path)
 
     def _build_config_wrapper(self, data, filename):
-        if sys.version_info.major >= 3:
-            fileconfig = configparser.RawConfigParser(
-                strict=False, inline_comment_prefixes=(";", "#")
-            )
-        else:
-            fileconfig = configparser.RawConfigParser()
+        access_tracking = {}
+        fileconfig = configparser.RawConfigParser(
+            strict=False,
+            inline_comment_prefixes=(";", "#"),
+            interpolation=SectionInterpolation(access_tracking),
+        )
+
         self._parse_config(data, filename, fileconfig, set())
-        return ConfigWrapper(self.printer, fileconfig, {}, "printer")
+        return ConfigWrapper(
+            self.printer, fileconfig, access_tracking, "printer"
+        )
 
     def _build_config_string(self, config):
         sfile = io.StringIO()
@@ -450,7 +485,7 @@ def check_unused_options(self, config, error_on_unused):
             for option in self.autosave.fileconfig.options(section):
                 access_tracking[(section.lower(), option.lower())] = 1
         # Validate that there are no undefined parameters in the config file
-        valid_sections = {s: 1 for s, o in access_tracking}
+        valid_sections = {s for s, o in access_tracking}
         for section_name in fileconfig.sections():
             section = section_name.lower()
             if section not in valid_sections and section not in objects:
@@ -464,7 +499,7 @@ def check_unused_options(self, config, error_on_unused):
             for option in fileconfig.options(section_name):
                 option = option.lower()
                 if (section, option) not in access_tracking:
-                    if error_on_unused:
+                    if error_on_unused and section != "constants":
                         raise error(
                             "Option '%s' is not valid in section '%s'"
                             % (option, section)
@@ -520,7 +555,10 @@ def _build_status(self, config):
 
         for section, option in self.unused_options:
             _type = "unused_option"
-            msg = f"Option '{option}' in section '{section}' is invalid"
+            if section == "constants":
+                msg = f"Constant '{option}' is unused"
+            else:
+                msg = f"Option '{option}' in section '{section}' is invalid"
             self.warn(_type, msg, section, option)
         for section in self.unused_sections:
             _type = "unused_section"

test/klippy/danger_options.cfg

@@ -1,9 +1,11 @@
+[constants]
+false: False
 
 [poopybutt]
 [danger_options]
 log_statistics: False
 log_config_file_at_startup: False
-error_on_unused_config_options: False
+error_on_unused_config_options: ${constants.false}
 log_bed_mesh_at_startup: False
 log_shutdown_info: False
 allow_plugin_override: True