Add a context modifier registry

Author: Andy Babic

pattern_library/cm_utils.py

@@ -0,0 +1,16 @@
+
+import inspect
+from typing import Callable
+
+
+def accepts_kwarg(func: Callable, kwarg: str) -> bool:
+    """
+    Returns a boolean indicating whether the callable  ``func`` has
+    a signature that accepts the keyword argument ``kwarg``.
+    """
+    signature = inspect.signature(func)
+    try:
+        signature.bind_partial(**{kwarg: None})
+        return True
+    except TypeError:
+        return False

pattern_library/context_modifiers.py

@@ -0,0 +1,53 @@
+from collections import defaultdict
+from operator import attrgetter
+from typing import Callable
+
+from django.core.exceptions import ImproperlyConfigured
+
+from .cm_utils import accepts_kwarg
+
+GENERIC_CM_KEY = "__generic__"
+ORDER_ATTR_NAME = "__cm_order"
+
+
+class ContextModifierRegistry(defaultdict):
+    def __init__(self):
+        super().__init__(list)
+
+    def register(self, func: Callable, template: str = None, order: int = 0) -> None:
+        """
+        Adds a context modifier to the registry.
+        """
+        if not callable(func):
+            raise ImproperlyConfigured(
+                f"Context modifiers must be callables. {func} is a {type(func).__name__}."
+            )
+        if not accepts_kwarg(func, "context"):
+            raise ImproperlyConfigured(
+                f"Context modifiers must accept a 'context' argument. {func} does not."
+            )
+        if not accepts_kwarg(func, "request"):
+            raise ImproperlyConfigured(
+                f"Context modifiers must accept a 'request' argument. {func} does not."
+            )
+
+        key = template or GENERIC_CM_KEY
+        if func not in self[key]:
+            setattr(func, ORDER_ATTR_NAME, order)
+            self[key].append(func)
+            self[key].sort(key=attrgetter(ORDER_ATTR_NAME))
+
+        return func
+
+    def register_decorator(self, func: Callable = None, **kwargs):
+        if func is None:
+            return lambda func: self.register(func, **kwargs)
+        return self.register(func, **kwargs)
+
+    def get_for_template(self, template: str):
+        modifiers = self[GENERIC_CM_KEY] + self[template]
+        return sorted(modifiers, key=attrgetter(ORDER_ATTR_NAME))
+
+
+registry = ContextModifierRegistry()
+register_context_modifier = registry.register_decorator