Provide a mechanism for adding forms and other complex django objects into template contexts

Author: Andy Babic

pattern_library/object_utils.py

@@ -0,0 +1,133 @@
+import functools
+import inspect
+from importlib import import_module
+from typing import Any, Callable, Dict, Type
+
+from django.conf import settings
+from django.http.request import HttpRequest
+
+
+@functools.lru_cache(maxsize=None)
+def import_from_path(path: str) -> Type:
+    module_path, object_name = path.rsplit(".", 1)
+    module = import_module(module_path)
+    return getattr(module, object_name)
+
+
+@functools.lru_cache(maxsize=None)
+def accepts_kwarg(func: Callable, kwarg: str):
+    """
+    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
+
+
+@functools.lru_cache(maxsize=None)
+def get_custom_factories():
+    return getattr(settings, "PATTERN_LIBRARY_CUSTOM_FACTORIES", {})
+
+
+def simple_object_factory(
+    klass: Type, definition: Dict[str, Any], request: HttpRequest, **kwargs
+):
+    """
+    Creates an instance of type ``klass`` using the remaining key/value pairs
+    from the ``definition`` and the current ``request``. Any key/value pairs
+    NOT accepted by the classes ``__init__`` method are set as attributes on
+    the object after initialisation.
+    """
+    init_kwargs = {}
+    attributes = {}
+    for key, value in definition.items():
+        if accepts_kwarg(klass.__init__, key):
+            init_kwargs[key] = value
+        else:
+            attributes[key] = value
+    if accepts_kwarg(klass.__init__, "request"):
+        init_kwargs["request"] = request
+    obj = klass(**init_kwargs)
+    for attr, value in attributes.items():
+        setattr(obj, attr, value)
+    return klass(**definition)
+
+
+def make_object(
+    definition: Dict[str, Any],
+    context: Dict[str, Any],
+    request: HttpRequest,
+    template_name: str,
+) -> Any:
+    """
+    Converts an object defintion to an instance of the relevant class,
+    by importing the class, finding a suitable 'factory' function and
+    passing everything on to it.
+    """
+    classname = definition.pop("klass")
+    custom_factories = get_custom_factories()
+    if classname in custom_factories:
+        factory = import_from_path(custom_factories[classname])
+    else:
+        factory = simple_object_factory
+    return factory(
+        import_from_path(classname),
+        definition=definition,
+        context=context,
+        request=request,
+        template_name=template_name,
+    )
+
+
+def get_common_objects(
+    context: Dict[str, Any], request: HttpRequest, template_name: str
+) -> Dict[str, Any]:
+    """
+    Returns a dictionary of objects to be added to all pattern template
+    contexts, as defined by the ``PATTERN_LIBRARY_COMMON_OBJECTS`` setting.
+
+    All arguments passed to this method are for 'passing through' to object
+    factories only. ``inject_python_objects()`` is responsible for working
+    the return value into the template context.
+    """
+    objects = {}
+    for key, value in getattr(settings, "PATTERN_LIBRARY_COMMON_OBJECTS", {}).items():
+        definition = {}
+        if isinstance(value, str):
+            definition["klass"] = value
+        if isinstance(value, dict):
+            definition.update(value)
+        if definition:
+            objects[key] = make_object(definition, context, request, template_name)
+    return objects
+
+
+def replace_object_definitions(
+    search_dict: Dict[str, Any],
+    context: Dict[str, Any],
+    request: HttpRequest,
+    template_name: str,
+) -> None:
+    """
+    Recursively evaluates ``search_dict``, looking for dictionaries that
+    look like object definitions, and replacing those defintions with
+    instances of the relevant class.
+    """
+    for key, value in dict(search_dict).items():
+        if isinstance(value, dict):
+            replace_object_definitions(value, context, request, template_name)
+            if "klass" in value:
+                search_dict[key] = make_object(value, context, request, template_name)
+
+
+def inject_python_objects(
+    context: Dict[str, Any],
+    request: HttpRequest,
+    template_name: str,
+) -> None:
+    context.update(get_common_objects(context, request, template_name))
+    replace_object_definitions(context, context, request, template_name)

pattern_library/utils.py

@@ -16,6 +16,7 @@
     get_pattern_context_var_name, get_pattern_template_suffix, get_sections
 )
 from pattern_library.exceptions import TemplateIsNotPattern
+from pattern_library.object_utils import inject_python_objects
 
 
 def path_to_section():
@@ -194,6 +195,7 @@ def render_pattern(request, template_name, allow_non_patterns=False):
 
     context = get_pattern_context(template_name)
     context[get_pattern_context_var_name()] = True
+    inject_python_objects(context, request, template_name)
     return render_to_string(template_name, request=request, context=context)