From 52f0fdba40b6cc19d7915565b822fbfa965caa12 Mon Sep 17 00:00:00 2001
From: Sven Augustin <sven.augustin@psi.ch>
Date: Mon, 13 Jul 2020 17:40:08 +0200
Subject: [PATCH] added Registry class and instances() function, which allow to
 automatically register/show instances of a class (and optionally its
 subclasses); for convenience, also added a combined Registry and ABC base
 class

---
 slic/utils/metaclasses.py | 35 +++++++++++++++++++++
 slic/utils/registry.py    | 66 +++++++++++++++++++++++++++++++++++++++
 2 files changed, 101 insertions(+)
 create mode 100644 slic/utils/metaclasses.py
 create mode 100644 slic/utils/registry.py

diff --git a/slic/utils/metaclasses.py b/slic/utils/metaclasses.py
new file mode 100644
index 000000000..98a89e3db
--- /dev/null
+++ b/slic/utils/metaclasses.py
@@ -0,0 +1,35 @@
+from abc import ABCMeta
+
+from .registry import RegistryMeta
+
+
+def combine_classes(*bases):
+    """
+    Convenience wrapper around type(name, bases, namespace)
+    for dynamically combining bases into a single class
+    """
+    name = "".join(a.__name__ for a in bases)
+    namespace = {}
+    return type(name, bases, namespace)
+
+
+
+class RegistryABC(object, metaclass=combine_classes(RegistryMeta, ABCMeta)):
+    """
+    Helper class that allows adding registry and ABC functionalities via inheritance
+    Does nothing but set a combined RegistryMeta and ABCMeta as metaclass
+    """
+    pass
+
+
+
+## The above corresponds to:
+
+#class RegistryMetaABCMeta(RegistryMeta, ABCMeta): # combine metaclasses
+#    pass
+
+#class RegistryABC(object, metaclass=RegistryMetaABCMeta):
+#    pass
+
+
+
diff --git a/slic/utils/registry.py b/slic/utils/registry.py
new file mode 100644
index 000000000..79fc33733
--- /dev/null
+++ b/slic/utils/registry.py
@@ -0,0 +1,66 @@
+import weakref
+
+from .utils import typename
+
+
+class RegistryMeta(type):
+    """
+    Metaclass containing the registry logic
+    In most cases, it will be more convenient to inherit from Registry instead of using RegistryMeta directly
+    """
+
+    def __init__(cls, name, bases, namespace):
+        super().__init__(name, bases, namespace) # creates the class
+        cls.__instances__ = weakref.WeakSet()
+
+    def __call__(cls, *args, **kwargs):
+        inst = super().__call__(*args, **kwargs) # creates the instance (calls __new__ and __init__ methods)
+        cls.__instances__.add(inst)
+        return inst
+
+
+
+class Registry(object, metaclass=RegistryMeta):
+    """
+    Helper class that allows adding registry functionality via inheritance
+    Does nothing but set RegistryMeta as metaclass
+    """
+    pass
+
+
+
+def instances(reg, recursive=True):
+    """
+    Return a set of all instances of reg
+    If recursive=True also include instances of subclasses of reg
+    """
+    res = _collect_instances(reg, recursive=recursive)
+    return set(res)
+
+
+def _collect_instances(reg, recursive=True):
+    """
+    Return a weakref.WeakSet of all instances of reg
+    If recursive=True also include instances of subclasses of reg
+    """
+    _check_registry(reg)
+    instances = reg.__instances__.copy()
+    if recursive:
+        for subcls in reg.__subclasses__():
+            instances |= _collect_instances(subcls, recursive=recursive)
+    return instances
+
+
+def _check_registry(reg):
+    """
+    Try whether reg has __instances__ attribute, if missing raise TypeError
+    """
+    try:
+        reg.__instances__
+    except AttributeError as exc:
+        tname = typename(reg)
+        cname = reg.__name__
+        raise TypeError(f"{tname} object '{cname}' has no registry functionality") from exc
+
+
+