Repository: fsspec/universal_pathlib (python)

Issue:
Add ability to register custom UPath implementations
It appears that the only way currently to register a custom implementation and avoid falling back on `upath.core` is to import and append to `upath.registry._register.known_implementations`. It would be helpful if there were a top-level registration, similar to `fsspec.register_implementation(name, filesystem)`

Pull request:
Implementation registration
Close #99

This PR adds:

- `upath.registry.available_implementations()` which returns a list of protocols for all available UPath implementations
- `upath.registry.register_implementation()` which let's a user register a UPath subclass and it's corresponding protocol dynamically

and it adds an `"universal_pathlib.implementations"` entry point, that let's users register their custom UPath subclasses dynamically via their package metadata.



Patch:
diff --git a/README.md b/README.md
index 503efdb..160588e 100644
--- a/README.md
+++ b/README.md
@@ -147,6 +147,39 @@ with fs.open(p.path) as f:
     data = f.read()
 ```
 
+### Register custom UPath implementations
+
+In case you develop a custom UPath implementation, feel free to open an issue to discuss integrating it
+in `universal_pathlib`. You can dynamically register your implementation too! Here are your options:
+
+#### Dynamic registration from Python
+
+```python
+# for example: mymodule/submodule.py
+from upath import UPath
+from upath.registry import register_implementation
+
+my_protocol = "myproto"
+class MyPath(UPath):
+    ...  # your custom implementation
+
+register_implementation(my_protocol, MyPath)
+```
+
+#### Registration via entry points
+
+```toml
+# pyproject.toml
+[project.entry-points."unversal_pathlib.implementations"]
+myproto = "my_module.submodule:MyPath"
+```
+
+```ini
+# setup.cfg
+[options.entry_points]
+universal_pathlib.implementations =
+    myproto = my_module.submodule:MyPath
+```
 
 ## Contributing
 
diff --git a/upath/registry.py b/upath/registry.py
index b30fd6d..43bf3cc 100644
--- a/upath/registry.py
+++ b/upath/registry.py
@@ -1,22 +1,63 @@
+"""upath.registry -- registry for file system specific implementations
+
+Retrieve UPath implementations via `get_upath_class`.
+Register custom UPath subclasses in one of two ways:
+
+### directly from Python
+
+>>> from upath import UPath
+>>> from upath.registry import register_implementation
+>>> my_protocol = "myproto"
+>>> class MyPath(UPath):
+...     pass
+>>> register_implementation(my_protocol, MyPath)
+
+### via entry points
+
+```toml
+# pyproject.toml
+[project.entry-points."unversal_pathlib.implementations"]
+myproto = "my_module.submodule:MyPath"
+```
+
+```ini
+# setup.cfg
+[options.entry_points]
+universal_pathlib.implementations =
+    myproto = my_module.submodule:MyPath
+```
+"""
 from __future__ import annotations
 
-import importlib
 import os
+import re
+import sys
 import warnings
+from collections import ChainMap
 from functools import lru_cache
-from typing import TYPE_CHECKING
+from importlib import import_module
+from importlib.metadata import entry_points
+from typing import Iterator
+from typing import MutableMapping
 
 from fsspec.core import get_filesystem_class
+from fsspec.registry import available_protocols
 
-if TYPE_CHECKING:
-    from upath.core import UPath
+import upath.core
 
 __all__ = [
     "get_upath_class",
+    "available_implementations",
+    "register_implementation",
 ]
 
 
-class _Registry:
+_ENTRY_POINT_GROUP = "universal_pathlib.implementations"
+
+
+class _Registry(MutableMapping[str, "type[upath.core.UPath]"]):
+    """internal registry for UPath subclasses"""
+
     known_implementations: dict[str, str] = {
         "abfs": "upath.implementations.cloud.AzurePath",
         "abfss": "upath.implementations.cloud.AzurePath",
@@ -35,26 +76,118 @@ class _Registry:
         "webdav+https": "upath.implementations.webdav.WebdavPath",
     }
 
-    def __getitem__(self, item: str) -> type[UPath] | None:
-        try:
-            fqn = self.known_implementations[item]
-        except KeyError:
-            return None
-        module_name, name = fqn.rsplit(".", 1)
-        mod = importlib.import_module(module_name)
-        return getattr(mod, name)  # type: ignore
+    def __init__(self) -> None:
+        if sys.version_info >= (3, 10):
+            eps = entry_points(group=_ENTRY_POINT_GROUP)
+        else:
+            eps = entry_points().get(_ENTRY_POINT_GROUP, [])
+        self._entries = {ep.name: ep for ep in eps}
+        self._m = ChainMap({}, self.known_implementations)  # type: ignore
+
+    def __contains__(self, item: object) -> bool:
+        return item in set().union(self._m, self._entries)
+
+    def __getitem__(self, item: str) -> type[upath.core.UPath]:
+        fqn = self._m.get(item)
+        if fqn is None:
+            if item in self._entries:
+                fqn = self._m[item] = self._entries[item].load()
+        if fqn is None:
+            raise KeyError(f"{item} not in registry")
+        if isinstance(fqn, str):
+            module_name, name = fqn.rsplit(".", 1)
+            mod = import_module(module_name)
+            cls = getattr(mod, name)  # type: ignore
+        else:
+            cls = fqn
+        return cls
+
+    def __setitem__(self, item: str, value: type[upath.core.UPath] | str) -> None:
+        if not (
+            (isinstance(value, type) and issubclass(value, upath.core.UPath))
+            or isinstance(value, str)
+        ):
+            raise ValueError(
+                f"expected UPath subclass or FQN-string, got: {type(value).__name__!r}"
+            )
+        self._m[item] = value
+
+    def __delitem__(self, __v: str) -> None:
+        raise NotImplementedError("removal is unsupported")
+
+    def __len__(self) -> int:
+        return len(set().union(self._m, self._entries))
+
+    def __iter__(self) -> Iterator[str]:
+        return iter(set().union(self._m, self._entries))
 
 
 _registry = _Registry()
 
 
-@lru_cache
-def get_upath_class(protocol: str) -> type[UPath] | None:
-    """Return the upath cls for the given protocol."""
-    cls: type[UPath] | None = _registry[protocol]
-    if cls is not None:
-        return cls
+def available_implementations(*, fallback: bool = False) -> list[str]:
+    """return a list of protocols for available implementations
+
+    Parameters
+    ----------
+    fallback:
+        If True, also return protocols for fsspec filesystems without
+        an implementation in universal_pathlib.
+    """
+    impl = list(_registry)
+    if not fallback:
+        return impl
     else:
+        return list({*impl, *available_protocols()})
+
+
+def register_implementation(
+    protocol: str,
+    cls: type[upath.core.UPath] | str,
+    *,
+    clobber: bool = False,
+) -> None:
+    """register a UPath implementation with a protocol
+
+    Parameters
+    ----------
+    protocol:
+        Protocol name to associate with the class
+    cls:
+        The UPath subclass for the protocol or a str representing the
+        full path to an implementation class like package.module.class.
+    clobber:
+        Whether to overwrite a protocol with the same name; if False,
+        will raise instead.
+    """
+    if not re.match(r"^[a-z][a-z0-9+_.]+$", protocol):
+        raise ValueError(f"{protocol!r} is not a valid URI scheme")
+    if not clobber and protocol in _registry:
+        raise ValueError(f"{protocol!r} is already in registry and clobber is False!")
+    _registry[protocol] = cls
+
+
+@lru_cache
+def get_upath_class(
+    protocol: str,
+    *,
+    fallback: bool = True,
+) -> type[upath.core.UPath] | None:
+    """Return the upath cls for the given protocol.
+
+    Returns `None` if no matching protocol can be found.
+
+    Parameters
+    ----------
+    protocol:
+        The protocol string
+    fallback:
+        If fallback is False, don't return UPath instances for fsspec
+        filesystems that don't have an implementation registered.
+    """
+    try:
+        return _registry[protocol]
+    except KeyError:
         if not protocol:
             if os.name == "nt":
                 from upath.implementations.local import WindowsUPath
@@ -64,6 +197,8 @@ def get_upath_class(protocol: str) -> type[UPath] | None:
                 from upath.implementations.local import PosixUPath
 
                 return PosixUPath
+        if not fallback:
+            return None
         try:
             _ = get_filesystem_class(protocol)
         except ValueError:
@@ -76,5 +211,4 @@ def get_upath_class(protocol: str) -> type[UPath] | None:
                 UserWarning,
                 stacklevel=2,
             )
-            mod = importlib.import_module("upath.core")
-            return mod.UPath  # type: ignore
+            return upath.core.UPath


Test patch:
diff --git a/upath/tests/test_registry.py b/upath/tests/test_registry.py
new file mode 100644
index 0000000..148f032
--- /dev/null
+++ b/upath/tests/test_registry.py
@@ -0,0 +1,126 @@
+import pytest
+from fsspec.registry import available_protocols
+
+from upath import UPath
+from upath.registry import available_implementations
+from upath.registry import get_upath_class
+from upath.registry import register_implementation
+
+IMPLEMENTATIONS = {
+    "abfs",
+    "abfss",
+    "adl",
+    "az",
+    "file",
+    "gcs",
+    "gs",
+    "hdfs",
+    "http",
+    "https",
+    "memory",
+    "s3",
+    "s3a",
+    "webdav+http",
+    "webdav+https",
+}
+
+
+@pytest.fixture(autouse=True)
+def reset_registry():
+    from upath.registry import _registry
+
+    try:
+        yield
+    finally:
+        _registry._m.maps[0].clear()  # type: ignore
+
+
+@pytest.fixture()
+def fake_entrypoint():
+    from importlib.metadata import EntryPoint
+
+    from upath.registry import _registry
+
+    ep = EntryPoint(
+        name="myeps",
+        value="upath.core:UPath",
+        group="universal_pathlib.implementations",
+    )
+    old_registry = _registry._entries.copy()
+
+    try:
+        _registry._entries["myeps"] = ep
+        yield
+    finally:
+        _registry._entries.clear()
+        _registry._entries.update(old_registry)
+
+
+def test_available_implementations():
+    impl = available_implementations()
+    assert len(impl) == len(set(impl))
+    assert set(impl) == IMPLEMENTATIONS
+
+
+def test_available_implementations_with_fallback():
+    impl = available_implementations(fallback=True)
+    assert set(impl) == IMPLEMENTATIONS.union(available_protocols())
+
+
+def test_available_implementations_with_entrypoint(fake_entrypoint):
+    impl = available_implementations()
+    assert set(impl) == IMPLEMENTATIONS.union({"myeps"})
+
+
+def test_register_implementation():
+    class MyProtoPath(UPath):
+        pass
+
+    register_implementation("myproto", MyProtoPath)
+
+    assert get_upath_class("myproto") is MyProtoPath
+
+
+def test_register_implementation_wrong_input():
+    with pytest.raises(TypeError):
+        register_implementation(None, UPath)  # type: ignore
+    with pytest.raises(ValueError):
+        register_implementation("incorrect**protocol", UPath)
+    with pytest.raises(ValueError):
+        register_implementation("myproto", object, clobber=True)  # type: ignore
+    with pytest.raises(ValueError):
+        register_implementation("file", UPath, clobber=False)
+    assert set(available_implementations()) == IMPLEMENTATIONS
+
+
+@pytest.mark.parametrize("protocol", IMPLEMENTATIONS)
+def test_get_upath_class(protocol):
+    upath_cls = get_upath_class("file")
+    assert issubclass(upath_cls, UPath)
+
+
+def test_get_upath_class_without_implementation(clear_registry):
+    with pytest.warns(
+        UserWarning, match="UPath 'mock' filesystem not explicitly implemented."
+    ):
+        upath_cls = get_upath_class("mock")
+    assert issubclass(upath_cls, UPath)
+
+
+def test_get_upath_class_without_implementation_no_fallback(clear_registry):
+    assert get_upath_class("mock", fallback=False) is None
+
+
+def test_get_upath_class_unknown_protocol(clear_registry):
+    assert get_upath_class("doesnotexist") is None
+
+
+def test_get_upath_class_from_entrypoint(fake_entrypoint):
+    assert issubclass(get_upath_class("myeps"), UPath)
+
+
+@pytest.mark.parametrize(
+    "protocol", [pytest.param("", id="empty-str"), pytest.param(None, id="none")]
+)
+def test_get_upath_class_falsey_protocol(protocol):
+    assert issubclass(get_upath_class(protocol), UPath)