Repository: fsspec/universal_pathlib (python)

Issue:
Behaviour of joinpath when the second path has protocol
```
UPath('a').joinpath(UPath('s3:///b'))
```
This call will return `PosixUPath('/b')`, but
```
UPath('a').joinuri(UPath('s3:///b'))
```
will return `S3Path('s3:///b')`.

Maybe in `joinpath`, we should return just the second path when the second path has protocol (like in `joinuri`)?

Pull request description:
UPath.joinpath raise error on protocol mismatch
Close #213 

This PR adds tests for `joinpath`, `joinuri`, and `__truediv__` to ensure an error is raised when UPaths with incompatible protocols are joined.

It also raises a ValueError for cloudpaths if the UPath is initialized without a bucket or container.

Patch:
diff --git a/upath/_protocol.py b/upath/_protocol.py
index a3827bd..d333dd6 100644
--- a/upath/_protocol.py
+++ b/upath/_protocol.py
@@ -3,11 +3,16 @@ from __future__ import annotations
 import os
 import re
 from pathlib import PurePath
+from typing import TYPE_CHECKING
 from typing import Any
 
+if TYPE_CHECKING:
+    from upath.core import UPath
+
 __all__ = [
     "get_upath_protocol",
     "normalize_empty_netloc",
+    "compatible_protocol",
 ]
 
 # Regular expression to match fsspec style protocols.
@@ -59,3 +64,15 @@ def normalize_empty_netloc(pth: str) -> str:
             path = m.group("path")
             pth = f"{protocol}:///{path}"
     return pth
+
+
+def compatible_protocol(protocol: str, *args: str | os.PathLike[str] | UPath) -> bool:
+    """check if UPath protocols are compatible"""
+    for arg in args:
+        other_protocol = get_upath_protocol(arg)
+        # consider protocols equivalent if they match up to the first "+"
+        other_protocol = other_protocol.partition("+")[0]
+        # protocols: only identical (or empty "") protocols can combine
+        if other_protocol and other_protocol != protocol:
+            return False
+    return True
diff --git a/upath/core.py b/upath/core.py
index 4c86e28..714bfa3 100644
--- a/upath/core.py
+++ b/upath/core.py
@@ -35,6 +35,7 @@ from upath._compat import str_remove_suffix
 from upath._flavour import LazyFlavourDescriptor
 from upath._flavour import upath_get_kwargs_from_url
 from upath._flavour import upath_urijoin
+from upath._protocol import compatible_protocol
 from upath._protocol import get_upath_protocol
 from upath._stat import UPathStatResult
 from upath.registry import get_upath_class
@@ -251,23 +252,12 @@ class UPath(PathlibPathShim, Path):
             self._storage_options = storage_options.copy()
 
         # check that UPath subclasses in args are compatible
-        # --> ensures items in _raw_paths are compatible
-        for arg in args:
-            if not isinstance(arg, UPath):
-                continue
-            # protocols: only identical (or empty "") protocols can combine
-            if arg.protocol and arg.protocol != self._protocol:
-                raise TypeError("can't combine different UPath protocols as parts")
-            # storage_options: args may not define other storage_options
-            if any(
-                self._storage_options.get(key) != value
-                for key, value in arg.storage_options.items()
-            ):
-                # TODO:
-                #   Future versions of UPath could verify that storage_options
-                #   can be combined between UPath instances. Not sure if this
-                #   is really necessary though. A warning might be enough...
-                pass
+        # TODO:
+        #   Future versions of UPath could verify that storage_options
+        #   can be combined between UPath instances. Not sure if this
+        #   is really necessary though. A warning might be enough...
+        if not compatible_protocol(self._protocol, *args):
+            raise ValueError("can't combine incompatible UPath protocols")
 
         # fill ._raw_paths
         if hasattr(self, "_raw_paths"):
diff --git a/upath/implementations/cloud.py b/upath/implementations/cloud.py
index e2f4cb9..455fca6 100644
--- a/upath/implementations/cloud.py
+++ b/upath/implementations/cloud.py
@@ -22,6 +22,13 @@ _CloudAccessor = _FSSpecAccessorShim
 class CloudPath(UPath):
     __slots__ = ()
 
+    def __init__(
+        self, *args, protocol: str | None = None, **storage_options: Any
+    ) -> None:
+        super().__init__(*args, protocol=protocol, **storage_options)
+        if not self.drive and len(self.parts) > 1:
+            raise ValueError("non key-like path provided (bucket/container missing)")
+
     @classmethod
     def _transform_init_args(
         cls,
diff --git a/upath/implementations/local.py b/upath/implementations/local.py
index 4552585..a0961ce 100644
--- a/upath/implementations/local.py
+++ b/upath/implementations/local.py
@@ -12,6 +12,7 @@ from typing import Collection
 from typing import MutableMapping
 from urllib.parse import SplitResult
 
+from upath._protocol import compatible_protocol
 from upath.core import UPath
 
 __all__ = [
@@ -141,6 +142,8 @@ class PosixUPath(PosixPath, LocalPath):  # type: ignore[misc]
                 raise NotImplementedError(
                     f"cannot instantiate {cls.__name__} on your system"
                 )
+            if not compatible_protocol("", *args):
+                raise ValueError("can't combine incompatible UPath protocols")
             obj = super().__new__(cls, *args)
             obj._protocol = ""
             return obj  # type: ignore[return-value]
@@ -152,6 +155,11 @@ class PosixUPath(PosixPath, LocalPath):  # type: ignore[misc]
             self._drv, self._root, self._parts = type(self)._parse_args(args)
             _upath_init(self)
 
+        def _make_child(self, args):
+            if not compatible_protocol(self._protocol, *args):
+                raise ValueError("can't combine incompatible UPath protocols")
+            return super()._make_child(args)
+
         @classmethod
         def _from_parts(cls, *args, **kwargs):
             obj = super(Path, cls)._from_parts(*args, **kwargs)
@@ -205,6 +213,8 @@ class WindowsUPath(WindowsPath, LocalPath):  # type: ignore[misc]
                 raise NotImplementedError(
                     f"cannot instantiate {cls.__name__} on your system"
                 )
+            if not compatible_protocol("", *args):
+                raise ValueError("can't combine incompatible UPath protocols")
             obj = super().__new__(cls, *args)
             obj._protocol = ""
             return obj  # type: ignore[return-value]
@@ -216,6 +226,11 @@ class WindowsUPath(WindowsPath, LocalPath):  # type: ignore[misc]
             self._drv, self._root, self._parts = self._parse_args(args)
             _upath_init(self)
 
+        def _make_child(self, args):
+            if not compatible_protocol(self._protocol, *args):
+                raise ValueError("can't combine incompatible UPath protocols")
+            return super()._make_child(args)
+
         @classmethod
         def _from_parts(cls, *args, **kwargs):
             obj = super(Path, cls)._from_parts(*args, **kwargs)


Test patch:
diff --git a/upath/tests/test_core.py b/upath/tests/test_core.py
index f52e6b5..92d608d 100644
--- a/upath/tests/test_core.py
+++ b/upath/tests/test_core.py
@@ -410,3 +410,32 @@ def test_query_string(uri, query_str):
     p = UPath(uri)
     assert str(p).endswith(query_str)
     assert p.path.endswith(query_str)
+
+
+@pytest.mark.parametrize(
+    "base,join",
+    [
+        ("/a", "s3://bucket/b"),
+        ("s3://bucket/a", "gs://b/c"),
+        ("gs://bucket/a", "memory://b/c"),
+        ("memory://bucket/a", "s3://b/c"),
+    ],
+)
+def test_joinpath_on_protocol_mismatch(base, join):
+    with pytest.raises(ValueError):
+        UPath(base).joinpath(UPath(join))
+    with pytest.raises(ValueError):
+        UPath(base) / UPath(join)
+
+
+@pytest.mark.parametrize(
+    "base,join",
+    [
+        ("/a", "s3://bucket/b"),
+        ("s3://bucket/a", "gs://b/c"),
+        ("gs://bucket/a", "memory://b/c"),
+        ("memory://bucket/a", "s3://b/c"),
+    ],
+)
+def test_joinuri_on_protocol_mismatch(base, join):
+    assert UPath(base).joinuri(UPath(join)) == UPath(join)