Add check for passed reader contribution

src/npe2/io_utils.py

@@ -154,9 +154,35 @@ def _read(
     if _pm is None:
         _pm = PluginManager.instance()
 
-    for rdr in _pm.iter_compatible_readers(paths):
-        if plugin_name and rdr.plugin_name != plugin_name:
-            continue
+    rdrs = list(_pm.iter_compatible_readers(paths))
+    if plugin_name:
+        # user might have passed 'plugin.reader_contrib' as the command
+        plugin, contrib = (
+            tuple(plugin_name.split(".")) if "." in plugin_name else (plugin_name, None)
+        )
+        chosen_compatible_readers = list(
+            filter(lambda rdr: rdr.plugin_name == plugin, rdrs)
+        )
+        if contrib:
+            chosen_compatible_readers = list(
+                filter(
+                    lambda rdr: rdr.command.split(".")[1] == contrib,
+                    chosen_compatible_readers,
+                )
+            )
+        if not chosen_compatible_readers:
+            raise ValueError(
+                f"Given reader '{plugin_name}' is not a compatible reader for {paths}. "
+                + (
+                    f"Available readers for {paths} are: {[rdr.command for rdr in rdrs]}"
+                    if rdrs
+                    else "No compatible readers are available."
+                )
+            )
+    else:
+        chosen_compatible_readers = rdrs
+
+    for rdr in chosen_compatible_readers:
         read_func = rdr.exec(
             kwargs={"path": paths, "stack": stack, "_registry": _pm.commands}
         )

tests/test__io_utils.py

@@ -23,10 +23,20 @@ def test_read(uses_sample_plugin):
 
 def test_read_with_unknown_plugin(uses_sample_plugin):
     # no such plugin name.... skips over the sample plugin & error is specific
-    with pytest.raises(ValueError, match="Plugin 'nope' was selected"):
+    with pytest.raises(
+        ValueError, match="Given reader 'nope' is not a compatible reader"
+    ):
         read(["some.fzzy"], plugin_name="nope", stack=False)
 
 
+def test_read_with_unknown_plugin_no_readers(uses_sample_plugin):
+    with pytest.raises(
+        ValueError, match="Given reader 'nope' is not a compatible reader"
+    ):
+        read(["some.nope"], plugin_name="nope", stack=False)
+    # assert 'No compatible readers are available.' in str(e)
+
+
 def test_read_with_no_plugin():
     # no plugin passed and none registered
     with pytest.raises(ValueError, match="No readers returned"):
@@ -62,6 +72,22 @@ def read(paths):
     io_utils._read(["some.fzzy"], plugin_name=short_name, stack=False, _pm=pm)
 
 
+def test_read_with_reader_contribution_plugin(uses_sample_plugin):
+    assert read(
+        ["some.fzzy"], stack=False, plugin_name=f"{SAMPLE_PLUGIN_NAME}.some_reader"
+    ) == [(None,)]
+
+    # if the wrong contribution is passed we get useful error message
+    with pytest.raises(
+        ValueError,
+        match="Given reader 'my-plugin.not_a_reader' is not a compatible reader",
+    ) as e:
+        read(
+            ["some.fzzy"], stack=False, plugin_name=f"{SAMPLE_PLUGIN_NAME}.not_a_reader"
+        )
+    assert "Available readers for" in str(e)
+
+
 def test_read_return_reader(uses_sample_plugin):
     data, reader = read_get_reader("some.fzzy")
     assert data == [(None,)]