feat(pytest): add an assert_error plugin method

Fixes #162

craft_cli/pytest_plugin.py

@@ -22,12 +22,12 @@
 import pathlib
 import re
 import tempfile
-from typing import TYPE_CHECKING
+from typing import TYPE_CHECKING, cast
 from unittest.mock import call
 
 import pytest
 
-from craft_cli import messages, printer
+from craft_cli import errors, messages, printer
 
 if TYPE_CHECKING:
     from unittest.mock import _Call
@@ -163,6 +163,18 @@ def assert_messages(self, texts):
         """
         self.assert_interactions([call("message", text) for text in texts])
 
+    def assert_error(self, error: errors.CraftError) -> errors.CraftError:
+        """Check that the 'error' method was called with the given error."""
+        # Error should be the last thing called, so start at the end.
+        errors_called = []
+        for stored_call in reversed(self.interactions):
+            if stored_call.args[0] != "error":
+                continue
+            errors_called.append(stored_call.args[1])
+            if stored_call.args[1] == error:
+                return cast(errors.CraftError, stored_call.args[1])
+        raise AssertionError(f"Error not emitted: {error!r}", errors_called[::-1])
+
     def assert_interactions(self, expected_call_list):
         """Check that the expected call list happen at some point between all stored calls.
 
@@ -205,7 +217,7 @@ def advance(self, *a, **k):
 def emitter(monkeypatch):
     """Provide a helper to test everything that was shown using the Emitter."""
     recording_emitter = RecordingEmitter()
-    for method_name in ("message", "progress", "verbose", "debug", "trace"):
+    for method_name in ("message", "progress", "verbose", "debug", "trace", "error"):
         monkeypatch.setattr(
             messages.emit,
             method_name,

tests/test_pytest_plugin.py

@@ -14,12 +14,13 @@
 # Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
 
 """Test the fixtures provided by Craft CLI."""
-
+import copy
 from unittest.mock import call
 
 import pytest
 
-from craft_cli import messages, printer
+from craft_cli import messages, printer, errors
+
 
 # -- tests for the `init_emitter` auto-fixture
 
@@ -183,6 +184,41 @@ def test_emitter_messages(emitter):
     )
 
 
+@pytest.mark.parametrize(
+    "error",
+    [
+        errors.CraftError("Basic error"),
+        errors.CraftError("Detailed", details="Very detailed"),
+        errors.CraftError("Resolved", resolution="640x480"),
+        errors.CraftError("Documented", docs_url="https://docs.ubuntu.com"),
+        errors.CraftError("Detailed Resolved", details="Extra fine", resolution="3840x2160"),
+    ],
+)
+def test_emitter_error_success(emitter, error):
+    """Verify that the checking the emitter for the correct error succeeds."""
+    error_copy = copy.deepcopy(error)  # Ensure we're not just checking identity.
+    messages.emit.error(error)
+    assert emitter.assert_error(error_copy) is error
+
+
+@pytest.mark.parametrize(
+    ("error", "assert_error"),
+    [
+        (errors.CraftError("Basic"), errors.CraftError("Very basic")),
+        (
+            errors.CraftError("Detailed", details="Case sensitive"),
+            errors.CraftError("Detailed", details="case Sensitive"),
+        ),
+    ],
+)
+def test_emitter_error_failure(emitter, error, assert_error):
+    """Verify that checking the emitter for the wrong error fails."""
+    messages.emit.error(error)
+    with pytest.raises(AssertionError, match="Error not emitted:") as exc_info:
+        emitter.assert_error(assert_error)
+    assert exc_info.value.args[1] == [error]
+
+
 def test_emitter_interactions_positive_complete(emitter):
     """All interactions can be verified, complete."""
     messages.emit.progress("foo")