remote/client: prepare handling of multiple Errors

A future commit will introduce ClientSession code that needs to
handle multiple Error exceptions, aggregated into a dedicated group.

Python 3.11 introduced support for exception groups [1]. This allows
raising multiple exceptions (not caused by one another) while keeping
their individual tracebacks etc. intact (i.e. `labgrid-client -d` will
still show all individual tracebacks).

Since labgrid will drop Python 3.10 support in October 2026 earliest,
use the backported version of exception groups until then.

While at it, add a Error.__str__() method, so Error and ErrorGroup can
use the same print to stderr statement in the except clause.

[1] https://docs.python.org/3/library/exceptions.html#exception-groups

Signed-off-by: Bastian Krause <bst@pengutronix.de>

Author: Bastian-Krause

labgrid/remote/client.py

@@ -28,6 +28,9 @@
 import attr
 import grpc
 
+# TODO: drop if Python >= 3.11 guaranteed
+from exceptiongroup import ExceptionGroup  # pylint: disable=redefined-builtin
+
 from .common import (
     ResourceEntry,
     ResourceMatch,
@@ -57,7 +60,8 @@
 
 
 class Error(Exception):
-    pass
+    def __str__(self):
+        return f"Error: {' '.join(self.args)}"
 
 
 class UserError(Error):
@@ -72,6 +76,13 @@ class InteractiveCommandError(Error):
     pass
 
 
+class ErrorGroup(ExceptionGroup):
+    def __str__(self):
+        # TODO: drop pylint disable once https://github.com/pylint-dev/pylint/issues/8985 is fixed
+        errors_combined = "\n".join(f"- {' '.join(e.args)}" for e in self.exceptions)  # pylint: disable=not-an-iterable
+        return f"{self.message}:\n{errors_combined}"
+
+
 @attr.s(eq=False)
 class ClientSession:
     """The ClientSession encapsulates all the actions a Client can invoke on
@@ -2212,11 +2223,11 @@ def main():
             if args.debug:
                 traceback.print_exc(file=sys.stderr)
             exitcode = e.exitcode
-        except Error as e:
+        except (Error, ErrorGroup) as e:
             if args.debug:
                 traceback.print_exc(file=sys.stderr)
             else:
-                print(f"{parser.prog}: error: {e}", file=sys.stderr)
+                print(f"{parser.prog}: {e}", file=sys.stderr)
             exitcode = 1
         except KeyboardInterrupt:
             exitcode = 1

pyproject.toml

@@ -33,6 +33,7 @@ classifiers = [
 ]
 dependencies = [
     "attrs>=21.4.0",
+    "exceptiongroup>=1.3.0",  # TODO: drop if Python >= 3.11 guaranteed
     "grpcio>=1.64.1, <2.0.0",
     "grpcio-reflection>=1.64.1, <2.0.0",
     "protobuf>=5.27.0",