fix: Stop exiting interpreter on error (#140)

guppylang/error.py

@@ -1,18 +1,18 @@
 import ast
 import functools
+import os
 import sys
 import textwrap
-from collections.abc import Callable, Sequence
+from collections.abc import Callable, Iterator, Sequence
+from contextlib import contextmanager
 from dataclasses import dataclass, field
+from types import TracebackType
 from typing import Any, TypeVar, cast
 
 from guppylang.ast_util import AstNode, get_file, get_line_offset, get_source
 from guppylang.gtypes import BoundTypeVar, ExistentialTypeVar, FunctionType, GuppyType
 from guppylang.hugr.hugr import Node, OutPortV
 
-# Whether the interpreter should exit when a Guppy error occurs
-EXIT_ON_ERROR: bool = True
-
 
 @dataclass(frozen=True)
 class SourceLoc:
@@ -136,6 +136,18 @@ def unsolved_vars(self) -> set[ExistentialTypeVar]:
         return set()
 
 
+ExceptHook = Callable[[type[BaseException], BaseException, TracebackType | None], Any]
+
+
+@contextmanager
+def exception_hook(hook: ExceptHook) -> Iterator[None]:
+    """Sets a custom `excepthook` for the scope of a 'with' block."""
+    old_hook = sys.excepthook
+    sys.excepthook = hook
+    yield
+    sys.excepthook = old_hook
+
+
 def format_source_location(
     loc: ast.AST,
     num_lines: int = 3,
@@ -169,27 +181,48 @@ def format_source_location(
 def pretty_errors(f: FuncT) -> FuncT:
     """Decorator to print custom error banners when a `GuppyError` occurs."""
 
+    def hook(
+        excty: type[BaseException], err: BaseException, traceback: TracebackType | None
+    ) -> None:
+        """Custom `excepthook` that intercepts `GuppyExceptions` for pretty printing."""
+        # Fall back to default hook if it's not a GuppyException or we're missing an
+        # error location
+        if not isinstance(err, GuppyError) or err.location is None:
+            sys.__excepthook__(excty, err, traceback)
+            return
+
+        loc = err.location
+        file, line_offset = get_file(loc), get_line_offset(loc)
+        assert file is not None
+        assert line_offset is not None
+        line = line_offset + loc.lineno - 1
+        sys.stderr.write(
+            f"Guppy compilation failed. Error in file {file}:{line}\n\n"
+            f"{format_source_location(loc)}\n"
+            f"{err.__class__.__name__}: {err.get_msg()}\n",
+        )
+
     @functools.wraps(f)
     def pretty_errors_wrapped(*args: Any, **kwargs: Any) -> Any:
-        try:
-            return f(*args, **kwargs)
-        except GuppyError as err:
-            # Reraise if we're missing a location
-            if not err.location:
+        with exception_hook(hook):
+            try:
+                return f(*args, **kwargs)
+            except GuppyError as err:
+                # For normal usage, this `try` block is not necessary since the
+                # excepthook is automatically invoked when the exception (which is being
+                # reraised below) is not handled. However, when running tests, we have
+                # to manually invoke the hook to print the error message, since the
+                # tests always have to capture exceptions.
+                if _pytest_running():
+                    hook(type(err), err, err.__traceback__)
                 raise
-            loc = err.location
-            file, line_offset = get_file(loc), get_line_offset(loc)
-            assert file is not None
-            assert line_offset is not None
-            line = line_offset + loc.lineno - 1
-            print(  # noqa: T201
-                f"Guppy compilation failed. Error in file {file}:{line}\n\n"
-                f"{format_source_location(loc)}\n"
-                f"{err.__class__.__name__}: {err.get_msg()}",
-                file=sys.stderr,
-            )
-            if EXIT_ON_ERROR:
-                sys.exit(1)
-            return None
 
     return cast(FuncT, pretty_errors_wrapped)
+
+
+def _pytest_running() -> bool:
+    """Checks if we are currently running pytest.
+
+    See https://docs.pytest.org/en/latest/example/simple.html#pytest-current-test-environment-variable
+    """
+    return "PYTEST_CURRENT_TEST" in os.environ

tests/error/util.py

@@ -2,9 +2,7 @@
 import pathlib
 import pytest
 
-from typing import Any
-from collections.abc import Callable
-
+from guppylang.error import GuppyError
 from guppylang.hugr import tys
 from guppylang.hugr.tys import TypeBound
 from guppylang.module import GuppyModule
@@ -17,7 +15,7 @@ def run_error_test(file, capsys):
     spec = importlib.util.spec_from_file_location("test_module", file)
     py_module = importlib.util.module_from_spec(spec)
 
-    with pytest.raises(SystemExit):
+    with pytest.raises(GuppyError):
         spec.loader.exec_module(py_module)
 
     err = capsys.readouterr().err