fix: Show Python traceback when constructing constraints fail (#1082)

Fixes #969

python/python-core/src/main/python/_timefold_java_interop.py

@@ -277,6 +277,22 @@ def register_java_class(python_object: Solution_,
     return python_object
 
 
+def wrap_errors(func):
+    def wrapped_func(*args, **kwargs):
+        nonlocal func
+        try:
+            return func(*args, **kwargs)
+        except Exception as e:
+            import traceback
+            msg = ''.join(traceback.TracebackException.from_exception(e).format())
+            raise RuntimeError(msg)
+
+    wrapped_func.__doc__ = func.__doc__
+    wrapped_func.__qualname__ = func.__qualname__
+    wrapped_func.__name__ = func.__name__
+    return wrapped_func
+
+
 unique_class_id = 0
 """A unique identifier; used to guarantee the generated class java name is unique"""
 

python/python-core/src/main/python/score/_annotations.py

@@ -1,8 +1,9 @@
-from ._constraint_factory import ConstraintFactory
-from ._constraint_builder import Constraint
-from .._timefold_java_interop import ensure_init, _generate_constraint_provider_class, register_java_class
 from typing import TypeVar, Callable, TYPE_CHECKING
 
+from ._constraint_builder import Constraint
+from ._constraint_factory import ConstraintFactory
+from .._timefold_java_interop import ensure_init, _generate_constraint_provider_class, register_java_class, wrap_errors
+
 if TYPE_CHECKING:
     from ..score import Score
 
@@ -46,7 +47,7 @@ def constraint_provider(constraint_provider_function: Callable[[ConstraintFactor
     def constraint_provider_wrapper(function):
         def wrapped_constraint_provider(constraint_factory):
             from ..score import ConstraintFactory
-            out = function(ConstraintFactory(constraint_factory))
+            out = wrap_errors(function)(ConstraintFactory(constraint_factory))
             return out
         java_class = _generate_constraint_provider_class(function, wrapped_constraint_provider)
         return register_java_class(wrapped_constraint_provider, java_class)

python/python-core/tests/test_user_error.py

@@ -1,14 +1,14 @@
+import re
+from dataclasses import dataclass, field
+from typing import Annotated, List
+
+import pytest
 from timefold.solver import *
 from timefold.solver.config import *
 from timefold.solver.domain import *
 from timefold.solver.heuristic import *
 from timefold.solver.score import *
 
-import pytest
-import re
-from typing import Annotated, List
-from dataclasses import dataclass, field
-
 
 @planning_entity
 @dataclass
@@ -106,6 +106,30 @@ def not_proxied():
         )._to_java_solver_config()
 
 
+def test_constraint_construction_failed():
+    import inspect
+    line = inspect.getframeinfo(inspect.stack()[0][0]).lineno + 4
+
+    def bad_constraints(constraint_factory: ConstraintFactory):
+        return [
+            constraint_factory.for_each(BadEntity)
+            .penalize(SimpleScore.ONE)
+            .as_constraint('Penalize each entity')
+        ]
+
+    bad_constraints = constraint_provider(bad_constraints)
+    solver_config = SolverConfig(
+        solution_class=Solution,
+        entity_class_list=[Entity],
+        score_director_factory_config=ScoreDirectorFactoryConfig(
+            constraint_provider_function=bad_constraints
+        )
+    )
+
+    with pytest.raises(RuntimeError, match=re.escape(f'line {line}, in bad_constraints')):
+        SolverFactory.create(solver_config).build_solver()
+
+
 def test_missing_enterprise():
     with pytest.raises(RequiresEnterpriseError, match=re.escape('multithreaded solving')):
         solver_config = SolverConfig(