python: hello-world / JavaException when using DEBUG log level

[Alex-K37]

Describe the bug

Turning on DEBUG logging causes exceptions with the hello-world example.

Expected behavior
More detailed log messages.

Actual behavior

The following error is shown (only first few lines):

Traceback (most recent call last):
File "DefaultSolver.java", line 200, in ai.timefold.solver.core.impl.solver.DefaultSolver.solve
Exception: Java Exception

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
File "/home/***/.virtualenvs/timefold/lib/python3.11/site-packages/timefold/solver/_solver.py", line 109, in solve
java_solution = self._delegate.solve(java_problem)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
ai.timefold.jpyinterpreter.types.errors.ai.timefold.jpyinterpreter.types.errors.AttributeError: ai.timefold.jpyinterpreter.types.errors.AttributeError: object '08:30' does not have attribute 'strftime'

To Reproduce

diff --git a/python/hello-world/src/hello_world/main.py b/python/hello-world/src/hello_world/main.py
index 660f7430..4b60fade 100644
--- a/python/hello-world/src/hello_world/main.py
+++ b/python/hello-world/src/hello_world/main.py
@@ -10,7 +10,7 @@ from .domain import *
from .constraints import define_constraints

-logging.basicConfig(level=logging.INFO)
+logging.basicConfig(level=logging.DEBUG)
LOGGER = logging.getLogger('app')

Environment

Timefold Solver Version or Git ref:
1.11.0b0

Output of java -version:
openjdk version "17.0.11" 2024-04-16
OpenJDK Runtime Environment (build 17.0.11+9-suse-150400.3.42.1-x8664)
OpenJDK 64-Bit Server VM (build 17.0.11+9-suse-150400.3.42.1-x8664, mixed mode, sharing)

Output of uname -a or ver:
Linux think14 5.14.21-150500.55.65-default TimefoldAI/timefold-solver#1 SMP PREEMPT_DYNAMIC Thu May 23 04:57:11 UTC 2024 (a46829d) x86_64 x86_64 x86_64 GNU/Linux

[triceo]

Thanks for reporting, @Alex-K37! We'll take a look when time permits.

[Christopher-Chianelli]

Caused by the issue this fixes: #101 ; The gist of it is because Python datetime classes are C-Native, we need to implement our own to avoid FFI calls (that is, Python calling Java or Java calling Python). The datetime classes were missing a few methods that deal with formatting. The hello-world and school-timetabling examples used said unimplemented method in their __str__ function. The __str__ function is only called when DEBUG logging or lower is used.

If you modified the __str__ function of Timeslot to not call strftime, no exception would occur.