The document describes finding and fixing a bug in the RustPython interpreter. Specifically, there was a bug where the divmod() function was not properly handling negative numbers. The steps taken to address this were: 1. Reproducing the bug by running an import datetime statement 2. Debugging the divmod() implementation 3. Adding a new test case to validate the expected output of divmod(-86340, 86400) 4. Fixing the divmod() implementation directly in the ObjInt source code 5. Creating a pull request on GitHub to submit the fixes

1. RustPython Sprint
How to find & fix a bug?

2. Example
➜ ~ cd ~/Projects/RustPython
(RustPython) ➜ RustPython git:(master) ✗ RUSTPYTHONPATH=Lib cargo run
Finished dev [unoptimized + debuginfo] target(s) in 0.10s
Running `target/debug/rustpython`
Welcome to the magnificent Rust Python 0.1.0 interpreter 😱 🖖
>>>>> import datetime
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "Lib/datetime.py", line 2100, in <module>
File "Lib/datetime.py", line 2194, in timezone
_maxoffset = timedelta(hours=23, minutes=59)
File "Lib/datetime.py", line 654, in __neg__
-self._microseconds)
File "Lib/datetime.py", line 564, in __new__
assert isinstance(s, int) and 0 <= s < 24*3600
AssertionError
>>>>>
•

3. Debug
timedelta.__new__ 0 -86340 0 0 0 0 0
divmod in: -86340 86400
divmod out: 0 -86340
<class 'int'> -86340
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "Lib/datetime.py", line 2100, in <module>
File "Lib/datetime.py", line 2194, in timezone
_maxoffset = timedelta(hours=23, minutes=59)
File "Lib/datetime.py", line 654, in __neg__
-self._microseconds)
File "Lib/datetime.py", line 564, in __new__
assert isinstance(s, int) and 0 <= s < 24*3600
AssertionError
>>>>>
•

4. Add a test case
diff --git a/tests/snippets/builtin_divmod.py b/tests/snippets/
builtin_divmod.py
index 939e1d2d..ce9bdfba 100644
--- a/tests/snippets/builtin_divmod.py
+++ b/tests/snippets/builtin_divmod.py
@@ -3,6 +3,7 @@ from testutils import assert_raises
assert divmod(11, 3) == (3, 2)
assert divmod(8,11) == (0, 8)
assert divmod(0.873, 0.252) == (3.0, 0.11699999999999999)
+assert divmod(-86340, 86400) == (-1, 60)
assert_raises(ZeroDivisionError, lambda: divmod(5, 0), 'divmod
by zero')
assert_raises(ZeroDivisionError, lambda: divmod(5.0, 0.0),
'divmod by zero')

5. Run test
(RustPython) ➜ RustPython git:(master) ✗ python tests/snippets/
builtin_divmod.py
(RustPython) ➜ RustPython git:(master) ✗ cargo run tests/snippets/
builtin_divmod.py
Finished dev [unoptimized + debuginfo] target(s) in 0.11s
Running `target/debug/rustpython tests/snippets/
builtin_divmod.py`
Traceback (most recent call last):
File "tests/snippets/builtin_divmod.py", line 6, in <module>
assert divmod(-86340, 86400) == (-1, 60)
AssertionError

6. Watch code
(RustPython) ➜ RustPython git:(master) ✗ vi ./vm/src/obj/objint.rs
…
#[pymethod(name = "__divmod__")]
fn divmod(&self, other: PyObjectRef, vm: &VirtualMachine) -> PyResult {
if objtype::isinstance(&other, &vm.ctx.int_type()) {
let v2 = get_value(&other);
if *v2 != BigInt::zero() {
let (r1, r2) = self.value.div_rem(v2);
Ok(vm
.ctx
.new_tuple(vec![vm.ctx.new_int(r1), vm.ctx.new_int(r2)]))
} else {
Err(vm.new_zero_division_error("integer divmod by zero".to_str
ing()))
}
} else {
Ok(vm.ctx.not_implemented())
}
}
…

7. Fix - Add divmod()
@@ -739,6 +735,17 @@ fn truediv_ints(vm: &VirtualMachine, i1: &BigInt, i2: &BigInt) -> PyResult
{
}
}
+#[inline]
+fn divmod(vm: &VirtualMachine, i1: &BigInt, i2: &BigInt) -> PyResult<(BigInt, BigInt)> {
+ if i2.is_zero() {
+ Err(vm.new_zero_division_error("integer division by zero".to_string()))
+ } else {
+ let modulo = (i1 % i2 + i2) % i2;
+ let division = (i1 - &modulo) / i2;
+ Ok((division, modulo))
+ }
+}
+
pub fn init(context: &PyContext) {
PyInt::extend_class(context, &context.int_type);
extend_class!(context, &context.int_type, {

8. Fix - bugfix
diff --git a/vm/src/obj/objint.rs b/vm/src/obj/objint.rs
index 000dcafd..f90d069a 100644
--- a/vm/src/obj/objint.rs
+++ b/vm/src/obj/objint.rs
@@ -405,14 +405,10 @@ impl PyInt {
fn divmod(&self, other: PyObjectRef, vm: &VirtualMachine) -> PyResult {
if objtype::isinstance(&other, &vm.ctx.int_type()) {
let v2 = get_value(&other);
- if *v2 != BigInt::zero() {
- let (r1, r2) = self.value.div_rem(v2);
- Ok(vm
- .ctx
- .new_tuple(vec![vm.ctx.new_int(r1), vm.ctx.new_int(r2)]))
- } else {
- Err(vm.new_zero_division_error("integer divmod by zero".to_string()))
- }
+ let (div, modulo) = divmod(vm, &self.value, v2)?;
+ Ok(vm
+ .ctx
+ .new_tuple(vec![vm.ctx.new_int(div), vm.ctx.new_int(modulo)]))
} else {
Ok(vm.ctx.not_implemented())
}

9. Pull request
(RustPython) ➜ RustPython git:(master) ✗ git push origin master:fix-divmod
Enumerating objects: 36, done.
Counting objects: 100% (36/36), done.
Delta compression using up to 12 threads.
Compressing objects: 100% (27/27), done.
Writing objects: 100% (27/27), 2.50 KiB | 2.50 MiB/s, done.
Total 27 (delta 24), reused 0 (delta 0)
remote: Resolving deltas: 100% (24/24), completed with 9 local objects.
remote:
remote: Create a pull request for 'fix-divmod' on GitHub by visiting:
remote: https://github.com/youknowone/RustPython/pull/new/fix-divmod
remote:
To https://github.com/youknowone/RustPython.git
* [new branch] master -> fix-divmod

10. Pull request
(RustPython) ➜ RustPython git:(master) ✗ git push origin master:fix-divmod
Enumerating objects: 36, done.
Counting objects: 100% (36/36), done.
Delta compression using up to 12 threads.
Compressing objects: 100% (27/27), done.
Writing objects: 100% (27/27), 2.50 KiB | 2.50 MiB/s, done.
Total 27 (delta 24), reused 0 (delta 0)
remote: Resolving deltas: 100% (24/24), completed with 9 local objects.
remote:
remote: Create a pull request for 'fix-divmod' on GitHub by visiting:
remote: https://github.com/youknowone/RustPython/pull/new/fix-divmod
remote:
To https://github.com/youknowone/RustPython.git
* [new branch] master -> fix-divmod

11. Pull request

12. Original issue
(RustPython) ➜ RustPython git:(master) ✗ RUSTPYTHONPATH=Lib cargo run
Compiling rustpython-vm v0.1.0 (/Users/youknowone/Projects/
RustPython/vm)
Compiling rustpython v0.1.0 (/Users/youknowone/Projects/RustPython)
Finished dev [unoptimized + debuginfo] target(s) in 15.14s
Running `target/debug/rustpython`
Welcome to the magnificent Rust Python 0.1.0 interpreter 😱 🖖
>>>>> import datetime
divmod in: 0 86400
divmod out: 0 0
divmod in: 86399 86400
divmod out: 0 86399
divmod in: 86340 86400
divmod out: 0 86340
divmod in: -86340 86400
divmod out: -1 60
divmod in: 0 86400
divmod out: 0 0
>>>>>
😱 해결!
보통 이런 일은 잘 없으니
기대하지 맙시다 ㅋㅋㅋㅋ