Fix negative_option conflict in NESTED argument generation mode

simple_parsing/wrappers/field_wrapper.py

@@ -385,7 +385,7 @@ def enum_to_str(e):
                 # NOTE: also pass the prefix to the boolean optional action, because it needs to add it
                 # to the generated negative flags as well.
                 _arg_options["action"] = BooleanOptionalAction
-                _arg_options["_conflict_prefix"] = self.prefix
+                _arg_options["_conflict_prefix"] = self.negative_option_prefix
 
         else:
             # "Plain" / simple argument.
@@ -659,6 +659,24 @@ def add_args(dash: str, candidates: list[str]) -> None:
     # def prefix(self) -> str:
     #     return self._prefix
 
+    @property
+    def negative_option_prefix(self) -> str:
+        """Prefix to add to an explicit `negative_option` so it doesn't conflict.
+
+        In `ArgumentGenerationMode.FLAT`, conflicts are resolved by setting `self.prefix`, so that
+        is the prefix the negative option needs. In `ArgumentGenerationMode.NESTED`, the option
+        strings are built from `self.dest` instead of `self.prefix`, which stays empty, so the
+        prefix has to be recovered from the generated (dotted) option string, otherwise two fields
+        of the same dataclass type generate the same negative flag and argparse raises a conflict.
+        """
+        if self.prefix:
+            return self.prefix
+        for option_string in self.option_strings:
+            name = option_string.lstrip("-")
+            if "." in name:
+                return name.rsplit(".", 1)[0] + "."
+        return ""
+
     @property
     def aliases(self) -> list[str]:
         return self.field.metadata.get("alias", [])

test/test_bools.py

@@ -8,6 +8,7 @@
 
 from simple_parsing import helpers
 from simple_parsing.helpers.fields import field, flag
+from simple_parsing.wrappers.field_wrapper import ArgumentGenerationMode
 
 from .testutils import (
     TestSetup,
@@ -307,6 +308,36 @@ class Config(TestSetup):
     assert "--silent" not in help_text
 
 
+@pytest.mark.parametrize("default_value", [True, False])
+def test_nested_bool_field_negative_option_conflict_nested_mode(default_value: bool):
+    """The negative option also needs a prefix with `ArgumentGenerationMode.NESTED`.
+
+    Reproduces https://github.com/lebrice/SimpleParsing/issues/364, where adding the same
+    dataclass twice raised `argparse.ArgumentError: conflicting option string: --silent`.
+    """
+
+    @dataclass
+    class Options:
+        verbose: bool = field(default=default_value, negative_option="silent")
+
+    @dataclass
+    class Config(TestSetup):
+        train: Options = field(default_factory=Options)
+        valid: Options = field(default_factory=Options)
+
+    help_text = Config.get_help_text(
+        argument_generation_mode=ArgumentGenerationMode.NESTED,
+    )
+    assert "--config.train.silent" in help_text
+    assert "--config.valid.silent" in help_text
+
+    config = Config.setup(
+        "--config.train.silent", argument_generation_mode=ArgumentGenerationMode.NESTED
+    )
+    assert config.train.verbose is False
+    assert config.valid.verbose is default_value
+
+
 @pytest.mark.xfail(
     reason=(
         "TODO: When a field has a single letter name, we usually add `-{name}` and `--{name}`. "