fix: raise error when both --frozen and --no-sync are set

Copilot · nathanjmcdougall · web-flow · commit 7a4549aa0685 · 2026-04-02T21:59:20.000Z
Instead of silently having --frozen take precedence, raise a UsethisError when both flags are set simultaneously. These flags are mutually exclusive since --frozen prevents both lockfile updates and syncing, while --no-sync only prevents syncing. Agent-Logs-Url: https://github.com/usethis-python/usethis-python/sessions/bbee573e-012b-4649-8923-7e4e46490fcf Co-authored-by: nathanjmcdougall <18602289+nathanjmcdougall@users.noreply.github.com>
diff --git a/src/usethis/_backend/uv/call.py b/src/usethis/_backend/uv/call.py
@@ -53,15 +53,10 @@ def call_uv_subprocess(args: list[str], change_toml: bool) -> str:
         "run",
     }:
         new_args = ["uv", args[0], "--frozen", *args[1:]]
-    elif (
-        not usethis_config.frozen
-        and usethis_config.no_sync
-        and args[0]
-        in {
-            "add",
-            "remove",
-        }
-    ):
+    elif usethis_config.no_sync and args[0] in {
+        "add",
+        "remove",
+    }:
         new_args = ["uv", args[0], "--no-sync", *args[1:]]
     else:
         new_args = ["uv", *args]
diff --git a/src/usethis/_config.py b/src/usethis/_config.py
@@ -9,6 +9,7 @@
 
 from usethis._types.backend import BackendEnum
 from usethis._types.build_backend import BuildBackendEnum
+from usethis.errors import UsethisError
 
 if TYPE_CHECKING:
     from collections.abc import Generator
@@ -114,6 +115,10 @@ def set(  # noqa: PLR0912, PLR0913, PLR0915
         if project_dir is None:
             project_dir = old_project_dir
 
+        if frozen and no_sync:
+            msg = "Cannot use both --frozen and --no-sync at the same time."
+            raise UsethisError(msg)
+
         self.offline = offline
         self.quiet = quiet
         self.frozen = frozen
diff --git a/tests/usethis/_backend/uv/test_call.py b/tests/usethis/_backend/uv/test_call.py
@@ -8,6 +8,7 @@
 from usethis._config import usethis_config
 from usethis._file.pyproject_toml.io_ import PyprojectTOMLManager
 from usethis._test import change_cwd
+from usethis.errors import UsethisError
 
 
 class TestCallUVSubprocess:
@@ -84,9 +85,7 @@ def mock_call_subprocess(args: list[str], *, cwd: Path | None = None) -> str:
                 ["remove", "--group", "test", "pytest"], change_toml=False
             ) == ("uv remove --quiet --no-sync --group test pytest")
 
-    def test_frozen_takes_precedence_over_no_sync(
-        self, monkeypatch: pytest.MonkeyPatch
-    ):
+    def test_frozen_and_no_sync_raises(self, monkeypatch: pytest.MonkeyPatch):
         # Arrange
         def mock_call_subprocess(args: list[str], *, cwd: Path | None = None) -> str:
             _ = cwd
@@ -98,20 +97,11 @@ def mock_call_subprocess(args: list[str], *, cwd: Path | None = None) -> str:
             mock_call_subprocess,
         )
 
-        with usethis_config.set(no_sync=True, frozen=True, offline=False):
-            # Act, Assert
-            # frozen should take precedence, --no-sync should not appear
-            result = call_uv_subprocess(
-                ["add", "--group", "test", "pytest"], change_toml=False
-            )
-            assert "--frozen" in result
-            assert "--no-sync" not in result
-
-            result = call_uv_subprocess(
-                ["remove", "--group", "test", "pytest"], change_toml=False
-            )
-            assert "--frozen" in result
-            assert "--no-sync" not in result
+        with (
+            pytest.raises(UsethisError, match="Cannot use both --frozen and --no-sync"),
+            usethis_config.set(no_sync=True, frozen=True, offline=False),
+        ):
+            call_uv_subprocess(["add", "--group", "test", "pytest"], change_toml=False)
 
     @pytest.mark.usefixtures("_vary_network_conn")
     def test_handle_missing_version(
diff --git a/tests/usethis/test_config.py b/tests/usethis/test_config.py
@@ -6,7 +6,7 @@
 from usethis._config import UsethisConfig, usethis_config
 from usethis._test import change_cwd
 from usethis._types.backend import BackendEnum
-from usethis.errors import ForbiddenBackendError
+from usethis.errors import ForbiddenBackendError, UsethisError
 
 
 class TestUsethisConfig:
@@ -55,3 +55,23 @@ def test_does_not_raise_error_when_enabled(self):
                 output = call_uv_subprocess(["python", "list"], change_toml=False)
 
             assert output is not None
+
+    class TestFrozenAndNoSync:
+        def test_raises_error_when_both_set(self):
+            with (
+                pytest.raises(
+                    UsethisError, match="Cannot use both --frozen and --no-sync"
+                ),
+                usethis_config.set(frozen=True, no_sync=True),
+            ):
+                pass
+
+        def test_no_error_when_only_frozen(self):
+            with usethis_config.set(frozen=True, no_sync=False):
+                assert usethis_config.frozen
+                assert not usethis_config.no_sync
+
+        def test_no_error_when_only_no_sync(self):
+            with usethis_config.set(frozen=False, no_sync=True):
+                assert not usethis_config.frozen
+                assert usethis_config.no_sync
