Use _CONDA_ROOT to detect conda base environments (#15680)

While investigating https://github.com/astral-sh/uv/pull/15679, I
noticed this variable was available in the environment and seems like a
nice additional heuristic.

crates/uv-python/src/lib.rs

@@ -1208,6 +1208,7 @@ mod tests {
             &[
                 ("CONDA_PREFIX", Some(baseenv.as_os_str())),
                 ("CONDA_DEFAULT_ENV", Some(&OsString::from("base"))),
+                (EnvVars::CONDA_ROOT, None),
             ],
             || {
                 find_python_installation(
@@ -1231,6 +1232,7 @@ mod tests {
                 &[
                     ("CONDA_PREFIX", Some(baseenv.as_os_str())),
                     ("CONDA_DEFAULT_ENV", Some(&OsString::from("base"))),
+                    (EnvVars::CONDA_ROOT, None),
                 ],
                 || {
                     find_python_installation(
@@ -1275,6 +1277,66 @@ mod tests {
             "We should find the conda environment"
         );
 
+        // Test _CONDA_ROOT detection of base environment
+        let conda_root_env = context.tempdir.child("conda-root");
+        TestContext::mock_conda_prefix(&conda_root_env, "3.12.2")?;
+
+        // When _CONDA_ROOT matches CONDA_PREFIX, it should be treated as a base environment
+        let result = context.run_with_vars(
+            &[
+                (EnvVars::CONDA_PREFIX, Some(conda_root_env.as_os_str())),
+                (EnvVars::CONDA_ROOT, Some(conda_root_env.as_os_str())),
+                (
+                    EnvVars::CONDA_DEFAULT_ENV,
+                    Some(&OsString::from("custom-name")),
+                ),
+            ],
+            || {
+                find_python_installation(
+                    &PythonRequest::Default,
+                    EnvironmentPreference::OnlyVirtual,
+                    PythonPreference::OnlySystem,
+                    &context.cache,
+                    Preview::default(),
+                )
+            },
+        )?;
+
+        assert!(
+            matches!(result, Err(PythonNotFound { .. })),
+            "Base environment detected via _CONDA_ROOT should be excluded from virtual environments; got {result:?}"
+        );
+
+        // When _CONDA_ROOT doesn't match CONDA_PREFIX, it should be treated as a regular conda environment
+        let other_conda_env = context.tempdir.child("other-conda");
+        TestContext::mock_conda_prefix(&other_conda_env, "3.12.3")?;
+
+        let python = context.run_with_vars(
+            &[
+                (EnvVars::CONDA_PREFIX, Some(other_conda_env.as_os_str())),
+                (EnvVars::CONDA_ROOT, Some(conda_root_env.as_os_str())),
+                (
+                    EnvVars::CONDA_DEFAULT_ENV,
+                    Some(&OsString::from("custom-env")),
+                ),
+            ],
+            || {
+                find_python_installation(
+                    &PythonRequest::Default,
+                    EnvironmentPreference::OnlyVirtual,
+                    PythonPreference::OnlySystem,
+                    &context.cache,
+                    Preview::default(),
+                )
+            },
+        )??;
+
+        assert_eq!(
+            python.interpreter().python_full_version().to_string(),
+            "3.12.3",
+            "Non-base conda environment should be available for virtual environment preference"
+        );
+
         Ok(())
     }
 

crates/uv-python/src/virtualenv.rs

@@ -79,19 +79,27 @@ pub(crate) enum CondaEnvironmentKind {
 impl CondaEnvironmentKind {
     /// Whether the given `CONDA_PREFIX` path is the base Conda environment.
     ///
-    /// When the base environment is used, `CONDA_DEFAULT_ENV` will be set to a name, i.e., `base` or
-    /// `root` which does not match the prefix, e.g. `/usr/local` instead of
-    /// `/usr/local/conda/envs/<name>`.
+    /// The base environment is typically stored in a location matching the `_CONDA_ROOT` path.
     ///
-    /// Note the name `CONDA_DEFAULT_ENV` is misleading, it's the current environment, not the base
-    /// environment name.
+    /// Additionally, when the base environment is active, `CONDA_DEFAULT_ENV` will be set to a
+    /// name, e.g., `base`, which does not match the `CONDA_PREFIX`, e.g., `/usr/local` instead of
+    /// `/usr/local/conda/envs/<name>`. Note the name `CONDA_DEFAULT_ENV` is misleading, it's the
+    /// active environment name, not a constant base environment name.
     fn from_prefix_path(path: &Path) -> Self {
-        // If we cannot read `CONDA_DEFAULT_ENV`, there's no way to know if the base environment
+        // If `_CONDA_ROOT` is set and matches `CONDA_PREFIX`, it's the base environment.
+        if let Ok(conda_root) = env::var(EnvVars::CONDA_ROOT) {
+            if path == Path::new(&conda_root) {
+                return Self::Base;
+            }
+        }
+
+        // Next, we'll use a heuristic based on `CONDA_DEFAULT_ENV`
         let Ok(current_env) = env::var(EnvVars::CONDA_DEFAULT_ENV) else {
             return Self::Child;
         };
 
-        // These are the expected names for the base environment
+        // These are the expected names for the base environment; we may want to remove this
+        // restriction in the future as it's not strictly necessary.
         if current_env != "base" && current_env != "root" {
             return Self::Child;
         }