From ee5f155f7ef2cf10b87920b366e7e912f2182dc4 Mon Sep 17 00:00:00 2001
From: Zanie Blue <contact@zanie.dev>
Date: Wed, 17 Sep 2025 06:04:29 -0500
Subject: [PATCH] Invert the logic for determining if a path is a base conda
 environment (#15679)

Closes https://github.com/astral-sh/uv/issues/15604

The previous logic does not match the discussion in the original issue
about this feature, nor does it match the comment for the function. I'm
confused because I know this logic is working for some people? I'm
consequently a little wary of making this change. I'm following up with
some additional changes that should ensure this is robust, e.g., #15680
---
 crates/uv-python/src/lib.rs        | 66 ++++++++++++++++--------------
 crates/uv-python/src/virtualenv.rs | 15 ++++---
 2 files changed, 45 insertions(+), 36 deletions(-)

diff --git a/crates/uv-python/src/lib.rs b/crates/uv-python/src/lib.rs
index 7362c710d..51ecbda8c 100644
--- a/crates/uv-python/src/lib.rs
+++ b/crates/uv-python/src/lib.rs
@@ -1200,7 +1200,7 @@ mod tests {
             "We should allow the active conda python"
         );
 
-        let baseenv = context.tempdir.child("base");
+        let baseenv = context.tempdir.child("conda");
         TestContext::mock_conda_prefix(&baseenv, "3.12.1")?;
 
         // But not if it's a base environment
@@ -1226,21 +1226,23 @@ mod tests {
         );
 
         // Unless, system interpreters are included...
-        let python = context.run_with_vars(
-            &[
-                ("CONDA_PREFIX", Some(baseenv.as_os_str())),
-                ("CONDA_DEFAULT_ENV", Some(&OsString::from("base"))),
-            ],
-            || {
-                find_python_installation(
-                    &PythonRequest::Default,
-                    EnvironmentPreference::OnlySystem,
-                    PythonPreference::OnlySystem,
-                    &context.cache,
-                    Preview::default(),
-                )
-            },
-        )??;
+        let python = context
+            .run_with_vars(
+                &[
+                    ("CONDA_PREFIX", Some(baseenv.as_os_str())),
+                    ("CONDA_DEFAULT_ENV", Some(&OsString::from("base"))),
+                ],
+                || {
+                    find_python_installation(
+                        &PythonRequest::Default,
+                        EnvironmentPreference::OnlySystem,
+                        PythonPreference::OnlySystem,
+                        &context.cache,
+                        Preview::default(),
+                    )
+                },
+            )?
+            .unwrap();
 
         assert_eq!(
             python.interpreter().python_full_version().to_string(),
@@ -1249,21 +1251,23 @@ mod tests {
         );
 
         // If the environment name doesn't match the default, we should not treat it as system
-        let python = context.run_with_vars(
-            &[
-                ("CONDA_PREFIX", Some(condaenv.as_os_str())),
-                ("CONDA_DEFAULT_ENV", Some(&OsString::from("base"))),
-            ],
-            || {
-                find_python_installation(
-                    &PythonRequest::Default,
-                    EnvironmentPreference::OnlyVirtual,
-                    PythonPreference::OnlySystem,
-                    &context.cache,
-                    Preview::default(),
-                )
-            },
-        )??;
+        let python = context
+            .run_with_vars(
+                &[
+                    ("CONDA_PREFIX", Some(condaenv.as_os_str())),
+                    ("CONDA_DEFAULT_ENV", Some(&OsString::from("condaenv"))),
+                ],
+                || {
+                    find_python_installation(
+                        &PythonRequest::Default,
+                        EnvironmentPreference::OnlyVirtual,
+                        PythonPreference::OnlySystem,
+                        &context.cache,
+                        Preview::default(),
+                    )
+                },
+            )?
+            .unwrap();
 
         assert_eq!(
             python.interpreter().python_full_version().to_string(),
diff --git a/crates/uv-python/src/virtualenv.rs b/crates/uv-python/src/virtualenv.rs
index f4314ed5f..a4e7d5509 100644
--- a/crates/uv-python/src/virtualenv.rs
+++ b/crates/uv-python/src/virtualenv.rs
@@ -82,14 +82,17 @@ impl CondaEnvironmentKind {
     /// 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>`.
+    ///
+    /// Note the name `CONDA_DEFAULT_ENV` is misleading, it's the current environment, not the 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
-        let Ok(default_env) = env::var(EnvVars::CONDA_DEFAULT_ENV) else {
+        let Ok(current_env) = env::var(EnvVars::CONDA_DEFAULT_ENV) else {
             return Self::Child;
         };
 
         // These are the expected names for the base environment
-        if default_env != "base" && default_env != "root" {
+        if current_env != "base" && current_env != "root" {
             return Self::Child;
         }
 
@@ -97,10 +100,12 @@ impl CondaEnvironmentKind {
             return Self::Child;
         };
 
-        if name.to_str().is_some_and(|name| name == default_env) {
-            Self::Base
-        } else {
+        // If the environment is in a directory matching the name of the environment, it's not
+        // usually a base environment.
+        if name.to_str().is_some_and(|name| name == current_env) {
             Self::Child
+        } else {
+            Self::Base
         }
     }
 }