Use python from python -m uv as default

Python tools run with `python -m <tool>` will use this `python` as their default python, including pip, virtualenv and the built-in venv. Calling Python tools this way is common, the [pip docs](https://pip.pypa.io/en/stable/user_guide/) use `python -m pip` exclusively, the built-in venv can only be called this way and certain project layouts require `python -m pytest` over `pytest`. This python interpreter takes precedence over the currently active (v)env.

These tools are all written in python and read `sys.executable`. To emulate this, we're setting `UV_DEFAULT_PYTHON` in the python module-launcher shim and read it in the default python discovery, prioritizing it over the active venv. User can also set `UV_DEFAULT_PYTHON` for their own purposes.

The test covers only half of the feature since we don't build the python package before running the tests.

Fixes #2058
Fixes #2222

crates/uv-interpreter/src/find_python.rs

@@ -6,7 +6,7 @@ use std::path::PathBuf;
 use tracing::{debug, instrument};
 
 use uv_cache::Cache;
-use uv_fs::normalize_path;
+use uv_fs::{normalize_path, Simplified};
 
 use crate::interpreter::InterpreterInfoError;
 use crate::python_environment::{detect_python_executable, detect_virtual_env};
@@ -61,10 +61,16 @@ pub fn find_requested_python(request: &str, cache: &Cache) -> Result<Option<Inte
 ///
 /// We prefer the test overwrite `UV_TEST_PYTHON_PATH` if it is set, otherwise `python3`/`python` or
 /// `python.exe` respectively.
+///
+/// When `system` is set, we ignore the `python -m uv` due to the `--system` flag.
 #[instrument(skip_all)]
-pub fn find_default_python(cache: &Cache) -> Result<Interpreter, Error> {
-    debug!("Starting interpreter discovery for default Python");
-    try_find_default_python(cache)?.ok_or(if cfg!(windows) {
+pub fn find_default_python(cache: &Cache, system: bool) -> Result<Interpreter, Error> {
+    let selector = if system {
+        PythonVersionSelector::System
+    } else {
+        PythonVersionSelector::Default
+    };
+    find_python(selector, cache)?.ok_or(if cfg!(windows) {
         Error::NoPythonInstalledWindows
     } else if cfg!(unix) {
         Error::NoPythonInstalledUnix
@@ -73,11 +79,6 @@ pub fn find_default_python(cache: &Cache) -> Result<Interpreter, Error> {
     })
 }
 
-/// Same as [`find_default_python`] but returns `None` if no python is found instead of returning an `Err`.
-pub(crate) fn try_find_default_python(cache: &Cache) -> Result<Option<Interpreter>, Error> {
-    find_python(PythonVersionSelector::Default, cache)
-}
-
 /// Find a Python version matching `selector`.
 ///
 /// It searches for an existing installation in the following order:
@@ -95,6 +96,20 @@ fn find_python(
     selector: PythonVersionSelector,
     cache: &Cache,
 ) -> Result<Option<Interpreter>, Error> {
+    if selector != PythonVersionSelector::System {
+        // `python -m uv` passes `sys.executable` as `UV_DEFAULT_PYTHON`. Users expect that this Python
+        // version is used as it is the recommended or sometimes even only way to use tools, e.g. pip
+        // (`python3.10 -m pip`) and venv (`python3.10 -m venv`).
+        if let Some(default_python) = env::var_os("UV_DEFAULT_PYTHON") {
+            debug!(
+                "Trying UV_DEFAULT_PYTHON at {}",
+                default_python.simplified_display()
+            );
+            let interpreter = Interpreter::query(default_python, cache)?;
+            return Ok(Some(interpreter));
+        }
+    }
+
     #[allow(non_snake_case)]
     let UV_TEST_PYTHON_PATH = env::var_os("UV_TEST_PYTHON_PATH");
 
@@ -299,7 +314,7 @@ impl PythonInstallation {
         cache: &Cache,
     ) -> Result<Option<Interpreter>, Error> {
         let selected = match selector {
-            PythonVersionSelector::Default => true,
+            PythonVersionSelector::Default | PythonVersionSelector::System => true,
 
             PythonVersionSelector::Major(major) => self.major() == major,
 
@@ -339,9 +354,11 @@ impl PythonInstallation {
     }
 }
 
-#[derive(Copy, Clone, Debug)]
+#[derive(Copy, Clone, Debug, Eq, PartialEq)]
 enum PythonVersionSelector {
     Default,
+    /// Like default, but skip over the `python` from `python -m uv`.
+    System,
     Major(u8),
     MajorMinor(u8, u8),
     MajorMinorPatch(u8, u8, u8),
@@ -360,7 +377,7 @@ impl PythonVersionSelector {
         };
 
         match self {
-            Self::Default => [Some(python3), Some(python), None, None],
+            Self::Default | Self::System => [Some(python3), Some(python), None, None],
             Self::Major(major) => [
                 Some(Cow::Owned(format!("python{major}{extension}"))),
                 Some(python),
@@ -386,7 +403,7 @@ impl PythonVersionSelector {
 
     fn major(self) -> Option<u8> {
         match self {
-            Self::Default => None,
+            Self::Default | Self::System => None,
             Self::Major(major) => Some(major),
             Self::MajorMinor(major, _) => Some(major),
             Self::MajorMinorPatch(major, _, _) => Some(major),
@@ -471,6 +488,21 @@ fn find_version(
         }
     };
 
+    // `python -m uv` passes `sys.executable` as `UV_DEFAULT_PYTHON`. Users expect that this Python
+    // version is used as it is the recommended or sometimes even only way to use tools, e.g. pip
+    // (`python3.10 -m pip`) and venv (`python3.10 -m venv`). This is duplicated in
+    // `find_requested_python`, but we need to do it here to take precedence over the active venv.
+    if let Some(default_python) = env::var_os("UV_DEFAULT_PYTHON") {
+        debug!(
+            "Trying UV_DEFAULT_PYTHON at {}",
+            default_python.simplified_display()
+        );
+        let interpreter = Interpreter::query(default_python, cache)?;
+        if version_matches(&interpreter) {
+            return Ok(Some(interpreter));
+        }
+    }
+
     // Check if the venv Python matches.
     if let Some(venv) = detect_virtual_env()? {
         let executable = detect_python_executable(venv);
@@ -486,7 +518,7 @@ fn find_version(
     let interpreter = if let Some(python_version) = python_version {
         find_requested_python(&python_version.string, cache)?
     } else {
-        try_find_default_python(cache)?
+        find_python(PythonVersionSelector::Default, cache)?
     };
 
     if let Some(interpreter) = interpreter {

crates/uv-interpreter/src/python_environment.rs

@@ -51,8 +51,8 @@ impl PythonEnvironment {
     }
 
     /// Create a [`PythonEnvironment`] for the default Python interpreter.
-    pub fn from_default_python(cache: &Cache) -> Result<Self, Error> {
-        let interpreter = find_default_python(cache)?;
+    pub fn from_default_python(cache: &Cache, system: bool) -> Result<Self, Error> {
+        let interpreter = find_default_python(cache, system)?;
         Ok(Self {
             root: interpreter.prefix().to_path_buf(),
             interpreter,

crates/uv-resolver/tests/resolver.rs

@@ -120,8 +120,8 @@ async fn resolve(
     let flat_index = FlatIndex::default();
     let index = InMemoryIndex::default();
     // TODO(konstin): Should we also use the bootstrapped pythons here?
-    let real_interpreter =
-        find_default_python(&Cache::temp().unwrap()).expect("Expected a python to be installed");
+    let real_interpreter = find_default_python(&Cache::temp().unwrap(), false)
+        .expect("Expected a python to be installed");
     let interpreter = Interpreter::artificial(real_interpreter.platform().clone(), markers.clone());
     let build_context = DummyContext::new(Cache::temp()?, interpreter.clone());
     let resolver = Resolver::new(

crates/uv-virtualenv/src/main.rs

@@ -39,7 +39,7 @@ fn run() -> Result<(), uv_virtualenv::Error> {
             uv_interpreter::Error::NoSuchPython(python_request.to_string()),
         )?
     } else {
-        find_default_python(&cache)?
+        find_default_python(&cache, false)?
     };
     create_bare_venv(
         &location,

crates/uv/src/commands/pip_freeze.rs

@@ -26,12 +26,12 @@ pub(crate) fn pip_freeze(
     let venv = if let Some(python) = python {
         PythonEnvironment::from_requested_python(python, cache)?
     } else if system {
-        PythonEnvironment::from_default_python(cache)?
+        PythonEnvironment::from_default_python(cache, true)?
     } else {
         match PythonEnvironment::from_virtualenv(cache) {
             Ok(venv) => venv,
             Err(uv_interpreter::Error::VenvNotFound) => {
-                PythonEnvironment::from_default_python(cache)?
+                PythonEnvironment::from_default_python(cache, false)?
             }
             Err(err) => return Err(err.into()),
         }

crates/uv/src/commands/pip_install.rs

@@ -110,7 +110,7 @@ pub(crate) async fn pip_install(
     let venv = if let Some(python) = python.as_ref() {
         PythonEnvironment::from_requested_python(python, &cache)?
     } else if system {
-        PythonEnvironment::from_default_python(&cache)?
+        PythonEnvironment::from_default_python(&cache, true)?
     } else {
         PythonEnvironment::from_virtualenv(&cache)?
     };

crates/uv/src/commands/pip_list.rs

@@ -37,12 +37,12 @@ pub(crate) fn pip_list(
     let venv = if let Some(python) = python {
         PythonEnvironment::from_requested_python(python, cache)?
     } else if system {
-        PythonEnvironment::from_default_python(cache)?
+        PythonEnvironment::from_default_python(cache, true)?
     } else {
         match PythonEnvironment::from_virtualenv(cache) {
             Ok(venv) => venv,
             Err(uv_interpreter::Error::VenvNotFound) => {
-                PythonEnvironment::from_default_python(cache)?
+                PythonEnvironment::from_default_python(cache, false)?
             }
             Err(err) => return Err(err.into()),
         }

crates/uv/src/commands/pip_show.rs

@@ -42,12 +42,12 @@ pub(crate) fn pip_show(
     let venv = if let Some(python) = python {
         PythonEnvironment::from_requested_python(python, cache)?
     } else if system {
-        PythonEnvironment::from_default_python(cache)?
+        PythonEnvironment::from_default_python(cache, true)?
     } else {
         match PythonEnvironment::from_virtualenv(cache) {
             Ok(venv) => venv,
             Err(uv_interpreter::Error::VenvNotFound) => {
-                PythonEnvironment::from_default_python(cache)?
+                PythonEnvironment::from_default_python(cache, false)?
             }
             Err(err) => return Err(err.into()),
         }

crates/uv/src/commands/pip_sync.rs

@@ -74,7 +74,7 @@ pub(crate) async fn pip_sync(
     let venv = if let Some(python) = python.as_ref() {
         PythonEnvironment::from_requested_python(python, &cache)?
     } else if system {
-        PythonEnvironment::from_default_python(&cache)?
+        PythonEnvironment::from_default_python(&cache, true)?
     } else {
         PythonEnvironment::from_virtualenv(&cache)?
     };

crates/uv/src/commands/pip_uninstall.rs

@@ -44,7 +44,7 @@ pub(crate) async fn pip_uninstall(
     let venv = if let Some(python) = python.as_ref() {
         PythonEnvironment::from_requested_python(python, &cache)?
     } else if system {
-        PythonEnvironment::from_default_python(&cache)?
+        PythonEnvironment::from_default_python(&cache, true)?
     } else {
         PythonEnvironment::from_virtualenv(&cache)?
     };

crates/uv/src/commands/venv.rs

@@ -102,7 +102,7 @@ async fn venv_impl(
             .ok_or(Error::NoSuchPython(python_request.to_string()))
             .into_diagnostic()?
     } else {
-        find_default_python(cache).into_diagnostic()?
+        find_default_python(cache, false).into_diagnostic()?
     };
 
     writeln!(

crates/uv/tests/venv.rs

@@ -1,5 +1,6 @@
 #![cfg(feature = "python")]
 
+use std::path::PathBuf;
 use std::process::Command;
 
 use anyhow::Result;
@@ -731,3 +732,61 @@ fn verify_nested_pyvenv_cfg() -> Result<()> {
 
     Ok(())
 }
+
+#[test]
+fn uv_default_python() -> Result<()> {
+    let temp_dir = assert_fs::TempDir::new()?;
+    let cache_dir = assert_fs::TempDir::new()?;
+    // The path to a Python 3.12 interpreter
+    let bin312 =
+        create_bin_with_executables(&temp_dir, &["3.12"]).expect("Failed to create bin dir");
+    // The path to a Python 3.10 interpreter
+    let bin310 =
+        create_bin_with_executables(&temp_dir, &["3.10"]).expect("Failed to create bin dir");
+    let python310 = PathBuf::from(bin310).join(if cfg!(unix) {
+        "python3"
+    } else if cfg!(windows) {
+        "python.exe"
+    } else {
+        unimplemented!("Only Windows and Unix are supported")
+    });
+    let venv = temp_dir.child(".venv");
+
+    // Create a virtual environment at `.venv`.
+    let filter_venv = regex::escape(&venv.simplified_display().to_string());
+    let filter_prompt = r"Activate with: (?:.*)\\Scripts\\activate";
+    let filters = &[
+        (r"interpreter at: .+", "interpreter at: [PATH]"),
+        (&filter_venv, "/home/ferris/project/.venv"),
+        (
+            filter_prompt,
+            "Activate with: source /home/ferris/project/.venv/bin/activate",
+        ),
+    ];
+    uv_snapshot!(filters, Command::new(get_bin())
+        .arg("venv")
+        .arg(venv.as_os_str())
+        .arg("--cache-dir")
+        .arg(cache_dir.path())
+        .arg("--exclude-newer")
+        .arg(EXCLUDE_NEWER)
+        // Simulate a PATH the user may have with Python 3.12 being the default.
+        .env("UV_TEST_PYTHON_PATH", bin312.clone())
+        // Simulate `python3.10 -m uv`.
+        .env("UV_DEFAULT_PYTHON", python310)
+        .current_dir(&temp_dir), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    Using Python 3.10.13 interpreter at: [PATH]
+    Creating virtualenv at: /home/ferris/project/.venv
+    Activate with: source /home/ferris/project/.venv/bin/activate
+    "###
+    );
+
+    venv.assert(predicates::path::is_dir());
+
+    Ok(())
+}

python/uv/__main__.py

@@ -22,6 +22,7 @@ def _detect_virtualenv() -> str:
 
     return ""
 
+
 def _run() -> None:
     uv = os.fsdecode(find_uv_bin())
 
@@ -30,6 +31,9 @@ def _run() -> None:
     if venv:
         env.setdefault("VIRTUAL_ENV", venv)
 
+    # When running with `python -m uv`, use this `python` as default.
+    env.setdefault("UV_DEFAULT_PYTHON", sys.executable)
+
     if sys.platform == "win32":
         import subprocess
 
@@ -39,6 +43,5 @@ def _run() -> None:
         os.execvpe(uv, [uv, *sys.argv[1:]], env=env)
 
 
-
 if __name__ == "__main__":
     _run()