Use sitecustomize.py to implement environment layering

crates/uv/src/commands/project/run.rs

@@ -3,7 +3,7 @@ use std::ffi::OsString;
 use std::fmt::Write;
 use std::path::{Path, PathBuf};
 
-use anyhow::{bail, Context, Result};
+use anyhow::{anyhow, bail, Context, Result};
 use itertools::Itertools;
 use owo_colors::OwoColorize;
 use tokio::process::Command;
@@ -14,7 +14,7 @@ use uv_cache::Cache;
 use uv_cli::ExternalCommand;
 use uv_client::{BaseClientBuilder, Connectivity};
 use uv_configuration::{Concurrency, ExtrasSpecification, PreviewMode};
-use uv_fs::Simplified;
+use uv_fs::{PythonExt, Simplified};
 use uv_installer::{SatisfiesResult, SitePackages};
 use uv_normalize::PackageName;
 use uv_python::{
@@ -412,6 +412,27 @@ pub(crate) async fn run(
         }
     };
 
+    // If we're running in an ephemeral environment, add a `sitecustomize.py` to enable loading of
+    // the base environment's site packages. Setting `PYTHONPATH` is insufficient, as it doesn't
+    // resolve `.pth` files in the base environment.
+    if let Some(ephemeral_env) = ephemeral_env.as_ref() {
+        if let Some(base_interpreter) = base_interpreter.as_ref() {
+            let ephemeral_site_packages = ephemeral_env
+                .site_packages()
+                .next()
+                .ok_or_else(|| anyhow!("Ephemeral environment has no site packages directory"))?;
+            let base_site_packages = base_interpreter
+                .site_packages()
+                .next()
+                .ok_or_else(|| anyhow!("Base environment has no site packages directory"))?;
+
+            fs_err::write(
+                ephemeral_site_packages.join("sitecustomize.py"),
+                format!("import site; site.addsitedir(\"{}\")", base_site_packages.escape_for_python()),
+            )?;
+        }
+    }
+
     debug!("Running `{command}`");
     let mut process = Command::from(&command);
 
@@ -437,30 +458,6 @@ pub(crate) async fn run(
     )?;
     process.env("PATH", new_path);
 
-    // Construct the `PYTHONPATH` environment variable.
-    let new_python_path = std::env::join_paths(
-        ephemeral_env
-            .as_ref()
-            .map(PythonEnvironment::site_packages)
-            .into_iter()
-            .flatten()
-            .chain(
-                base_interpreter
-                    .as_ref()
-                    .map(Interpreter::site_packages)
-                    .into_iter()
-                    .flatten(),
-            )
-            .map(PathBuf::from)
-            .chain(
-                std::env::var_os("PYTHONPATH")
-                    .as_ref()
-                    .iter()
-                    .flat_map(std::env::split_paths),
-            ),
-    )?;
-    process.env("PYTHONPATH", new_python_path);
-
     // Spawn and wait for completion
     // Standard input, output, and error streams are all inherited
     // TODO(zanieb): Throw a nicer error message if the command is not found

crates/uv/src/commands/tool/run.rs

@@ -123,17 +123,6 @@ pub(crate) async fn run(
     )?;
     process.env("PATH", new_path);
 
-    // Construct the `PYTHONPATH` environment variable.
-    let new_python_path = std::env::join_paths(
-        environment.site_packages().map(PathBuf::from).chain(
-            std::env::var_os("PYTHONPATH")
-                .as_ref()
-                .iter()
-                .flat_map(std::env::split_paths),
-        ),
-    )?;
-    process.env("PYTHONPATH", new_python_path);
-
     // Spawn and wait for completion
     // Standard input, output, and error streams are all inherited
     // TODO(zanieb): Throw a nicer error message if the command is not found

crates/uv/tests/run.rs

@@ -769,3 +769,57 @@ fn run_requirements_txt_arguments() -> Result<()> {
 
     Ok(())
 }
+
+/// Ensure that we can import from the root project when layering `--with` requirements.
+#[test]
+fn run_editable() -> Result<()> {
+    let context = TestContext::new("3.12");
+
+    let pyproject_toml = context.temp_dir.child("pyproject.toml");
+    pyproject_toml.write_str(indoc! { r#"
+        [build-system]
+        requires = ["hatchling"]
+        build-backend = "hatchling.build"
+
+        [project]
+        name = "foo"
+        version = "1.0.0"
+        requires-python = ">=3.8"
+        dependencies = []
+        "#
+    })?;
+
+    let src = context.temp_dir.child("src").child("foo");
+    src.create_dir_all()?;
+
+    let init = src.child("__init__.py");
+    init.touch()?;
+
+    let main = context.temp_dir.child("main.py");
+    main.write_str(indoc! { r"
+        import foo
+        print('Hello, world!')
+       "
+    })?;
+
+    // We treat arguments before the command as uv arguments
+    uv_snapshot!(context.filters(), context.run().arg("--with").arg("iniconfig").arg("main.py"), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    Hello, world!
+
+    ----- stderr -----
+    warning: `uv run` is experimental and may change without warning
+    Resolved 1 package in [TIME]
+    Prepared 1 package in [TIME]
+    Installed 1 package in [TIME]
+     + foo==1.0.0 (from file://[TEMP_DIR]/)
+    Resolved 1 package in [TIME]
+    Prepared 1 package in [TIME]
+    Installed 1 package in [TIME]
+     + iniconfig==2.0.0
+    "###);
+
+    Ok(())
+}

crates/uv/tests/sync.rs

@@ -177,15 +177,19 @@ fn empty() -> Result<()> {
 
     // Running `uv sync` should generate an empty lockfile.
     uv_snapshot!(context.filters(), context.sync(), @r###"
-    success: true
-    exit_code: 0
+    success: false
+    exit_code: 2
     ----- stdout -----
 
     ----- stderr -----
     warning: `uv sync` is experimental and may change without warning
-    warning: No `requires-python` value found in the workspace. Defaulting to `>=3.12`.
-    Resolved 0 packages in [TIME]
-    Audited 0 packages in [TIME]
+    error: Failed to parse: `pyproject.toml`
+      Caused by: TOML parse error at line 2, column 9
+      |
+    2 |         [project]
+      |         ^^^^^^^^^
+    missing field `name`
+
     "###);
 
     assert!(context.temp_dir.child("uv.lock").exists());