[ty]: Search PYTHONPATH to find modules

This brings ty in line with other type checkers and python interpreters,
including on by default and no options to disable. Can be disabled by
clearing the PYTHONPATH environment variable or specifying paths in the
--exta-paths.

Co-authored-by: Manuel Mendez <[email protected]>
Co-authored-by: Micha Reiser <[email protected]>

Author: 3 people

crates/ty_project/src/metadata/options.rs

@@ -277,14 +277,52 @@ impl Options {
             roots
         };
 
+        // collect the existing site packages
+        let mut extra_paths: Vec<SystemPathBuf> = Vec::new();
+
+        match (&environment.extra_paths, system.env_var("PYTHONPATH")) {
+            (Some(extra_paths_env), Ok(_)) => {
+                tracing::debug!(
+                    "Both --extra-paths and `PYTHONPATH` are set, ignoring `PYTHONPATH`"
+                );
+                extra_paths.extend(
+                    extra_paths_env
+                        .iter()
+                        .map(|path| path.absolute(project_root, system)),
+                );
+            }
+            (Some(extra_paths_env), Err(_)) => {
+                extra_paths.extend(
+                    extra_paths_env
+                        .iter()
+                        .map(|path| path.absolute(project_root, system)),
+                );
+            }
+            (None, Ok(python_path)) => {
+                // read all the paths off the PYTHONPATH environment variable,
+                //  check they exist as a directory, and add them to the vec of
+                //  extra_paths as they should be checked before site-packages
+                //  just like python interpreter does
+                for path in python_path.split(':') {
+                    let possible_path = SystemPathBuf::from(path);
+
+                    if system.is_directory(&possible_path) {
+                        tracing::debug!(
+                            "Adding `{possible_path}` from the `PYTHONPATH` environment variable to `extra_paths`"
+                        );
+                        extra_paths.push(possible_path);
+                    } else {
+                        tracing::debug!(
+                            "Skipping `{possible_path}` listed in `PYTHONPATH` because the path doesn't exist or isn't a directory"
+                        );
+                    }
+                }
+            }
+            (None, Err(_)) => {}
+        }
+
         let settings = SearchPathSettings {
-            extra_paths: environment
-                .extra_paths
-                .as_deref()
-                .unwrap_or_default()
-                .iter()
-                .map(|path| path.absolute(project_root, system))
-                .collect(),
+            extra_paths,
             src_roots,
             custom_typeshed: environment
                 .typeshed