Abstract over platform differences

Author: AlexWaygood

crates/red_knot_test/README.md

@@ -314,6 +314,45 @@ typeshed = "/typeshed"
 
 For more details, take a look at the [custom-typeshed Markdown test].
 
+### Mocking a virtual environment
+
+Mdtest supports mocking a virtual environment for a specific test at an arbitrary location, again
+using the `[environment]` configuration option:
+
+````markdown
+```toml
+[environment]
+python = ".venv"
+```
+````
+
+Red-knot will reject virtual environments that do not have valid `pyvenv.cfg` files at the
+virtual-environment directory root (here, `.venv/pyvenv.cfg`). However, if a `pyvenv.cfg` file does
+not have its contents specified by the test, mdtest will automatically generate one for you, to
+make mocking a virtual environment more ergonomic.
+
+Mdtest also makes it easy to write Python packages to the mock virtual environment's
+`site-packages` directory using the `<path-to-site-packages>` magic path separator. This would
+otherwise be hard, due to the fact that the `site-packages` subdirectory in a virtual environment
+is located at a different relative path depending on the platform the virtual environment was
+created on. In the following test, mdtest will write the Python file to
+`.venv/Lib/site-packages/foo.py` in its in-memory filesystem used for the test if the test is being
+executed on Windows, and `.venv/lib/python3.13/site-packages/foo.py` otherwise:
+
+````markdown
+```toml
+[environment]
+python = ".venv"
+python-version = "3.13"
+```
+
+`.venv/<path-to-site-packages>/foo.py`:
+
+```py
+X = 1
+```
+````
+
 ## Documentation of tests
 
 Arbitrary Markdown syntax (including of course normal prose paragraphs) is permitted (and ignored by

crates/red_knot_test/src/lib.rs

@@ -162,6 +162,7 @@ fn run_test(
     let custom_typeshed_path = test.configuration().typeshed();
     let python_path = test.configuration().python();
     let python_version = test.configuration().python_version().unwrap_or_default();
+
     let mut typeshed_files = vec![];
     let mut has_custom_versions_file = false;
     let mut has_custom_pyvenv_cfg_file = false;
@@ -178,7 +179,7 @@ fn run_test(
                 "Supported file types are: py (or python), pyi, text, cfg and ignore"
             );
 
-            let full_path = embedded.full_path(&project_root);
+            let mut full_path = embedded.full_path(&project_root);
 
             if let Some(typeshed_path) = custom_typeshed_path {
                 if let Ok(relative_path) = full_path.strip_prefix(typeshed_path.join("stdlib")) {
@@ -192,6 +193,24 @@ fn run_test(
                 if let Ok(relative_path) = full_path.strip_prefix(python_path) {
                     if relative_path.as_str() == "pyvenv.cfg" {
                         has_custom_pyvenv_cfg_file = true;
+                    } else {
+                        let mut new_path = SystemPathBuf::new();
+                        for component in full_path.components() {
+                            let component = component.as_str();
+                            if component == "<path-to-site-packages>" {
+                                if cfg!(target_os = "windows") {
+                                    new_path.push("Lib");
+                                    new_path.push("site-packages");
+                                } else {
+                                    new_path.push("lib");
+                                    new_path.push(format!("python{python_version}"));
+                                    new_path.push("site-packages");
+                                }
+                            } else {
+                                new_path.push(component);
+                            }
+                        }
+                        full_path = new_path;
                     }
                 }
             }