Update web-platform-tests to revision 77fa639e94d19ad846db71da960d49c89578fb52

tests/wpt/web-platform-tests/docs/writing-tests/python-handlers/index.md

@@ -43,6 +43,44 @@ The wptserver implements a number of Python APIs for controlling traffic.
    /tools/wptserve/docs/stash
 ```
 
+### Importing local helper scripts
+
+Python file handlers may import local helper scripts, e.g. to share logic
+across multiple handlers. To avoid module name collision, however, imports must
+be relative to the root of WPT. For example, in an imaginary
+`cookies/resources/myhandler.py`:
+
+```python
+# DON'T DO THIS
+import myhelper
+
+# DO THIS
+from cookies.resources import myhelper
+```
+
+Only absolute imports are allowed; do not use relative imports. If the path to
+your helper script includes a hyphen (`-`), you can use `import_module` from
+`importlib` to import it. For example:
+
+```python
+import importlib
+myhelper = importlib.import_module('common.security-features.myhelper')
+```
+
+**Note on __init__ files**: Until WPT moves to Python 3 only (~Q1 2021),
+importing helper scripts like this requires a 'path' of empty `__init__.py`
+files in every directory down to the helper. For example, if your helper is
+`css/css-align/resources/myhelper.py`, you need to have:
+
+```
+css/__init__.py
+css/css-align/__init__.py
+css/css-align/resources/__init__.py
+```
+
+This requirement will be removed once we move to Python 3, due to
+[PEP 420](https://www.python.org/dev/peps/pep-0420/).
+
 ### Python3 compatibility
 
 Even though Python3 is not fully supported at this point, some work is being