allow modify poll_delay_ms argument using an environment variable (#303)

Co-authored-by: Samuel Colvin <s@muelcolvin.com>

Author: joelpelaez

tests/test_force_polling.py

@@ -42,6 +42,17 @@ def test_watch_polling_env(mocker, env: SetEnv):
     m.assert_called_once_with(['.'], False, True, 300, True, False)
 
 
+def test_watch_polling_env_with_custom_delay(mocker, env: SetEnv):
+    env('WATCHFILES_FORCE_POLLING', '1')
+    env('WATCHFILES_POLL_DELAY_MS', '1000')
+    m = mocker.patch('watchfiles.main.RustNotify', return_value=MockRustNotify())
+
+    for _ in watch('.'):
+        pass
+
+    m.assert_called_once_with(['.'], False, True, 1000, True, False)
+
+
 @pytest.mark.parametrize(
     'env_var,arg,expected',
     [

watchfiles/main.py

@@ -81,6 +81,12 @@ def watch(
         * otherwise, force polling is enabled
     * otherwise, we enable force polling only if we detect we're running on WSL (Windows Subsystem for Linux)
 
+    It is also possible to change the poll delay between iterations, it can be changed to maintain a good response time
+    and an appropiate CPU consumption using the `poll_delay_ms` argument, we change poll delay thus:
+
+    * if file polling is enabled and the `WATCHFILES_POLL_DELAY_MS` env var exists and it is numeric, we use that
+    * otherwise, we use the argument value
+
     Args:
         *paths: filesystem paths to watch.
         watch_filter: callable used to filter out changes which are not important, you can either use a raw callable
@@ -114,6 +120,7 @@ def watch(
     ```
     """
     force_polling = _default_force_polling(force_polling)
+    poll_delay_ms = _default_poll_delay_ms(poll_delay_ms)
     ignore_permission_denied = _default_ignore_permission_denied(ignore_permission_denied)
     debug = _default_debug(debug)
     with RustNotify(
@@ -184,6 +191,7 @@ async def awatch(  # C901
         force_polling: if true, always use polling instead of file system notifications, default is `None` where
             `force_polling` is set to `True` if the `WATCHFILES_FORCE_POLLING` environment variable exists.
         poll_delay_ms: delay between polling for changes, only used if `force_polling=True`.
+            `poll_delay_ms` can be changed via the `WATCHFILES_POLL_DELAY_MS` environment variable.
         recursive: if `True`, watch for changes in sub-directories recursively, otherwise watch only for changes in the
             top-level directory, default is `True`.
         ignore_permission_denied: if `True`, will ignore permission denied errors, otherwise will raise them by default.
@@ -242,6 +250,7 @@ async def stop_soon():
         stop_event_ = stop_event
 
     force_polling = _default_force_polling(force_polling)
+    poll_delay_ms = _default_poll_delay_ms(poll_delay_ms)
     ignore_permission_denied = _default_ignore_permission_denied(ignore_permission_denied)
     debug = _default_debug(debug)
     with RustNotify(
@@ -327,6 +336,17 @@ def _default_force_polling(force_polling: Optional[bool]) -> bool:
         return _auto_force_polling()
 
 
+def _default_poll_delay_ms(poll_delay_ms: int) -> int:
+    """
+    See docstring for `watch` above for details.
+    """
+    env_var = os.getenv('WATCHFILES_POLL_DELAY_MS')
+    if env_var and env_var.isdecimal():
+        return int(env_var)
+    else:
+        return poll_delay_ms
+
+
 def _default_debug(debug: Optional[bool]) -> bool:
     if debug is not None:
         return debug