[watchmedo] Lazy loading of the PyYAML module

The module is loaded only when strictly necessary.
Simple usages of watchmedo are possible without the module being installed.

changelog.rst

@@ -8,8 +8,8 @@ Changelog
 
 2021-xx-xx • `full history <https://github.com/gorakhargosh/watchdog/compare/v2.1.6...master>`__
 
-- 
-- Thanks to our beloved contributors: @
+- [watchmedo] ``PyYAML`` is loaded only when strictly necessary. Simple usages of ``watchmedo`` are possible without the module being installed. (`#847 <https://github.com/gorakhargosh/watchdog/pull/847>`_)
+- Thanks to our beloved contributors: @BoboTiG
 
 2.1.6
 ~~~~~

src/watchdog/watchmedo.py

@@ -31,7 +31,6 @@
 from io import StringIO
 from textwrap import dedent
 
-import yaml
 from watchdog.utils import WatchdogShutdown, load_class
 from watchdog.version import VERSION_STRING
 
@@ -136,8 +135,10 @@ def load_config(tricks_file_pathname):
     :returns:
         A dictionary of configuration information.
     """
+    from yaml import safe_load
+
     with open(tricks_file_pathname, 'rb') as f:
-        return yaml.safe_load(f.read())
+        return safe_load(f.read())
 
 
 def parse_patterns(patterns_spec, ignore_patterns_spec, separator=';'):
@@ -309,6 +310,8 @@ def tricks_generate_yaml(args):
     """
     Command to generate Yaml configuration for tricks named on the command line.
     """
+    from yaml import dump
+
     python_paths = path_split(args.python_path)
     add_to_sys_path(python_paths)
     output = StringIO()
@@ -320,7 +323,7 @@ def tricks_generate_yaml(args):
     content = output.getvalue()
     output.close()
 
-    header = yaml.dump({CONFIG_KEY_PYTHON_PATH: python_paths})
+    header = dump({CONFIG_KEY_PYTHON_PATH: python_paths})
     header += "%s:\n" % CONFIG_KEY_TRICKS
     if args.append_to_file is None:
         # Output to standard output.