Fixes issue (127) where append_date_dirs wasn't working properly. (#130)

* Fixed bug where partition_keys was being modified in-place, and causing problems laters.

* Adjusted how target_path is constructed so that trailing forward slashes in target_path are all removed.

* Added typing.cast to comply with pytype.

weather_dl/download_pipeline/parsers.py

@@ -328,8 +328,9 @@ def require(condition: bool, message: str) -> None:
     num_partition_keys = len(partition_keys)
     if use_date_as_directory(config):
         num_partition_keys -= 1
-        if str(params.get('target_path')).endswith('/'):
-            params['target_path'] = params.get('target_path')[:-1]
+        target_path = t.cast(str, params.get('target_path', ''))
+        if target_path != '':
+            params['target_path'] = target_path.rstrip('/')
 
     require(num_template_replacements == num_partition_keys,
             """
@@ -352,10 +353,12 @@ def prepare_target_name(config: Config) -> str:
 
     target_path = t.cast(str, parameters.get('target_path', ''))
     target_filename = t.cast(str, parameters.get('target_filename', ''))
-    partition_keys = t.cast(t.List[str], parameters.get('partition_keys', list()))
+    partition_keys = t.cast(t.List[str],
+                            cp.copy(parameters.get('partition_keys', list())))
 
     if use_date_as_directory(config):
-        date_vals = config['selection']['date'][0].split('-')
+        date = t.cast(str, config['selection']['date'][0])
+        date_vals = date.split('-')
         target_path = os.path.join(target_path, *date_vals)
         partition_keys.remove('date')