maint: add relevant code comment + minimal stylistic changes [skip ci]

Author: oesteban

nipype/pipeline/engine/nodes.py

@@ -389,17 +389,17 @@ def hash_exists(self, updatehash=False):
         return cached, self._hashvalue, hashfile, self._hashed_inputs
 
     def run(self, updatehash=False):
-        """Execute the node in its directory.
+        """
+        Execute the node in its directory.
 
         Parameters
         ----------
-
         updatehash: boolean
             When the hash stored in the output directory as a result of a previous run
             does not match that calculated for this execution, updatehash=True only
             updates the hash without re-running.
-        """
 
+        """
         if self.config is None:
             self.config = {}
         self.config = merge_dict(deepcopy(config._sections), self.config)
@@ -443,6 +443,10 @@ def run(self, updatehash=False):
             for outdatedhash in glob(op.join(self.output_dir(), '_0x*.json')):
                 os.remove(outdatedhash)
 
+        # _get_hashval needs to be called before running. When there is a valid (or seemingly
+        # valid cache), the is_cached() member updates the hashval via _get_hashval.
+        # However, if this node's folder doesn't exist or the result file is not found, then
+        # the hashval needs to be generated here. See #3026 for a larger context.
         self._get_hashval()
         # Hashfile while running
         hashfile_unfinished = op.join(