bpo-32989: IDLE - fix bad editor call of pyparse method (GH-5968)

Fix comments and add tests for editor newline_and_indent_event method.
Remove unused None default for function parameter of pyparse find_good_parse_start method
and code triggered by that default.

Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>

Lib/idlelib/editor.py

@@ -1342,38 +1342,51 @@ class EditorWindow(object):
             text.undo_block_stop()
 
     def newline_and_indent_event(self, event):
+        """Insert a newline and indentation after Enter keypress event.
+
+        Properly position the cursor on the new line based on information
+        from the current line.  This takes into account if the current line
+        is a shell prompt, is empty, has selected text, contains a block
+        opener, contains a block closer, is a continuation line, or
+        is inside a string.
+        """
         text = self.text
         first, last = self.get_selection_indices()
         text.undo_block_start()
-        try:
+        try:  # Close undo block and expose new line in finally clause.
             if first and last:
                 text.delete(first, last)
                 text.mark_set("insert", first)
             line = text.get("insert linestart", "insert")
+
+            # Count leading whitespace for indent size.
             i, n = 0, len(line)
             while i < n and line[i] in " \t":
-                i = i+1
+                i += 1
             if i == n:
-                # the cursor is in or at leading indentation in a continuation
-                # line; just inject an empty line at the start
+                # The cursor is in or at leading indentation in a continuation
+                # line; just inject an empty line at the start.
                 text.insert("insert linestart", '\n')
                 return "break"
             indent = line[:i]
-            # strip whitespace before insert point unless it's in the prompt
+
+            # Strip whitespace before insert point unless it's in the prompt.
             i = 0
             while line and line[-1] in " \t" and line != self.prompt_last_line:
                 line = line[:-1]
-                i = i+1
+                i += 1
             if i:
                 text.delete("insert - %d chars" % i, "insert")
-            # strip whitespace after insert point
+
+            # Strip whitespace after insert point.
             while text.get("insert") in " \t":
                 text.delete("insert")
-            # start new line
+
+            # Insert new line.
             text.insert("insert", '\n')
 
-            # adjust indentation for continuations and block
-            # open/close first need to find the last stmt
+            # Adjust indentation for continuations and block open/close.
+            # First need to find the last statement.
             lno = index2line(text.index('insert'))
             y = pyparse.Parser(self.indentwidth, self.tabwidth)
             if not self.prompt_last_line:
@@ -1383,7 +1396,7 @@ class EditorWindow(object):
                     rawtext = text.get(startatindex, "insert")
                     y.set_code(rawtext)
                     bod = y.find_good_parse_start(
-                              self._build_char_in_string_func(startatindex))
+                            self._build_char_in_string_func(startatindex))
                     if bod is not None or startat == 1:
                         break
                 y.set_lo(bod or 0)
@@ -1399,26 +1412,26 @@ class EditorWindow(object):
 
             c = y.get_continuation_type()
             if c != pyparse.C_NONE:
-                # The current stmt hasn't ended yet.
+                # The current statement hasn't ended yet.
                 if c == pyparse.C_STRING_FIRST_LINE:
-                    # after the first line of a string; do not indent at all
+                    # After the first line of a string do not indent at all.
                     pass
                 elif c == pyparse.C_STRING_NEXT_LINES:
-                    # inside a string which started before this line;
-                    # just mimic the current indent
+                    # Inside a string which started before this line;
+                    # just mimic the current indent.
                     text.insert("insert", indent)
                 elif c == pyparse.C_BRACKET:
-                    # line up with the first (if any) element of the
+                    # Line up with the first (if any) element of the
                     # last open bracket structure; else indent one
                     # level beyond the indent of the line with the
-                    # last open bracket
+                    # last open bracket.
                     self.reindent_to(y.compute_bracket_indent())
                 elif c == pyparse.C_BACKSLASH:
-                    # if more than one line in this stmt already, just
+                    # If more than one line in this statement already, just
                     # mimic the current indent; else if initial line
                     # has a start on an assignment stmt, indent to
                     # beyond leftmost =; else to beyond first chunk of
-                    # non-whitespace on initial line
+                    # non-whitespace on initial line.
                     if y.get_num_lines_in_stmt() > 1:
                         text.insert("insert", indent)
                     else:
@@ -1427,9 +1440,9 @@ class EditorWindow(object):
                     assert 0, "bogus continuation type %r" % (c,)
                 return "break"
 
-            # This line starts a brand new stmt; indent relative to
+            # This line starts a brand new statement; indent relative to
             # indentation of initial line of closest preceding
-            # interesting stmt.
+            # interesting statement.
             indent = y.get_base_indent_string()
             text.insert("insert", indent)
             if y.is_block_opener():