Bugfix for flow_map_start.

To my best understanding, in 4e5edc9, the `flow_map_*` and `flow_seq_*`
class variables were added to allow different formatting for flow
mappings and flow sequences. However, it seems that these are currently
not functional due to an assert.

This commit makes a small change to allow, in particular, the
`flow_map_start` variable to be set and adds a test to help prevent a
future regression.

Author: burnison

_test/test_z_check_debug_leftovers.py

@@ -5,6 +5,7 @@
 
 import pytest  # type: ignore # NOQA
 from roundtrip import dedent, round_trip_dump, round_trip_load  # type: ignore
+from ruyaml.emitter import Emitter
 
 
 class TestLeftOverDebug:
@@ -38,3 +39,43 @@ def test_01(self, capsys: Any) -> None:
         round_trip_dump(d, sys.stdout)
         out, err = capsys.readouterr()
         assert out == s
+
+    def test_02(self, capsys: Any) -> None:
+        s = dedent(
+            """
+        - 1
+        - { f: 3.14 , g: 42 }
+        - {  }
+        - [ 3.14 , 42 ]
+        - [  ]
+        """)
+        d = round_trip_load(s)
+
+        current_map_start = Emitter.flow_map_start
+        current_map_end = Emitter.flow_map_end
+        current_map_separator = Emitter.flow_map_separator
+        current_seq_start = Emitter.flow_seq_start
+        current_seq_end = Emitter.flow_seq_end
+        current_seq_separator = Emitter.flow_seq_separator
+
+        Emitter.flow_map_start = '{ '
+        Emitter.flow_map_end = ' }'
+        Emitter.flow_map_separator = ' ,'
+
+        Emitter.flow_seq_start = '[ '
+        Emitter.flow_seq_end = ' ]'
+        Emitter.flow_seq_separator = ' ,'
+
+        try:
+            round_trip_dump(d, sys.stdout)
+        finally:
+            Emitter.flow_map_start = current_map_start
+            Emitter.flow_map_end = current_map_end
+            Emitter.flow_map_separator = current_map_separator
+
+            Emitter.flow_seq_start = current_seq_start
+            Emitter.flow_seq_end = current_seq_end
+            Emitter.flow_seq_separator = current_seq_separator
+
+        out, err = capsys.readouterr()
+        assert out == s

lib/ruyaml/emitter.py

@@ -596,7 +596,7 @@ def expect_first_flow_mapping_key(self) -> None:
         if isinstance(self.event, MappingEndEvent):
             self.indent = self.indents.pop()
             popped = self.flow_context.pop()
-            assert popped == '{'  # empty flow mapping
+            assert popped == self.flow_map_start  # empty flow mapping
             self.write_indicator(self.flow_map_end, False)
             if self.event.comment and self.event.comment[0]:
                 # eol comment on empty mapping
@@ -621,7 +621,7 @@ def expect_flow_mapping_key(self) -> None:
             #     self.write_pre_comment(self.event)
             self.indent = self.indents.pop()
             popped = self.flow_context.pop()
-            assert popped in ['{', '']
+            assert popped in [self.flow_map_start, '']
             if self.canonical:
                 self.write_indicator(self.flow_map_separator, False)
                 self.write_indent()