Updates on pg.format.

1) Delegates `pg.Formattable.__str__()/__repr__()` to `pg.format`, which allows more top-level features to be supported without involving subclass formatting logics. E.g. (`custom_format` and `markdown` arguments).
2) Upgrades the `custom_format` argument of `pg.format` with `Callable` type: This allows users to plugin custom formatting logics more flexibly without intrusively adding method to existing types.
3) Add `pg.Formatting.__str_kwargs__` and `pg.Formatting.__repr_kwargs__` to allow subclasses to override the kwargs arbitration process.

PiperOrigin-RevId: 678422747

langfun/core/eval/matching.py

@@ -266,20 +266,27 @@ def _render_matches(self, s: io.StringIO) -> None:
         '<td>Prompt/Response Chain</td>'
         '</tr>'
     )
+    def _maybe_html(v, root_indent: int):
+      del root_indent
+      if hasattr(v, '_repr_html_'):
+        return v._repr_html_()  # pylint: disable=protected-access
+      # Fall back to the default format.
+      return None
+
     for i, (example, output, message) in enumerate(self.matches):
       bgcolor = 'white' if i % 2 == 0 else '#DDDDDD'
       s.write(f'<tr style="background-color: {bgcolor}"><td>{i + 1}</td>')
       input_str = lf.repr_utils.escape_quoted(
           pg.format(
               example, verbose=False, max_bytes_len=32,
-              custom_format='_repr_html_'
+              custom_format=_maybe_html
           )
       )
       s.write(f'<td style="color:green;white-space:pre-wrap">{input_str}</td>')
       output_str = lf.repr_utils.escape_quoted(
           pg.format(
               output, verbose=False, max_bytes_len=32,
-              custom_format='_repr_html_'
+              custom_format=_maybe_html
           )
       )
       s.write(f'<td style="color:blue;white-space:pre-wrap">{output_str}</td>')

langfun/core/modality.py

@@ -49,6 +49,13 @@ def format(self, *args, **kwargs) -> str:
       return super().format(*args, **kwargs)
     return Modality.text_marker(self.referred_name)
 
+  def __str_kwargs__(self) -> dict[str, Any]:
+    # For modality objects, we don't want to use markdown format when they
+    # are rendered as parts of the prompt.
+    kwargs = super().__str_kwargs__()
+    kwargs.pop('markdown', None)
+    return kwargs
+
   @abc.abstractmethod
   def to_bytes(self) -> bytes:
     """Returns content in bytes."""

langfun/core/repr_utils.py

@@ -121,8 +121,15 @@ def html_repr(
   s.write('<table style="border-top: 1px solid #EEEEEE;">')
   item_color = item_color or (lambda k, v: (None, '#F1C40F', None, None))
 
-  with (pg.str_format(custom_format='_repr_html_'),
-        pg.repr_format(custom_format='_repr_html_')):
+  def maybe_html_format(v: Any, root_indent: int) -> str | None:
+    del root_indent
+    if hasattr(v, '_repr_html_'):
+      return v._repr_html_()  # pylint: disable=protected-access
+    # Fall back to the default format.
+    return None
+
+  with (pg.str_format(custom_format=maybe_html_format),
+        pg.repr_format(custom_format=maybe_html_format)):
     for k, v in pg.object_utils.flatten(value).items():
       if isinstance(v, pg.Ref):
         v = v.value