chore: add experimental image blob preview in DataFrame._repr_html_ (#1276)

Author: GarrettWu

bigframes/dataframe.py

@@ -739,10 +739,23 @@ def _repr_html_(self) -> str:
         if opts.repr_mode == "deferred":
             return formatter.repr_query_job(self._compute_dry_run())
 
+        df = self.copy()
+        if bigframes.options.experiments.blob:
+            import bigframes.bigquery as bbq
+
+            blob_cols = [
+                col
+                for col in df.columns
+                if df[col].dtype == bigframes.dtypes.OBJ_REF_DTYPE
+            ]
+            for col in blob_cols:
+                df[col] = df[col]._apply_unary_op(ops.ObjGetAccessUrl(mode="R"))
+                df[col] = bbq.json_extract(df[col], "$.access_urls.read_url")
+
         # TODO(swast): pass max_columns and get the true column count back. Maybe
         # get 1 more column than we have requested so that pandas can add the
         # ... for us?
-        pandas_df, row_count, query_job = self._block.retrieve_repr_request_results(
+        pandas_df, row_count, query_job = df._block.retrieve_repr_request_results(
             max_results
         )
 
@@ -751,8 +764,31 @@ def _repr_html_(self) -> str:
         column_count = len(pandas_df.columns)
 
         with display_options.pandas_repr(opts):
-            # _repr_html_ stub is missing so mypy thinks it's a Series. Ignore mypy.
-            html_string = pandas_df._repr_html_()  # type:ignore
+            # Allows to preview images in the DataFrame. The implementation changes the string repr as well, that it doesn't truncate strings or escape html charaters such as "<" and ">". We may need to implement a full-fledged repr module to better support types not in pandas.
+            if bigframes.options.experiments.blob:
+
+                def url_to_image_html(url: str) -> str:
+                    # url is a json string, which already contains double-quotes ""
+                    return f"<img src={url}>"
+
+                formatters = {blob_col: url_to_image_html for blob_col in blob_cols}
+
+                # set max_colwidth so not to truncate the image url
+                with pandas.option_context("display.max_colwidth", None):
+                    max_rows = pandas.get_option("display.max_rows")
+                    max_cols = pandas.get_option("display.max_columns")
+                    show_dimensions = pandas.get_option("display.show_dimensions")
+                    html_string = pandas_df.to_html(
+                        escape=False,
+                        notebook=True,
+                        max_rows=max_rows,
+                        max_cols=max_cols,
+                        show_dimensions=show_dimensions,
+                        formatters=formatters,  # type: ignore
+                    )
+            else:
+                # _repr_html_ stub is missing so mypy thinks it's a Series. Ignore mypy.
+                html_string = pandas_df._repr_html_()  # type:ignore
 
         html_string += f"[{row_count} rows x {column_count} columns in total]"
         return html_string