fix: write and read figure JSON as UTF-8

write_json wrote figure JSON with Path.write_text(json_str) and read_json
read it back with Path.read_text(), both omitting the encoding. On platforms
whose default text encoding is not UTF-8 (e.g. cp1252 on Windows), writing a
figure containing non-ASCII text raised UnicodeEncodeError and reading produced
mojibake. write_html already passes "utf-8" explicitly; apply the same to the
JSON I/O path so figures round-trip everywhere.

Update the existing pathlib mock tests to assert the UTF-8 encoding.

Author: LukeTheoJohnson

CHANGELOG.md

@@ -6,6 +6,7 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 
 ### Fixed
 - Raise a clear `ValueError` when an unsupported marginal plot type is passed to Plotly Express, instead of failing later with a cryptic `'NoneType' object has no attribute 'constructor'` message [[#5625](https://github.com/plotly/plotly.py/pull/5625)], with thanks to @eugen-goebel for the contribution!
+- Read and write figure JSON files as UTF-8 in `read_json`/`write_json` so figures containing non-ASCII text round-trip correctly on platforms whose default encoding is not UTF-8 (e.g. cp1252 on Windows), matching the existing behavior of `write_html` [[#XXXX](https://github.com/plotly/plotly.py/pull/XXXX)]
 
 
 ## [6.8.0] - 2026-06-03

plotly/io/_json.py

@@ -295,7 +295,7 @@ def write_json(fig, file, validate=True, pretty=False, remove_uids=True, engine=
     else:
         # We previously succeeded in interpreting `file` as a pathlib object.
         # Now we can use `write_bytes()`.
-        path.write_text(json_str)
+        path.write_text(json_str, "utf-8")
 
 
 def from_json_plotly(value, engine=None):
@@ -464,7 +464,7 @@ def read_json(file, output_type="Figure", skip_invalid=False, engine=None):
     # Read file contents into JSON string
     # -----------------------------------
     if path is not None:
-        json_str = path.read_text()
+        json_str = path.read_text("utf-8")
     else:
         json_str = file.read()
 

tests/test_io/test_to_from_json.py

@@ -179,6 +179,11 @@ def test_read_json_from_pathlib(fig1, fig_type_spec, fig_type):
     # read_json on mock file
     fig1_loaded = pio.read_json(filemock, output_type=fig_type_spec)
 
+    # The file must be read as UTF-8 (JSON is UTF-8 per RFC 8259); otherwise a
+    # figure with non-ASCII text is mangled on platforms whose default codec is
+    # not UTF-8 (e.g. cp1252 on Windows).
+    filemock.read_text.assert_called_once_with("utf-8")
+
     # Check return type
     assert isinstance(fig1_loaded, fig_type)
 
@@ -240,7 +245,10 @@ def test_write_json_pathlib(fig1, pretty, remove_uids):
 
     # check write contents
     expected = pio.to_json(fig1, pretty=pretty, remove_uids=remove_uids)
-    filemock.write_text.assert_called_once_with(expected)
+    # The figure must be written as UTF-8 so non-ASCII text does not raise
+    # UnicodeEncodeError on platforms whose default codec is not UTF-8
+    # (e.g. cp1252 on Windows).
+    filemock.write_text.assert_called_once_with(expected, "utf-8")
 
 
 @pytest.mark.parametrize("pretty", [True, False])