Fix Plotly Express mutating lists passed to x or y in wide mode

CHANGELOG.md

@@ -14,6 +14,7 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 - Fix the sphinx-gallery scraper so that it generates thumbnails for figures shown with `fig.show()` or displayed as the last expression of a code block [[#5701](https://github.com/plotly/plotly.py/pull/5701)], with thanks to @larsoner for the contribution!
   - The scaper now warns once (instead of failing the build) when static image export is unavailable
 - The sphinx-gallery scraper no longer scrapes files belonging to other examples during parallel builds [[#5701](https://github.com/plotly/plotly.py/pull/5701)], with thanks to @larsoner for the contribution!
+- Fix Plotly Express mutating lists passed to the `x` or `y` arguments in wide mode [[#4117](https://github.com/plotly/plotly.py/issues/4117)]
 
 ### Updated
 - Update plotly.js from version 4.0.0 to version 4.1.1 [[#5722](https://github.com/plotly/plotly.py/pull/5722), [#5730](https://github.com/plotly/plotly.py/pull/5730)]. See the plotly.js release notes for [v4.1.0](https://github.com/plotly/plotly.js/releases/tag/v4.1.0) and [v4.1.1](https://github.com/plotly/plotly.js/releases/tag/v4.1.1) for details. Notable changes include:

plotly/express/_core.py

@@ -1687,8 +1687,9 @@ def build_dataframe(args, constructor):
             args["wide_variable"] = args["y"] if wide_y else args["x"]
             if df_provided and is_pd_like and args["wide_variable"] is columns:
                 var_name = columns.name
-            if is_pd_like and isinstance(args["wide_variable"], native_namespace.Index):
-                args["wide_variable"] = list(args["wide_variable"])
+            # copy into a new list so that the list provided by the user for
+            # x or y is not mutated when wide_variable's entries are replaced
+            args["wide_variable"] = list(args["wide_variable"])
             if var_name in [None, "value", "index"] or (
                 df_provided and var_name in columns
             ):

tests/test_optional/test_px/test_px_wide.py

@@ -890,3 +890,16 @@ def test_no_pd_perf_warning():
         if issubclass(warn.category, pd.errors.PerformanceWarning)
     ]
     assert len(performance_warnings) == 0, "PerformanceWarning(s) raised!"
+
+
+def test_wide_mode_does_not_mutate_x_or_y():
+    # https://github.com/plotly/plotly.py/issues/4117
+    df = pd.DataFrame(dict(a=[1, 2, 3], b=[4, 5, 6], c=[7, 8, 9]))
+    for arg in ["x", "y"]:
+        cols = ["a", "b"]
+        px.bar(df, **{arg: cols})
+        assert cols == ["a", "b"]
+    cols = [0, 1]
+    df_int = pd.DataFrame([[1, 2], [3, 4]])
+    px.histogram(df_int, x=cols)
+    assert cols == [0, 1]