Improve px error for unknown column names in a list passed to x or y

CHANGELOG.md

@@ -21,6 +21,7 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 - Fix `convert_dash` mapping dotted lines (`linestyle=':'`) to invalid `'circle'` in Plotly, correctly converting them to `'dot'` and preserving custom dash patterns as pixel lists, [[#5719](https://github.com/plotly/plotly.py/pull/5719)] with thanks to @robertoffmoura for the contribution!
 - Fix `px.sunburst`, `px.treemap` and `px.icicle` listing sectors in a different order on every run when `path` is used with a Polars DataFrame; sectors now follow their order of first appearance for all dataframe backends [[#5766](https://github.com/plotly/plotly.py/pull/5766)], with thanks to @Irahan2 for the contribution!
 - Fix `mpl_to_plotly` conversion of matplotlib contour lines and line collections: close contour rings ending with `Z` codes, support dash styles, convert date x-axes, separate disjoint subpaths with `None` separators, hide line collection traces from the legend by default, and group consecutive same-style lines into single traces [[#5770](https://github.com/plotly/plotly.py/pull/5770)], with thanks to @robertoffmoura for the contribution!
+- Fix the misleading "All arguments should have the same length" error raised by Plotly Express when a list of column names passed to `x` or `y` contains a name that is not a column of `data_frame` (e.g. a typo); the error now names the missing column(s) and lists the available ones [[#5840](https://github.com/plotly/plotly.py/pull/5840)]
 
 ## [7.1.0] - 2026-09-15
 

plotly/express/_core.py

@@ -1188,6 +1188,39 @@ def _is_col_list(columns, arg, is_pd_like, native_namespace):
     return True
 
 
+def _check_col_list_typos(field, argument, df_input, length):
+    """Raise a helpful error if `argument` looks like a list of column names of
+    `df_input` (e.g. `y=["a", "b"]`) but some of them are not columns, instead of
+    letting it be treated as a list of values with a confusing length error.
+    `length` is the length of the previously-processed arguments (0 if none).
+    """
+    if (
+        not isinstance(argument, (list, tuple))
+        or len(argument) == 0
+        or not all(isinstance(c, str) for c in argument)
+    ):
+        return
+    missing = [c for c in argument if c not in df_input.columns]
+    if not missing or len(argument) == len(df_input):
+        return
+    # Only raise where a length error would be raised anyway, so that lists of
+    # strings used as data keep working even if some of them match column names
+    if length and len(argument) != length:
+        raise ValueError(
+            "Value of '%s' looks like a list of column names, but %s %s in "
+            "'data_frame'. Expected one of %s but received: %s"
+            % (
+                field,
+                ", ".join(repr(c) for c in missing),
+                "is not the name of a column"
+                if len(missing) == 1
+                else "are not names of columns",
+                str(list(df_input.columns)),
+                list(argument),
+            )
+        )
+
+
 def _isinstance_listlike(x):
     """Returns True if x is an iterable which can be transformed into a pandas Series,
     False for the other types of possible values of a `hover_data` dict.
@@ -1421,6 +1454,9 @@ def process_args_into_dataframe(
                 if col_name is None:  # numpy array, list...
                     col_name = _check_name_not_reserved(field, reserved_names)
 
+                if df_provided and field_name in ("x", "y"):
+                    _check_col_list_typos(field, argument, df_input, length)
+
                 if length and (len_arg := len(argument)) != length:
                     raise ValueError(
                         "All arguments should have the same length. "

tests/test_optional/test_px/test_px_input.py

@@ -756,3 +756,36 @@ def test_no_futurewarning():
         warn for warn in warn_list if issubclass(warn.category, FutureWarning)
     ]
     assert len(future_warnings) == 0, "FutureWarning(s) raised!"
+
+
+def test_wrong_column_name_in_list(constructor):  # issue 4040
+    df = constructor(dict(x=[1, 2, 3], data1=[1, 4, 9], data2=[1, 8, 27]))
+    # a typo in a list of column names used to raise a misleading
+    # "All arguments should have the same length" error
+    with pytest.raises(ValueError) as err_msg:
+        px.line(df, x="x", y=["data1", "Data2"])
+    msg = str(err_msg.value)
+    assert "Value of 'y' looks like a list of column names" in msg
+    assert "'Data2' is not the name of a column in 'data_frame'" in msg
+    assert "same length" not in msg
+    if isinstance(df, pd.DataFrame):
+        # without x, y is compared with the length of the index
+        with pytest.raises(ValueError, match="'Data2' is not the name of a column"):
+            px.line(df, y=["data1", "Data2"])
+    # all names wrong
+    with pytest.raises(ValueError, match="'Data1', 'Data2' are not names of columns"):
+        px.line(df, x="x", y=["Data1", "Data2"])
+
+
+def test_list_of_strings_as_values_with_data_frame():
+    # lists of strings whose length matches are still treated as data
+    df = pd.DataFrame(dict(a=[1, 2, 3], b=[4, 5, 6]))
+    fig = px.scatter(df, x=["u", "v", "w"], y="b")
+    assert list(fig.data[0].x) == ["u", "v", "w"]
+    fig = px.scatter(df, x=["u", "v"], y=["w", "z"])
+    assert list(fig.data[0].y) == ["w", "z"]
+    # even if some of the strings are also column names
+    fig = px.scatter(df, x=["a", "u"], y=[1, 2])
+    assert list(fig.data[0].x) == ["a", "u"]
+    fig = px.histogram(df, x=["a", "u"])
+    assert list(fig.data[0].x) == ["a", "u"]