CLN: Deprecate non-keyword arguments in concat #41485

pandas-dev · jreback · Jun 7, 2021 · May 29, 2021 · May 29, 2021 · May 29, 2021
commit 717fd398b96d617952ff259aca480c3584136261
diff --git a/doc/source/whatsnew/v1.3.0.rst b/doc/source/whatsnew/v1.3.0.rst
@@ -701,6 +701,7 @@ Deprecations
 - Deprecated passing arguments as positional (other than ``filepath_or_buffer``) in :func:`read_csv` (:issue:`41485`)
 - Deprecated passing arguments as positional in :meth:`DataFrame.drop` (other than ``"labels"``) and :meth:`Series.drop` (:issue:`41485`)
 - Deprecated passing arguments as positional (other than ``filepath_or_buffer``) in :func:`read_table` (:issue:`41485`)
+- Deprecated passing arguments as positional (other than ``objs``) in :func:`concat` (:issue:`41485`)
 
 
 .. _whatsnew_130.deprecations.nuisance_columns:

diff --git a/pandas/core/reshape/concat.py b/pandas/core/reshape/concat.py
@@ -16,7 +16,10 @@
 import numpy as np
 
 from pandas._typing import FrameOrSeriesUnion
-from pandas.util._decorators import cache_readonly
+from pandas.util._decorators import (
+    cache_readonly,
+    deprecate_nonkeyword_arguments,
+)
 
 from pandas.core.dtypes.concat import concat_compat
 from pandas.core.dtypes.generic import (
@@ -52,6 +55,7 @@
 # Concatenate DataFrame objects
 
 
+@deprecate_nonkeyword_arguments(version=None, allowed_args=["objs"])
 @overload
 def concat(
     objs: Iterable[DataFrame] | Mapping[Hashable, DataFrame],

diff --git a/pandas/tests/reshape/concat/test_concat.py b/pandas/tests/reshape/concat/test_concat.py
@@ -638,3 +638,20 @@ def test_concat_multiindex_with_empty_rangeindex():
     result = concat([df1, df2])
     expected = DataFrame([[1, 2], [np.nan, np.nan]], columns=mi)
     tm.assert_frame_equal(result, expected)
+
+
+def test_concat_posargs_deprecation(all_parsers):
+    # https://github.com/pandas-dev/pandas/issues/41485
+    df = pd.DataFrame([[1,2,3]], index=["a"])
+    df2 = pd.DataFrame([[4,5,6]], index=["b"])
+
+    msg = (
+        "In a future version of pandas all arguments of concat"
+        "except for the argument 'objs' will be keyword-only"
+    )
+    with tm.assert_produces_warning(FutureWarning, match=msg):
+        concat([df, df2], " ")
+    expected = DataFrame([[1,2,3],[4,5,6]], index=["a","b"])
+    tm.assert_frame_equal(result, expected)
+
+