Adding FutureWarning if Series.plot is called with positional arguments

Author: Marc Garcia

pandas/plotting/_core.py

@@ -1,5 +1,6 @@
 import importlib
 from typing import List, Type  # noqa
+import warnings
 
 from pandas.util._decorators import Appender
 
@@ -518,11 +519,6 @@ def _get_call_args(data, args, kwargs):
         signatures, since `DataFramePlotMethods` accepted `x` and `y`
         parameters.
         """
-        if args and isinstance(data, ABCSeries):
-            # TODO raise warning here, positional arguments shouldn't be
-            # used anymore, so we can add x, y and kind to the signature
-            pass
-
         if isinstance(data, ABCSeries):
             arg_def = [
                 ('kind', 'line'), ('ax', None), ('figsize', None),
@@ -550,6 +546,18 @@ def _get_call_args(data, args, kwargs):
                               'Series or DataFrame').format(
                                   type(data).__name__))
 
+        if args and isinstance(data, ABCSeries):
+            msg = ('`Series.plot()` should not be called with positional '
+                   'arguments, only keyword arguments. The order of '
+                   'positional arguments will change in the future. '
+                   'Use `Series.plot({})` instead of `Series.plot({})`.')
+            positional_args = str(args)[1:-1]
+            keyword_args = ', '.join('{}={!r}'.format(name, value)
+                                     for (name, default), value
+                                     in zip(arg_def, args))
+            warnings.warn(msg.format(keyword_args, positional_args),
+                          FutureWarning, stacklevel=3)
+
         pos_args = {name: value for value, (name, _) in zip(args, arg_def)}
         kwargs = dict(arg_def, **pos_args, **kwargs)
 

pandas/tests/plotting/test_misc.py

@@ -9,7 +9,7 @@
 
 import pandas.util._test_decorators as td
 
-from pandas import DataFrame
+from pandas import DataFrame, Series
 from pandas.tests.plotting.common import TestPlotBase, _check_plot_works
 import pandas.util.testing as tm
 
@@ -25,6 +25,12 @@ def test_import_error_message():
         df.plot()
 
 
+@td.skip_if_no_mpl
+def test_series_plot_with_positional_arguments_warns():
+    with tm.assert_produces_warning(FutureWarning):
+        Series([1, 2, 3]).plot('line', None)
+
+
 @td.skip_if_no_mpl
 class TestSeriesPlots(TestPlotBase):