BUG: wide_to_long no longer changes the stubnames passed as an argument to the function call (GH9204)

Author: jtorcasso

doc/source/whatsnew/v0.16.0.txt

@@ -196,3 +196,4 @@ Bug Fixes
 
 - Fixed issue in the ``xlsxwriter`` engine where it added a default 'General' format to cells if no other format wass applied. This prevented other row or column formatting being applied. (:issue:`9167`)
 - Fixes issue with ``index_col=False`` when ``usecols`` is also specified in ``read_csv``. (:issue:`9082`)
+- Bug where ``wide_to_long`` would modify the input stubnames list (:issue:`9204`)

pandas/core/reshape.py

@@ -930,10 +930,9 @@ def melt_stub(df, stub, i, j):
     if i not in id_vars:
         id_vars += [i]
 
-    stub = stubnames.pop(0)
-    newdf = melt_stub(df, stub, id_vars, j)
+    newdf = melt_stub(df, stubnames[0], id_vars, j)
 
-    for stub in stubnames:
+    for stub in stubnames[1:]:
         new = melt_stub(df, stub, id_vars, j)
         newdf = newdf.merge(new, how="outer", on=id_vars + [j], copy=False)
     return newdf.set_index([i, j])

pandas/tests/test_reshape.py

@@ -443,6 +443,14 @@ def test_simple(self):
         long_frame = wide_to_long(df, ["A", "B"], i="id", j="year")
         tm.assert_frame_equal(long_frame, exp_frame)
 
+    def test_stubs(self):
+        # GH9204
+        df = pd.DataFrame([[0,1,2,3,8],[4,5,6,7,9]])
+        df.columns = ['id', 'inc1', 'inc2', 'edu1', 'edu2']
+        stubs = ['inc', 'edu']
+        df_long = pd.wide_to_long(df, stubs, i='id', j='age')
+
+        self.assertEqual(stubs,['inc', 'edu'])
 
 if __name__ == '__main__':
     nose.runmodule(argv=[__file__, '-vvs', '-x', '--pdb', '--pdb-failure'],