BUG: Thoroughly dedup column names in read_csv (#17095)

doc/source/whatsnew/v0.21.0.txt

@@ -279,6 +279,7 @@ I/O
 ^^^
 
 - Bug in :func:`read_csv` in which columns were not being thoroughly de-duplicated (:issue:`17060`)
+- Bug in :func:`read_csv` in which specified column names were not being thoroughly de-duplicated (:issue:`17095`)
 - Bug in :func:`read_csv` in which non integer values for the header argument generated an unhelpful / unrelated error message (:issue:`16338`)
 - Bug in :func:`read_csv` in which memory management issues in exception handling, under certain conditions, would cause the interpreter to segfault (:issue:`14696`, :issue:`16798`).
 - Bug in :func:`read_csv` when called with ``low_memory=False`` in which a CSV with at least one column > 2GB in size would incorrectly raise a ``MemoryError`` (:issue:`16798`).

pandas/io/parsers.py

@@ -1318,14 +1318,18 @@ def _maybe_dedup_names(self, names):
         # would be nice!
         if self.mangle_dupe_cols:
             names = list(names)  # so we can index
-            counts = {}
+            counts = defaultdict(int)
 
             for i, col in enumerate(names):
-                cur_count = counts.get(col, 0)
+                cur_count = counts[col]
 
-                if cur_count > 0:
-                    names[i] = '%s.%d' % (col, cur_count)
+                while cur_count > 0:
+                    counts[col] = cur_count + 1
 
+                    col = '%s.%d' % (col, cur_count)
+                    cur_count = counts[col]
+
+                names[i] = col
                 counts[col] = cur_count + 1
 
         return names
@@ -2330,15 +2334,15 @@ def _infer_columns(self):
                         this_columns.append(c)
 
                 if not have_mi_columns and self.mangle_dupe_cols:
-                    counts = {}
+                    counts = defaultdict(int)
 
                     for i, col in enumerate(this_columns):
-                        cur_count = counts.get(col, 0)
+                        cur_count = counts[col]
 
                         while cur_count > 0:
                             counts[col] = cur_count + 1
                             col = "%s.%d" % (col, cur_count)
-                            cur_count = counts.get(col, 0)
+                            cur_count = counts[col]
 
                         this_columns[i] = col
                         counts[col] = cur_count + 1

pandas/tests/io/parser/mangle_dupes.py

@@ -25,7 +25,7 @@ def test_basic(self):
                                        mangle_dupe_cols=True)
             assert list(df.columns) == expected
 
-    def test_thorough_mangle(self):
+    def test_thorough_mangle_columns(self):
         # see gh-17060
         data = "a,a,a.1\n1,2,3"
         df = self.read_csv(StringIO(data), sep=",", mangle_dupe_cols=True)
@@ -40,3 +40,25 @@ def test_thorough_mangle(self):
         df = self.read_csv(StringIO(data), sep=",", mangle_dupe_cols=True)
         assert list(df.columns) == ["a", "a.1", "a.3", "a.1.1",
                                     "a.2", "a.2.1", "a.3.1"]
+
+    def test_thorough_mangle_names(self):
+        # see gh-17095
+        data = "a,b,b\n1,2,3"
+        names = ["a.1", "a.1", "a.1.1"]
+        df = self.read_csv(StringIO(data), sep=",", names=names,
+                           mangle_dupe_cols=True)
+        assert list(df.columns) == ["a.1", "a.1.1", "a.1.1.1"]
+
+        data = "a,b,c,d,e,f\n1,2,3,4,5,6"
+        names = ["a", "a", "a.1", "a.1.1", "a.1.1.1", "a.1.1.1.1"]
+        df = self.read_csv(StringIO(data), sep=",", names=names,
+                           mangle_dupe_cols=True)
+        assert list(df.columns) == ["a", "a.1", "a.1.1", "a.1.1.1",
+                                    "a.1.1.1.1", "a.1.1.1.1.1"]
+
+        data = "a,b,c,d,e,f,g\n1,2,3,4,5,6,7"
+        names = ["a", "a", "a.3", "a.1", "a.2", "a", "a"]
+        df = self.read_csv(StringIO(data), sep=",", names=names,
+                           mangle_dupe_cols=True)
+        assert list(df.columns) == ["a", "a.1", "a.3", "a.1.1",
+                                    "a.2", "a.2.1", "a.3.1"]