BUG: Updated _pprint_seq to use enumerate instead of next

doc/source/whatsnew/v3.0.0.rst

@@ -137,6 +137,7 @@ Performance improvements
 Bug fixes
 ~~~~~~~~~
 - Fixed bug in :meth:`DataFrame.join` inconsistently setting result index name (:issue:`55815`)
+- Fixed bug in :meth:`DataFrame.to_string` that raised ``StopIteration`` with nested DataFrames. (:issue:`16098`)
 - Fixed bug in :meth:`Series.diff` allowing non-integer values for the ``periods`` argument. (:issue:`56607`)
 
 Categorical

pandas/io/formats/printing.py

@@ -111,19 +111,22 @@ def _pprint_seq(
         fmt = "[{body}]" if hasattr(seq, "__setitem__") else "({body})"
 
     if max_seq_items is False:
-        nitems = len(seq)
+        max_items = None
     else:
-        nitems = max_seq_items or get_option("max_seq_items") or len(seq)
+        max_items = max_seq_items or get_option("max_seq_items") or len(seq)
 
     s = iter(seq)
     # handle sets, no slicing
-    r = [
-        pprint_thing(next(s), _nest_lvl + 1, max_seq_items=max_seq_items, **kwds)
-        for i in range(min(nitems, len(seq)))
-    ]
+    r = []
+    max_items_reached = False
+    for i, item in enumerate(s):
+        if (max_items is not None) and (i >= max_items):
+            max_items_reached = True
+            break
+        r.append(pprint_thing(item, _nest_lvl + 1, max_seq_items=max_seq_items, **kwds))
     body = ", ".join(r)
 
-    if nitems < len(seq):
+    if max_items_reached:
         body += ", ..."
     elif isinstance(seq, tuple) and len(seq) == 1:
         body += ","

pandas/tests/io/formats/test_to_string.py

@@ -945,6 +945,13 @@ def test_to_string_repr_unicode(self):
         finally:
             sys.stdin = _stdin
 
+    def test_nested_dataframe(self):
+        df1 = DataFrame({"level1": [["row1"], ["row2"]]})
+        df2 = DataFrame({"level3": [{"level2": df1}]})
+        result = df2.to_string()
+        expected = "                   level3\n0  {'level2': ['level1']}"
+        assert result == expected
+
 
 class TestSeriesToString:
     def test_to_string_without_index(self):