BUG: Bug in DataFrame update where non-specified values could cause dtype changes

RELEASE.rst

@@ -143,6 +143,7 @@ pandas 0.11.0
   - Bug in ``icol`` with negative indicies was incorrect producing incorrect return values (see GH2922_)
   - Bug in DataFrame column insertion when the column creation fails, existing frame is left in
     an irrecoverable state (GH3010_)
+  - Bug in DataFrame update where non-specified values could cause dtype changes (GH3016_)
 
 .. _GH622: https://github.com/pydata/pandas/issues/622
 .. _GH797: https://github.com/pydata/pandas/issues/797

pandas/core/frame.py

@@ -3843,8 +3843,13 @@ def update(self, other, join='left', overwrite=True, filter_func=None,
 
                 if overwrite:
                     mask = isnull(that)
+
+                    # don't overwrite columns unecessarily
+                    if mask.all():
+                        continue
                 else:
                     mask = notnull(this)
+
             self[col] = np.where(mask, this, that)
 
     #----------------------------------------------------------------------

pandas/tests/test_frame.py

@@ -7265,6 +7265,19 @@ def test_update(self):
                               [1.5, nan, 7.]])
         assert_frame_equal(df, expected)
 
+    def test_update_dtypes(self):
+
+        # gh 3016
+        df = DataFrame([[1.,2.,False, True],[4.,5.,True,False]],
+                       columns=['A','B','bool1','bool2'])
+
+        other = DataFrame([[45,45]],index=[0],columns=['A','B'])
+        df.update(other)
+
+        expected = DataFrame([[45.,45.,False, True],[4.,5.,True,False]],
+                             columns=['A','B','bool1','bool2'])
+        assert_frame_equal(df, expected)
+
     def test_update_nooverwrite(self):
         df = DataFrame([[1.5, nan, 3.],
                         [1.5, nan, 3.],