From ee999f76f515bf2ad8c77a9b3f2c7d9c9a0493cc Mon Sep 17 00:00:00 2001
From: cluo <cluoren@gmail.com>
Date: Mon, 22 May 2017 15:35:40 -0400
Subject: [PATCH] BUG: handle nan values in DataFrame.update when
 overwrite=False (#15593)

BUG: handle nan values in DataFrame.update when overwrite=False (#15593)

add nan test for DataFrame.update

update whatsnew v0.20.2
---
 doc/source/whatsnew/v0.20.2.txt           |  1 +
 pandas/core/frame.py                      |  8 ++++----
 pandas/tests/frame/test_combine_concat.py | 10 ++++++++++
 3 files changed, 15 insertions(+), 4 deletions(-)

diff --git a/doc/source/whatsnew/v0.20.2.txt b/doc/source/whatsnew/v0.20.2.txt
index e0857019d2fd4c..21d3643641f1b4 100644
--- a/doc/source/whatsnew/v0.20.2.txt
+++ b/doc/source/whatsnew/v0.20.2.txt
@@ -37,6 +37,7 @@ Bug Fixes
 ~~~~~~~~~
 
 - Bug in using ``pathlib.Path`` or ``py.path.local`` objects with io functions (:issue:`16291`)
+- Bug in using ``DataFrame.update`` when overwrite=False and the second value is nan
 
 Conversion
 ^^^^^^^^^^
diff --git a/pandas/core/frame.py b/pandas/core/frame.py
index 78a369761afc11..beea0e9d3d8adc 100644
--- a/pandas/core/frame.py
+++ b/pandas/core/frame.py
@@ -3921,13 +3921,13 @@ 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)
 
+            # don't overwrite columns unecessarily
+            if mask.all():
+                continue
+
             self[col] = expressions.where(mask, this, that,
                                           raise_on_error=True)
 
diff --git a/pandas/tests/frame/test_combine_concat.py b/pandas/tests/frame/test_combine_concat.py
index 688cacdee263ee..e3b153b9b2c0fb 100644
--- a/pandas/tests/frame/test_combine_concat.py
+++ b/pandas/tests/frame/test_combine_concat.py
@@ -763,3 +763,13 @@ def test_concat_datetime_datetime64_frame(self):
 
         # it works!
         pd.concat([df1, df2_obj])
+
+
+class TestDataFrameUpdate(TestData):
+
+    def test_update_nan(self):
+        # #15593 #15617
+        df1 = DataFrame({'A': [1, None, 3], 'B': date_range('2000', periods=3)})
+        df2 = DataFrame({'A': [None, 2, 3]})
+        df1.update(df2, overwrite=False)
+        # it works!