handle nan values in DataFrame.update when overwrite=False pandas-dev#15593

pandas-dev#15593

Author: pcluo

pandas/core/frame.py

@@ -3846,13 +3846,13 @@ 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)