BUG: Fix combine_first converts other columns type into floats unexpectedly

Author: Licht-T

pandas/core/frame.py

@@ -4870,14 +4870,24 @@ def combine(self, other, func, fill_value=None, overwrite=True):
                 series[this_mask] = fill_value
                 otherSeries[other_mask] = fill_value
 
-            # if we have different dtypes, possibly promote
-            new_dtype = this_dtype
-            if not is_dtype_equal(this_dtype, other_dtype):
-                new_dtype = find_common_type([this_dtype, other_dtype])
-                if not is_dtype_equal(this_dtype, new_dtype):
+            if col not in self.columns:
+                # If self DataFrame does not have col in other DataFrame,
+                # try to promote series, which is all NaN, as other_dtype.
+                new_dtype = other_dtype
+                try:
                     series = series.astype(new_dtype)
-                if not is_dtype_equal(other_dtype, new_dtype):
-                    otherSeries = otherSeries.astype(new_dtype)
+                except ValueError:
+                    # e.g. new_dtype is integer types
+                    pass
+            else:
+                # if we have different dtypes, possibly promote
+                new_dtype = this_dtype
+                if not is_dtype_equal(this_dtype, other_dtype):
+                    new_dtype = find_common_type([this_dtype, other_dtype])
+                    if not is_dtype_equal(this_dtype, new_dtype):
+                        series = series.astype(new_dtype)
+                    if not is_dtype_equal(other_dtype, new_dtype):
+                        otherSeries = otherSeries.astype(new_dtype)
 
             # see if we need to be represented as i8 (datetimelike)
             # try to keep us at this dtype
@@ -4950,6 +4960,11 @@ def combiner(x, y, needs_i8_conversion=False):
             else:
                 mask = isna(x_values)
 
+            # If the column y in other DataFrame is not in first DataFrame,
+            # just return y_values.
+            if y.name not in self.columns:
+                return y_values
+
             return expressions.where(mask, y_values, x_values)
 
         return self.combine(other, combiner, overwrite=False)