From 1f7518b231184066439ced1f5408ef831511659b Mon Sep 17 00:00:00 2001
From: Patrick Hoefler <61934744+phofl@users.noreply.github.com>
Date: Thu, 22 Jun 2023 12:59:51 +0200
Subject: [PATCH 1/2] BUG: combine_first ignoring others columns if other is
 empty

---
 doc/source/whatsnew/v2.1.0.rst                   | 1 +
 pandas/core/frame.py                             | 8 +++++++-
 pandas/tests/frame/methods/test_combine_first.py | 8 ++++++++
 3 files changed, 16 insertions(+), 1 deletion(-)

diff --git a/doc/source/whatsnew/v2.1.0.rst b/doc/source/whatsnew/v2.1.0.rst
index 511e5793608bc..2436d91690ed3 100644
--- a/doc/source/whatsnew/v2.1.0.rst
+++ b/doc/source/whatsnew/v2.1.0.rst
@@ -491,6 +491,7 @@ Reshaping
 - Bug in :func:`merge_asof` raising ``KeyError`` for extension dtypes (:issue:`52904`)
 - Bug in :func:`merge_asof` raising ``ValueError`` for data backed by read-only ndarrays (:issue:`53513`)
 - Bug in :meth:`DataFrame.agg` and :meth:`Series.agg` on non-unique columns would return incorrect type when dist-like argument passed in (:issue:`51099`)
+- Bug in :meth:`DataFrame.combine_first` ignoring other's columns if ``other`` is empty (:issue:`53792`)
 - Bug in :meth:`DataFrame.idxmin` and :meth:`DataFrame.idxmax`, where the axis dtype would be lost for empty frames (:issue:`53265`)
 - Bug in :meth:`DataFrame.merge` not merging correctly when having ``MultiIndex`` with single level (:issue:`52331`)
 - Bug in :meth:`DataFrame.stack` losing extension dtypes when columns is a :class:`MultiIndex` and frame contains mixed dtypes (:issue:`45740`)
diff --git a/pandas/core/frame.py b/pandas/core/frame.py
index 352d353ef4dba..44808ce2c66fd 100644
--- a/pandas/core/frame.py
+++ b/pandas/core/frame.py
@@ -8344,7 +8344,13 @@ def combiner(x, y):
 
             return expressions.where(mask, y_values, x_values)
 
-        combined = self.combine(other, combiner, overwrite=False)
+        if other.empty:
+            combined = self.reindex(
+                self.columns.append(other.columns.difference(self.columns)), axis=1
+            )
+            combined = combined.astype(other.dtypes)
+        else:
+            combined = self.combine(other, combiner, overwrite=False)
 
         dtypes = {
             col: find_common_type([self.dtypes[col], other.dtypes[col]])
diff --git a/pandas/tests/frame/methods/test_combine_first.py b/pandas/tests/frame/methods/test_combine_first.py
index 9b9c3f88277e8..156e50d50a9ef 100644
--- a/pandas/tests/frame/methods/test_combine_first.py
+++ b/pandas/tests/frame/methods/test_combine_first.py
@@ -538,3 +538,11 @@ def test_midx_losing_dtype():
     )
     expected = DataFrame({"a": [np.nan, 4, 3, 3]}, index=expected_midx)
     tm.assert_frame_equal(result, expected)
+
+
+def test_combine_first_empty_columns():
+    left = DataFrame(columns=["a", "b"])
+    right = DataFrame(columns=["a", "c"])
+    result = left.combine_first(right)
+    expected = DataFrame(columns=["a", "b", "c"])
+    tm.assert_frame_equal(result, expected)

From d176ed46d179a5923b14bc942aeebc061c317fac Mon Sep 17 00:00:00 2001
From: Patrick Hoefler <61934744+phofl@users.noreply.github.com>
Date: Thu, 22 Jun 2023 14:14:27 +0200
Subject: [PATCH 2/2] Fix

---
 pandas/core/frame.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pandas/core/frame.py b/pandas/core/frame.py
index 44808ce2c66fd..cd554afc54e71 100644
--- a/pandas/core/frame.py
+++ b/pandas/core/frame.py
@@ -8344,7 +8344,7 @@ def combiner(x, y):
 
             return expressions.where(mask, y_values, x_values)
 
-        if other.empty:
+        if len(other) == 0:
             combined = self.reindex(
                 self.columns.append(other.columns.difference(self.columns)), axis=1
             )