From 35ca598b95c5fcad04518cbe5dd0ae7a5c164244 Mon Sep 17 00:00:00 2001
From: MarcoGorelli <33491632+MarcoGorelli@users.noreply.github.com>
Date: Thu, 7 Sep 2023 09:31:15 +0200
Subject: [PATCH 1/2] note uniqueness constraint on join

---
 spec/API_specification/dataframe_api/dataframe_object.py | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/spec/API_specification/dataframe_api/dataframe_object.py b/spec/API_specification/dataframe_api/dataframe_object.py
index 0fd0eed8..779e4c48 100644
--- a/spec/API_specification/dataframe_api/dataframe_object.py
+++ b/spec/API_specification/dataframe_api/dataframe_object.py
@@ -932,6 +932,10 @@ def join(
         """
         Join with other dataframe.
 
+        Other than the joining column name(s), no column name is allowed to appear in
+        both `self` and `other`. Rename columns before calling `join` if necessary
+        using :meth:`rename_columns`.
+
         Parameters
         ----------
         other : DataFrame

From bae1918be14a09b2aafffbdd45af32936a8271e9 Mon Sep 17 00:00:00 2001
From: MarcoGorelli <33491632+MarcoGorelli@users.noreply.github.com>
Date: Wed, 27 Sep 2023 13:39:19 +0100
Subject: [PATCH 2/2] clarify raises

---
 spec/API_specification/dataframe_api/dataframe_object.py | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/spec/API_specification/dataframe_api/dataframe_object.py b/spec/API_specification/dataframe_api/dataframe_object.py
index 779e4c48..bf9643e6 100644
--- a/spec/API_specification/dataframe_api/dataframe_object.py
+++ b/spec/API_specification/dataframe_api/dataframe_object.py
@@ -955,4 +955,10 @@ def join(
         Returns
         -------
         DataFrame
+
+        Raises
+        ------
+        ValueError
+            If, apart from `left_on` and `right_on`, there are any column names
+            present in both `self` and `other`.
         """