From 5e949f855b83f41201d794401dbce32767a41b7b Mon Sep 17 00:00:00 2001
From: "Thomas J. Fan" <thomasjpfan@gmail.com>
Date: Fri, 27 May 2022 13:20:48 -0400
Subject: [PATCH 1/2] ENH Uses more didactic x[i] to represent features

---
 doc/whats_new/v1.2.rst            |  3 +++
 sklearn/tree/_export.py           |  6 +++---
 sklearn/tree/tests/test_export.py | 16 ++++++++--------
 3 files changed, 14 insertions(+), 11 deletions(-)

diff --git a/doc/whats_new/v1.2.rst b/doc/whats_new/v1.2.rst
index b4804f3c9c8b9..610094a8d7c8b 100644
--- a/doc/whats_new/v1.2.rst
+++ b/doc/whats_new/v1.2.rst
@@ -111,6 +111,9 @@ Changelog
 :mod:`sklearn.tree`
 ...................
 
+- |Enhancement| :func:`tree.plot_tree`, :func:`tree.export_graphviz` now uses
+  a lower case `x[i]` to represent feature `i`. :pr:`xxxxx` by `Thomas Fan`_.
+
 - |Fix| Fixed invalid memory access bug during fit in
   :class:`tree.DecisionTreeRegressor` and :class:`tree.DecisionTreeClassifier`.
   :pr:`23273` by `Thomas Fan`_.
diff --git a/sklearn/tree/_export.py b/sklearn/tree/_export.py
index 4e2e8b58cc370..e3be3795e16e8 100644
--- a/sklearn/tree/_export.py
+++ b/sklearn/tree/_export.py
@@ -115,7 +115,7 @@ def plot_tree(
 
     feature_names : list of strings, default=None
         Names of each of the features.
-        If None, generic names will be used ("X[0]", "X[1]", ...).
+        If None, generic names will be used ("x[0]", "x[1]", ...).
 
     class_names : list of str or bool, default=None
         Names of each of the target classes in ascending numerical order.
@@ -291,7 +291,7 @@ def node_to_str(self, tree, node_id, criterion):
             if self.feature_names is not None:
                 feature = self.feature_names[tree.feature[node_id]]
             else:
-                feature = "X%s%s%s" % (
+                feature = "x%s%s%s" % (
                     characters[1],
                     tree.feature[node_id],
                     characters[2],
@@ -789,7 +789,7 @@ def export_graphviz(
 
     feature_names : list of str, default=None
         Names of each of the features.
-        If None generic names will be used ("feature_0", "feature_1", ...).
+        If None, generic names will be used ("x[0]", "x[1]", ...).
 
     class_names : list of str or bool, default=None
         Names of each of the target classes in ascending numerical order.
diff --git a/sklearn/tree/tests/test_export.py b/sklearn/tree/tests/test_export.py
index d3b082a927048..657860a435c05 100644
--- a/sklearn/tree/tests/test_export.py
+++ b/sklearn/tree/tests/test_export.py
@@ -35,7 +35,7 @@ def test_graphviz_toy():
         "digraph Tree {\n"
         'node [shape=box, fontname="helvetica"] ;\n'
         'edge [fontname="helvetica"] ;\n'
-        '0 [label="X[0] <= 0.0\\ngini = 0.5\\nsamples = 6\\n'
+        '0 [label="x[0] <= 0.0\\ngini = 0.5\\nsamples = 6\\n'
         'value = [3, 3]"] ;\n'
         '1 [label="gini = 0.0\\nsamples = 3\\nvalue = [3, 0]"] ;\n'
         "0 -> 1 [labeldistance=2.5, labelangle=45, "
@@ -75,7 +75,7 @@ def test_graphviz_toy():
         "digraph Tree {\n"
         'node [shape=box, fontname="helvetica"] ;\n'
         'edge [fontname="helvetica"] ;\n'
-        '0 [label="X[0] <= 0.0\\ngini = 0.5\\nsamples = 6\\n'
+        '0 [label="x[0] <= 0.0\\ngini = 0.5\\nsamples = 6\\n'
         'value = [3, 3]\\nclass = yes"] ;\n'
         '1 [label="gini = 0.0\\nsamples = 3\\nvalue = [3, 0]\\n'
         'class = yes"] ;\n'
@@ -106,7 +106,7 @@ def test_graphviz_toy():
         'node [shape=box, style="filled, rounded", color="black", '
         'fontname="sans"] ;\n'
         'edge [fontname="sans"] ;\n'
-        "0 [label=<X<SUB>0</SUB> &le; 0.0<br/>samples = 100.0%<br/>"
+        "0 [label=<x<SUB>0</SUB> &le; 0.0<br/>samples = 100.0%<br/>"
         'value = [0.5, 0.5]>, fillcolor="#ffffff"] ;\n'
         "1 [label=<samples = 50.0%<br/>value = [1.0, 0.0]>, "
         'fillcolor="#e58139"] ;\n'
@@ -127,7 +127,7 @@ def test_graphviz_toy():
         "digraph Tree {\n"
         'node [shape=box, fontname="helvetica"] ;\n'
         'edge [fontname="helvetica"] ;\n'
-        '0 [label="X[0] <= 0.0\\ngini = 0.5\\nsamples = 6\\n'
+        '0 [label="x[0] <= 0.0\\ngini = 0.5\\nsamples = 6\\n'
         'value = [3, 3]\\nclass = y[0]"] ;\n'
         '1 [label="(...)"] ;\n'
         "0 -> 1 ;\n"
@@ -147,7 +147,7 @@ def test_graphviz_toy():
         'node [shape=box, style="filled", color="black", '
         'fontname="helvetica"] ;\n'
         'edge [fontname="helvetica"] ;\n'
-        '0 [label="node #0\\nX[0] <= 0.0\\ngini = 0.5\\n'
+        '0 [label="node #0\\nx[0] <= 0.0\\ngini = 0.5\\n'
         'samples = 6\\nvalue = [3, 3]", fillcolor="#ffffff"] ;\n'
         '1 [label="(...)", fillcolor="#C0C0C0"] ;\n'
         "0 -> 1 ;\n"
@@ -170,14 +170,14 @@ def test_graphviz_toy():
         'node [shape=box, style="filled", color="black", '
         'fontname="helvetica"] ;\n'
         'edge [fontname="helvetica"] ;\n'
-        '0 [label="X[0] <= 0.0\\nsamples = 6\\n'
+        '0 [label="x[0] <= 0.0\\nsamples = 6\\n'
         "value = [[3.0, 1.5, 0.0]\\n"
         '[3.0, 1.0, 0.5]]", fillcolor="#ffffff"] ;\n'
         '1 [label="samples = 3\\nvalue = [[3, 0, 0]\\n'
         '[3, 0, 0]]", fillcolor="#e58139"] ;\n'
         "0 -> 1 [labeldistance=2.5, labelangle=45, "
         'headlabel="True"] ;\n'
-        '2 [label="X[0] <= 1.5\\nsamples = 3\\n'
+        '2 [label="x[0] <= 1.5\\nsamples = 3\\n'
         "value = [[0.0, 1.5, 0.0]\\n"
         '[0.0, 1.0, 0.5]]", fillcolor="#f1bd97"] ;\n'
         "0 -> 2 [labeldistance=2.5, labelangle=-45, "
@@ -215,7 +215,7 @@ def test_graphviz_toy():
         "graph [ranksep=equally, splines=polyline] ;\n"
         'edge [fontname="sans"] ;\n'
         "rankdir=LR ;\n"
-        '0 [label="X[0] <= 0.0\\nsquared_error = 1.0\\nsamples = 6\\n'
+        '0 [label="x[0] <= 0.0\\nsquared_error = 1.0\\nsamples = 6\\n'
         'value = 0.0", fillcolor="#f2c09c"] ;\n'
         '1 [label="squared_error = 0.0\\nsamples = 3\\'
         'nvalue = -1.0", '

From a843ec28db4478d24402863fcad3acd1b187c24f Mon Sep 17 00:00:00 2001
From: "Thomas J. Fan" <thomasjpfan@gmail.com>
Date: Fri, 27 May 2022 13:35:23 -0400
Subject: [PATCH 2/2] DOC Adds PR number

---
 doc/whats_new/v1.2.rst | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/doc/whats_new/v1.2.rst b/doc/whats_new/v1.2.rst
index 610094a8d7c8b..8778e74a011a3 100644
--- a/doc/whats_new/v1.2.rst
+++ b/doc/whats_new/v1.2.rst
@@ -112,7 +112,7 @@ Changelog
 ...................
 
 - |Enhancement| :func:`tree.plot_tree`, :func:`tree.export_graphviz` now uses
-  a lower case `x[i]` to represent feature `i`. :pr:`xxxxx` by `Thomas Fan`_.
+  a lower case `x[i]` to represent feature `i`. :pr:`23480` by `Thomas Fan`_.
 
 - |Fix| Fixed invalid memory access bug during fit in
   :class:`tree.DecisionTreeRegressor` and :class:`tree.DecisionTreeClassifier`.