From 75a8bb601c681ff5fc24f3d1728b85deb3f3d383 Mon Sep 17 00:00:00 2001
From: Tim Hoffmann <2836374+timhoffm@users.noreply.github.com>
Date: Sun, 9 Jan 2022 13:19:47 +0100
Subject: [PATCH] Backport PR #22144: Fix cl subgridspec

---
 lib/matplotlib/_constrained_layout.py         |  8 +++++---
 .../tests/test_constrainedlayout.py           | 20 +++++++++++++++++++
 2 files changed, 25 insertions(+), 3 deletions(-)

diff --git a/lib/matplotlib/_constrained_layout.py b/lib/matplotlib/_constrained_layout.py
index d360eb66164e..c998681b1db3 100644
--- a/lib/matplotlib/_constrained_layout.py
+++ b/lib/matplotlib/_constrained_layout.py
@@ -204,14 +204,16 @@ def make_layoutgrids_gs(layoutgrids, gs):
             layoutgrids = make_layoutgrids_gs(layoutgrids, parentgs)
         subspeclb = layoutgrids[parentgs]
         # gridspecfromsubplotspec need an outer container:
-        if f'{gs}top' not in layoutgrids:
-            layoutgrids[f'{gs}top'] = mlayoutgrid.LayoutGrid(
+        # get a unique representation:
+        rep = (gs, 'top')
+        if rep not in layoutgrids:
+            layoutgrids[rep] = mlayoutgrid.LayoutGrid(
                 parent=subspeclb,
                 name='top',
                 nrows=1, ncols=1,
                 parent_pos=(subplot_spec.rowspan, subplot_spec.colspan))
         layoutgrids[gs] = mlayoutgrid.LayoutGrid(
-                parent=layoutgrids[f'{gs}top'],
+                parent=layoutgrids[rep],
                 name='gridspec',
                 nrows=gs._nrows, ncols=gs._ncols,
                 width_ratios=gs.get_width_ratios(),
diff --git a/lib/matplotlib/tests/test_constrainedlayout.py b/lib/matplotlib/tests/test_constrainedlayout.py
index a717eace8348..8fd3cc5a35a7 100644
--- a/lib/matplotlib/tests/test_constrainedlayout.py
+++ b/lib/matplotlib/tests/test_constrainedlayout.py
@@ -115,6 +115,26 @@ def test_constrained_layout6():
                  ticks=ticker.MaxNLocator(nbins=5))
 
 
+def test_identical_subgridspec():
+
+    fig = plt.figure(constrained_layout=True)
+
+    GS = fig.add_gridspec(2, 1)
+
+    GSA = GS[0].subgridspec(1, 3)
+    GSB = GS[1].subgridspec(1, 3)
+
+    axa = []
+    axb = []
+    for i in range(3):
+        axa += [fig.add_subplot(GSA[i])]
+        axb += [fig.add_subplot(GSB[i])]
+
+    fig.draw_without_rendering()
+    # chech first row above second
+    assert axa[0].get_position().y0 > axb[0].get_position().y1
+
+
 def test_constrained_layout7():
     """Test for proper warning if fig not set in GridSpec"""
     with pytest.warns(