matplotlib · timhoffm · Mar 11, 2026 · Mar 3, 2026
diff --git a/doc/api/next_api_changes/behavior/31223-AS.rst b/doc/api/next_api_changes/behavior/31223-AS.rst
@@ -0,0 +1,28 @@
+``pyplot.subplot`` and ``pyplot.subplot_mosaic`` raise *ValueError* on existing figures
+----------------------------------------------------------------------------------------
+
+Passing a *num* argument to `~.pyplot.subplots` or `~.pyplot.subplot_mosaic`
+that refers to an existing figure or is a ``Figure`` instance now raises a
+`ValueError`.
+
+These utility functions are intended strictly for the creation of new figures and
+subplots. Previously, they accidentally allowed the reuse of existing figures because
+they internally called `~.pyplot.figure`. This change ensures that these functions
+strictly follow their documented purpose of creating new figures.
+
+To reuse an existing figure, clear it first using ``clear=True``:
+
+.. code-block:: python
+
+    fig, axs = plt.subplots(num=1, clear=True)
+    # or
+    fig, axd = plt.subplot_mosaic([['A', 'B']], num=1, clear=True)
+
+If you have a ``Figure`` instance and want to add subplots to it, use the
+object-oriented API:
+
+.. code-block:: python
+
+    fig.subplots(nrows=2, ncols=2)
+    # or
+    fig.subplot_mosaic([['A', 'B']])
diff --git a/lib/matplotlib/pyplot.py b/lib/matplotlib/pyplot.py
@@ -1178,6 +1178,25 @@ def fignum_exists(num: int | str) -> bool:
     )
 
 
+def _raise_if_figure_exists(num, func_name, clear=False):
+    """
+    Raise a ValueError if the figure *num* already exists.
+    """
+    if num is not None and not clear:
+        if isinstance(num, FigureBase):
+            raise ValueError(
+                f"num {num!r} cannot be a FigureBase instance. "
+                f"plt.{func_name}() is for creating new figures. "
+                f"To add to an existing figure, use fig.{func_name}() "
+                "instead.")
+
+        if fignum_exists(num):
+            raise ValueError(
+                f"Figure {num!r} already exists. Use plt.figure({num!r}) "
+                f"to get it or plt.close({num!r}) to close it. "
+                f"Alternatively, pass 'clear=True' to {func_name}().")
+
+
 def get_fignums() -> list[int]:
     """Return a list of existing figure numbers."""
     return sorted(_pylab_helpers.Gcf.figs)
@@ -1856,6 +1875,9 @@ def subplots(
         fig, ax = plt.subplots(num=10, clear=True)
 
     """
+    num = fig_kw.get('num')
+    _raise_if_figure_exists(fig_kw.get('num'), "subplots", fig_kw.get('clear'))
+
     fig = figure(**fig_kw)
     axs = fig.subplots(nrows=nrows, ncols=ncols, sharex=sharex, sharey=sharey,
                        squeeze=squeeze, subplot_kw=subplot_kw,
@@ -2029,6 +2051,9 @@ def subplot_mosaic(
        total layout.
 
     """
+    num = fig_kw.get('num')
+    _raise_if_figure_exists(fig_kw.get('num'), "subplot_mosaic", fig_kw.get('clear'))
+
     fig = figure(**fig_kw)
     ax_dict = fig.subplot_mosaic(  # type: ignore[misc]
         mosaic,  # type: ignore[arg-type]

diff --git a/lib/matplotlib/tests/test_pyplot.py b/lib/matplotlib/tests/test_pyplot.py
@@ -532,3 +532,42 @@ def assert_same_signature(func1, func2):
 
 def test_setloglevel_signature():
     assert_same_signature(plt.set_loglevel, mpl.set_loglevel)
+
+
+def test_subplots_reuse_existing_figure_error():
+    """Test interaction of plt.subplots(num=...) with existing figures."""
+    # Create a figure with a specific number first.
+    fig = plt.figure(1)
+
+    # Case 1: Reusing without clear=True should raise ValueError
+    with pytest.raises(ValueError, match="already exists"):
+        plt.subplots(num=1)
+
+    # Case 2: Reusing WITH clear=True should work fine (no error)
+    fig_new, axs = plt.subplots(num=1, clear=True)
+    assert fig_new is fig
+
+    # Case 3: Test passing the actual Figure object (The "Narrow Check")
+    with pytest.raises(ValueError, match="cannot be a FigureBase instance"):
+        plt.subplots(num=fig)
+
+    plt.close(1)
+
+
+def test_subplot_mosaic_reuse_existing_figure_error():
+    """Test that plt.subplot_mosaic raises ValueError when reusing a figure."""
+    fig = plt.figure(2)
+
+    # 1. Test passing the existing figure number
+    with pytest.raises(ValueError, match="already exists"):
+        plt.subplot_mosaic([['A']], num=2)
+
+    # 2. Test passing the actual Figure object
+    with pytest.raises(ValueError, match="cannot be a FigureBase instance"):
+        plt.subplot_mosaic([['A']], num=fig)
+
+    # 3. Test that clear=True allows reuse without error
+    fig_new, axd = plt.subplot_mosaic([['A']], num=2, clear=True)
+    assert fig_new is fig
+
+    plt.close(2)