MNT: Deprecate changing Figure.number

Historically, pyplot dynamically added a number attribute to figure.
However, this number must stay in sync with the figure manger.
AFAICS overwriting the number attribute does not have the desired
effect for pyplot. But there are some repos in GitHub that do change
number. So let's take it slow and properly migrate away from writing.

Making the dynamic attribute private and wrapping it in a property
allows to maintain current behavior and deprecate write-access.

When the deprecation expires, there's no need for duplicate state
anymore and the private _number attribute can be replaced by
`self.canvas.manager.num` if that exists and None otherwise.

Also closes matplotlib#28994.

doc/api/next_api_changes/deprecations/28007-TH.rst

@@ -0,0 +1,6 @@
+Changing ``Figure.number``
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Changing ``Figure.number`` is deprecated. This value is used by `.pyplot`
+to identify figures. It must stay in sync with the pyplot internal state
+and is not intended to be modified by the user.

lib/matplotlib/_pylab_helpers.py

@@ -108,7 +108,7 @@ def _set_new_active_manager(cls, manager):
             manager._cidgcf = manager.canvas.mpl_connect(
                 "button_press_event", lambda event: cls.set_active(manager))
         fig = manager.canvas.figure
-        fig.number = manager.num
+        fig._number = manager.num
         label = fig.get_label()
         if label:
             manager.set_window_title(label)

lib/matplotlib/figure.py

@@ -2808,6 +2808,33 @@ def axes(self):
 
     get_axes = axes.fget
 
+    @property
+    def number(self):
+        """The figure id, used to identify figures in `.pyplot`."""
+        # Historically, pyplot dynamically added a number attribute to figure.
+        # However, this number must stay in sync with the figure manager.
+        # AFAICS overwriting the number attribute does not have the desired
+        # effect for pyplot. But there are some repos in GitHub that do change
+        # number. So let's take it slow and properly migrate away from writing.
+        #
+        # Making the dynamic attribute private and wrapping it in a property
+        # allows to maintain current behavior and deprecate write-access.
+        #
+        # When the deprecation expires, there's no need for duplicate state
+        # anymore and the private _number attribute can be replaced by
+        # `self.canvas.manager.num` if that exists and None otherwise.
+        if hasattr(self, '_number'):
+            return self._number
+        else:
+            raise AttributeError(
+                "'Figure' object has no attribute 'number'. In the future this"
+                "will change to returning 'None' instead.")
+
+    @number.setter
+    def number(self, num):
+        _api.warn_deprecated("3.10", message="Changing 'Figure.number' is deprecated")
+        self._number = num
+
     def _get_renderer(self):
         if hasattr(self.canvas, 'get_renderer'):
             return self.canvas.get_renderer()

lib/matplotlib/figure.pyi

@@ -343,6 +343,10 @@ class Figure(FigureBase):
     def get_layout_engine(self) -> LayoutEngine | None: ...
     def _repr_html_(self) -> str | None: ...
     def show(self, warn: bool = ...) -> None: ...
+    @property
+    def number(self) -> int | str: ...
+    @number.setter
+    def number(self, num: int | str) -> None: ...
     @property  # type: ignore[misc]
     def axes(self) -> list[Axes]: ...  # type: ignore[override]
     def get_axes(self) -> list[Axes]: ...

lib/matplotlib/tests/test_figure.py

@@ -150,6 +150,26 @@ def test_figure_label():
     with pytest.raises(ValueError):
         plt.figure(Figure())
 
+def test_figure_label_replaced():
+    plt.close('all')
+    fig = plt.figure(1)
+    with pytest.warns(mpl.MatplotlibDeprecationWarning,
+                      match="Changing 'Figure.number' is deprecated"):
+        fig.number = 2
+    assert fig.number == 2
+
+def test_figure_no_label():
+    # standalone figures do not have a figure attribute
+    fig = matplotlib.figure.Figure()
+    with pytest.raises(AttributeError):
+        fig.number
+    # but one can set one
+    fig.number = 5
+    with pytest.warns(MatplotlibDeprecationWarning,
+                      match="Changing 'Figure.number' is deprecated"):
+        assert fig.number == 5
+    # even though it's not known by pyplot
+    assert not plt.fignum_exists(fig.number)
 
 def test_fignum_exists():
     # pyplot figure creation, selection and closing with fignum_exists