diff --git a/doc/api/next_api_changes/2018-02-15-AL.rst b/doc/api/next_api_changes/2018-02-15-AL.rst
new file mode 100644
index 000000000000..112b18e3e322
--- /dev/null
+++ b/doc/api/next_api_changes/2018-02-15-AL.rst
@@ -0,0 +1,17 @@
+Axis-dependent Locators and Formatters explicitly error out when used over multiple Axis
+````````````````````````````````````````````````````````````````````````````````````````
+
+Certain Locators and Formatters (e.g. the default `AutoLocator` and
+`ScalarFormatter`) can only be used meaningfully on one Axis object at a time
+(i.e., attempting to use a single `AutoLocator` instance on the x and the y
+axis of an Axes, or the x axis of two different Axes, would result in
+nonsensical results).
+
+Such "double-use" is now detected and raises a RuntimeError *at canvas draw
+time*. The exception is not raised when the second Axis is registered in order
+to avoid incorrectly raising exceptions for the Locators and Formatters that
+*can* be used on multiple Axis objects simultaneously (e.g. `NullLocator` and
+`FuncFormatter`).
+
+In case a Locator or a Formatter really needs to be reassigned from one axis to
+another, first set its axis to None to bypass this protection.
diff --git a/lib/matplotlib/figure.py b/lib/matplotlib/figure.py
index f86c0c78936c..cf1de4f265ed 100644
--- a/lib/matplotlib/figure.py
+++ b/lib/matplotlib/figure.py
@@ -1589,11 +1589,19 @@ def subplots(self, nrows=1, ncols=1, sharex=False, sharey=False,
return axarr
def _remove_ax(self, ax):
- def _reset_loc_form(axis):
- axis.set_major_formatter(axis.get_major_formatter())
- axis.set_major_locator(axis.get_major_locator())
- axis.set_minor_formatter(axis.get_minor_formatter())
- axis.set_minor_locator(axis.get_minor_locator())
+ def _reset_tickers(axis):
+ major_formatter = axis.get_major_formatter()
+ major_formatter.set_axis(None) # Bypass prevention of axis reset.
+ axis.set_major_formatter(major_formatter)
+ major_locator = axis.get_major_locator()
+ major_locator.set_axis(None)
+ axis.set_major_locator(major_locator)
+ minor_formatter = axis.get_minor_formatter()
+ minor_formatter.set_axis(None)
+ axis.set_minor_formatter(minor_formatter)
+ minor_locator = axis.get_minor_locator()
+ minor_locator.set_axis(None)
+ axis.set_minor_locator(minor_locator)
def _break_share_link(ax, grouper):
siblings = grouper.get_siblings(ax)
@@ -1607,11 +1615,11 @@ def _break_share_link(ax, grouper):
self.delaxes(ax)
last_ax = _break_share_link(ax, ax._shared_y_axes)
if last_ax is not None:
- _reset_loc_form(last_ax.yaxis)
+ _reset_tickers(last_ax.yaxis)
last_ax = _break_share_link(ax, ax._shared_x_axes)
if last_ax is not None:
- _reset_loc_form(last_ax.xaxis)
+ _reset_tickers(last_ax.xaxis)
def clf(self, keep_observers=False):
"""
diff --git a/lib/matplotlib/projections/polar.py b/lib/matplotlib/projections/polar.py
index 188ffe0708f2..fbdbdc8923a7 100644
--- a/lib/matplotlib/projections/polar.py
+++ b/lib/matplotlib/projections/polar.py
@@ -198,6 +198,14 @@ class _AxisWrapper(object):
def __init__(self, axis):
self._axis = axis
+ def __eq__(self, other):
+ # Needed so that assignment, as the locator.axis attribute, of another
+ # _AxisWrapper wrapping the same axis works.
+ return self._axis == other._axis
+
+ def __hash__(self):
+ return hash((type(self), *sorted(vars(self).items())))
+
def get_view_interval(self):
return np.rad2deg(self._axis.get_view_interval())
@@ -227,10 +235,11 @@ class ThetaLocator(mticker.Locator):
"""
def __init__(self, base):
self.base = base
- self.axis = self.base.axis = _AxisWrapper(self.base.axis)
+ self.set_axis(self.base.axis)
def set_axis(self, axis):
- self.axis = _AxisWrapper(axis)
+ super().set_axis(_AxisWrapper(axis))
+ self.base.set_axis(None) # Bypass prevention of axis resetting.
self.base.set_axis(self.axis)
def __call__(self):
@@ -383,7 +392,6 @@ def _wrap_locator_formatter(self):
def cla(self):
super().cla()
self.set_ticks_position('none')
- self._wrap_locator_formatter()
def _set_scale(self, value, **kwargs):
super()._set_scale(value, **kwargs)
diff --git a/lib/matplotlib/tests/test_ticker.py b/lib/matplotlib/tests/test_ticker.py
index 9dddb69cf764..99c3b0108193 100644
--- a/lib/matplotlib/tests/test_ticker.py
+++ b/lib/matplotlib/tests/test_ticker.py
@@ -897,3 +897,29 @@ def minorticksubplot(xminor, yminor, i):
minorticksubplot(True, False, 2)
minorticksubplot(False, True, 3)
minorticksubplot(True, True, 4)
+
+
+def test_multiple_assignment():
+ fig = plt.figure()
+
+ ax = fig.subplots()
+ fmt = mticker.NullFormatter()
+ ax.xaxis.set_major_formatter(fmt)
+ ax.yaxis.set_major_formatter(fmt)
+ fig.canvas.draw() # No error.
+ fig.clf()
+
+ ax = fig.subplots()
+ fmt = mticker.ScalarFormatter()
+ ax.xaxis.set_major_formatter(fmt)
+ ax.xaxis.set_minor_formatter(fmt)
+ fig.canvas.draw() # No error.
+ fig.clf()
+
+ ax = fig.subplots()
+ fmt = mticker.ScalarFormatter()
+ ax.xaxis.set_major_formatter(fmt)
+ ax.yaxis.set_major_formatter(fmt)
+ with pytest.raises(RuntimeError):
+ fig.canvas.draw()
+ fig.clf()
diff --git a/lib/matplotlib/ticker.py b/lib/matplotlib/ticker.py
index 9c2a1fb7e2db..6b2d36a738c9 100644
--- a/lib/matplotlib/ticker.py
+++ b/lib/matplotlib/ticker.py
@@ -217,11 +217,41 @@ def get_tick_space(self):
return 9
-class TickHelper(object):
- axis = None
+class TickHelper:
+ # TickHelpers that access their axis attribute can only be assigned to
+ # one Axis at a time, but we don't know a priori whether they will (e.g.,
+ # NullFormatter doesn't, but ScalarFormatter does). So keep track of all
+ # Axises that a TickHelper is assigned to (in a set: a TickHelper could be
+ # assigned both as major and minor helper on a single axis), but only error
+ # out after multiple assignment when the attribute is accessed.
- def set_axis(self, axis):
- self.axis = axis
+ # As an escape hatch, allow resetting the axis by first setting it to None.
+
+ @property
+ def axis(self):
+ # We can't set the '_set_axises' attribute in TickHelper.__init__
+ # (without a deprecation period) because subclasses didn't have to call
+ # super().__init__ so far so they likely didn't.
+ set_axises = getattr(self, "_set_axises", set())
+ if len(set_axises) == 0:
+ return None
+ elif len(set_axises) == 1:
+ axis, = set_axises
+ return axis
+ else:
+ raise RuntimeError(
+ f"The 'axis' attribute of this {type(self).__name__} object "
+ f"has been set multiple times, but a {type(self).__name__} "
+ f"can only be used for one Axis at a time")
+
+ @axis.setter
+ def axis(self, axis):
+ if not hasattr(self, "_set_axises") or axis is None:
+ self._set_axises = set()
+ if axis is not None:
+ self._set_axises.add(axis)
+
+ set_axis = axis.fset
def create_dummy_axis(self, **kwargs):
if self.axis is None:
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