Ignore transOffset if no offsets passed to Collection

This fixes a regression from #20717 in networkx (Fixes #21517), but we'll go forward with the change in a later release to give them time to fix it.
matplotlib · Nov 6, 2021 · 27ab206 · 27ab206
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diff --git a/doc/api/prev_api_changes/api_changes_3.5.0/deprecations.rst b/doc/api/prev_api_changes/api_changes_3.5.0/deprecations.rst
@@ -269,6 +269,10 @@ Miscellaneous deprecations
 - ``cm.LUTSIZE`` is deprecated. Use :rc:`image.lut` instead. This value only
   affects colormap quantization levels for default colormaps generated at
   module import time.
+- ``Collection.__init__`` previously ignored *transOffset* without *offsets* also
+  being specified. In the future, *transOffset* will begin having an effect
+  regardless of *offsets*. In the meantime, if you wish to set *transOffset*,
+  call `.Collection.set_offset_transform` explicitly.
 - ``Colorbar.patch`` is deprecated; this attribute is not correctly updated
   anymore.
 - ``ContourLabeler.get_label_width`` is deprecated.

diff --git a/lib/matplotlib/collections.py b/lib/matplotlib/collections.py
@@ -202,6 +202,18 @@ def __init__(self,
             if offsets.shape == (2,):
                 offsets = offsets[None, :]
             self._offsets = offsets
+        elif transOffset is not None:
+            _api.warn_deprecated(
+                '3.5',
+                removal='3.6',
+                message='Passing *transOffset* without *offsets* has no '
+                        'effect. This behavior is deprecated since %(since)s '
+                        'and %(removal)s, *transOffset* will begin having an '
+                        'effect regardless of *offsets*. In the meantime, if '
+                        'you wish to set *transOffset*, call '
+                        'collection.set_offset_transform(transOffset) '
+                        'explicitly.')
+            transOffset = None
 
         self._transOffset = transOffset
 

diff --git a/lib/matplotlib/tests/test_collections.py b/lib/matplotlib/tests/test_collections.py
@@ -1072,8 +1072,13 @@ def test_set_offsets_late():
 
 
 def test_set_offset_transform():
+    with pytest.warns(MatplotlibDeprecationWarning,
+                      match='.transOffset. without .offsets. has no effect'):
+        mcollections.Collection([],
+                                transOffset=mtransforms.IdentityTransform())
+
     skew = mtransforms.Affine2D().skew(2, 2)
-    init = mcollections.Collection([], transOffset=skew)
+    init = mcollections.Collection([], offsets=[], transOffset=skew)
 
     late = mcollections.Collection([])
     late.set_offset_transform(skew)