Implement Butler.get for matplotlib generated png files

python/lsst/daf/butler/configs/storageClasses.yaml

@@ -342,10 +342,14 @@ storageClasses:
       dict: lsst.utils.packages.Packages
   NumpyArray:
     pytype: numpy.ndarray
+    converters:
+      matplotlib.figure.Figure: lsst.daf.butler.formatters.matplotlib.MatplotlibFormatter.dummyConverter
   Thumbnail:
     pytype: numpy.ndarray
   Plot:
     pytype: matplotlib.figure.Figure
+    converters:
+      numpy.ndarray: lsst.daf.butler.formatters.matplotlib.MatplotlibFormatter.fromArray
   MetricValue:
     pytype: lsst.verify.Measurement
   StampsBase:

python/lsst/daf/butler/formatters/matplotlib.py

@@ -29,6 +29,9 @@
 
 from .file import FileFormatter
 
+import numpy as np
+import matplotlib.pyplot as plt
+
 
 class MatplotlibFormatter(FileFormatter):
     """Interface for writing matplotlib figures."""
@@ -38,8 +41,23 @@ class MatplotlibFormatter(FileFormatter):
 
     def _readFile(self, path: str, pytype: type[Any] | None = None) -> Any:
         # docstring inherited from FileFormatter._readFile
-        raise NotImplementedError(f"matplotlib figures cannot be read by the butler; path is {path}")
+        return plt.imread(path)
 
     def _writeFile(self, inMemoryDataset: Any) -> None:
         # docstring inherited from FileFormatter._writeFile
         inMemoryDataset.savefig(self.fileDescriptor.location.path)
+
+    @staticmethod
+    def fromArray(cls: np.ndarray) -> plt.Figure:
+        """Convert an array into a Figure.
+        """
+        fig = plt.figure()
+        plt.imshow(cls)
+        return fig
+
+    @staticmethod
+    def dummyCovnerter(cls):
+        """This converter exists to trick the Butler into allowing
+        a numpy array on read with ``storageClass='NumpyArray'``.
+        """
+        return cls

tests/test_matplotlibFormatter.py

@@ -26,6 +26,7 @@
 import tempfile
 import unittest
 from random import Random
+import numpy as np
 
 try:
     import matplotlib
@@ -39,6 +40,7 @@
 
 from lsst.daf.butler import Butler, DatasetType
 from lsst.daf.butler.tests.utils import makeTestTempDir, removeTestTempDir
+from lsst.daf.butler.formatters.matplotlib import MatplotlibFormatter
 
 TESTDIR = os.path.abspath(os.path.dirname(__file__))
 
@@ -78,8 +80,13 @@ def testMatplotlibFormatter(self):
             pyplot.gcf().savefig(file.name)
             self.assertTrue(filecmp.cmp(local.ospath, file.name, shallow=True))
         self.assertTrue(butler.exists(ref))
-        with self.assertRaises(ValueError):
-            butler.get(ref)
+
+        fig = butler.get(ref)
+        # Ensure that the result is a figure
+        self.assertTrue(isinstance(fig, pyplot.Figure))
+        image = butler.get(ref, storageClass="NumpyArray")
+        self.assertTrue(isinstance(image, np.ndarray))
+
         butler.pruneDatasets([ref], unstore=True, purge=True)
         self.assertFalse(butler.exists(ref))