BUG: warn when saving dtype with metadata (#14994)

Address gh-14142 for the 1.18 release: warn when saving a dtype with metadata that cannot be loaded.

2 files changed, 44 insertions, 0 deletions

diff --git a/numpy/lib/format.py b/numpy/lib/format.py
index 1ecd72815..20e2e9c72 100644
--- a/numpy/lib/format.py
+++ b/numpy/lib/format.py
@@ -242,6 +242,16 @@ def read_magic(fp):
         major, minor = magic_str[-2:]
     return major, minor
 
+def _has_metadata(dt):
+    if dt.metadata is not None:
+        return True
+    elif dt.names is not None:
+        return any(_has_metadata(dt[k]) for k in dt.names)
+    elif dt.subdtype is not None:
+        return _has_metadata(dt.base)
+    else:
+        return False
+
 def dtype_to_descr(dtype):
     """
     Get a serializable descriptor from the dtype.
@@ -265,6 +275,10 @@ def dtype_to_descr(dtype):
         replicate the input dtype.
 
     """
+    if _has_metadata(dtype):
+        warnings.warn("metadata on a dtype may be saved or ignored, but will "
+                      "raise if saved when read. Use another form of storage.",
+                      UserWarning, stacklevel=2)
     if dtype.names is not None:
         # This is a record array. The .descr is fine.  XXX: parts of the
         # record array with an empty name, like padding bytes, still get
diff --git a/numpy/lib/tests/test_format.py b/numpy/lib/tests/test_format.py
index 062c21725..0592e0b12 100644
--- a/numpy/lib/tests/test_format.py
+++ b/numpy/lib/tests/test_format.py
@@ -963,3 +963,33 @@ def test_unicode_field_names():
     with open(fname, 'wb') as f:
         with assert_warns(UserWarning):
             format.write_array(f, arr, version=None)
+
+
+@pytest.mark.parametrize('dt, fail', [
+    (np.dtype({'names': ['a', 'b'], 'formats':  [float, np.dtype('S3',
+                 metadata={'some': 'stuff'})]}), True),
+    (np.dtype(int, metadata={'some': 'stuff'}), False),
+    (np.dtype([('subarray', (int, (2,)))], metadata={'some': 'stuff'}), False),
+    # recursive: metadata on the field of a dtype
+    (np.dtype({'names': ['a', 'b'], 'formats': [
+        float, np.dtype({'names': ['c'], 'formats': [np.dtype(int, metadata={})]})
+    ]}), False)
+    ])
+def test_metadata_dtype(dt, fail):
+    # gh-14142
+    arr = np.ones(10, dtype=dt)
+    buf = BytesIO()
+    with assert_warns(UserWarning):
+        np.save(buf, arr)
+    buf.seek(0)
+    if fail:
+        with assert_raises(ValueError):
+            np.load(buf)
+    else:
+        arr2 = np.load(buf)
+        # BUG: assert_array_equal does not check metadata
+        from numpy.lib.format import _has_metadata
+        assert_array_equal(arr, arr2)
+        assert _has_metadata(arr.dtype)
+        assert not _has_metadata(arr2.dtype)
+