Give more informative errors for unsupported dtypes

Author: michaelosthege

Diff for: mcbackend/backends/clickhouse.py

@@ -49,6 +49,11 @@ def create_runs_table(client: clickhouse_driver.Client):
 
 
 def column_spec_for(var: Variable, is_stat: bool = False):
+    if var.dtype not in CLICKHOUSE_DTYPES:
+        raise KeyError(
+            f"Don't know how to store dtype {var.dtype} "
+            f"of '{var.name}' (is_stat={is_stat}) in ClickHouse."
+        )
     cdt = CLICKHOUSE_DTYPES[var.dtype]
     ndim = len(var.shape)
     for _ in range(ndim):

Diff for: mcbackend/test_backend_clickhouse.py

@@ -13,6 +13,7 @@
     ClickHouseBackend,
     ClickHouseChain,
     ClickHouseRun,
+    column_spec_for,
     create_chain_table,
 )
 from mcbackend.core import Run, chain_id
@@ -27,6 +28,17 @@
     HAS_REAL_DB = False
 
 
+def test_column_spec_for():
+    assert column_spec_for(Variable("A", "float32", [])) == "`A` Float32"
+    assert column_spec_for(Variable("A", "float32", []), is_stat=True) == "`__stat_A` Float32"
+    assert column_spec_for(Variable("A", "float32", [2])) == "`A` Array(Float32)"
+    assert column_spec_for(Variable("A", "float32", [2, 3])) == "`A` Array(Array(Float32))"
+    with pytest.raises(KeyError, match="float16 of 'A'"):
+        column_spec_for(Variable("A", "float16", []))
+    pass
+
+
+
 def fully_initialized(
     cbackend: ClickHouseBackend, rmeta: RunMeta, *, nchains: int = 1
 ) -> Tuple[ClickHouseRun, Sequence[ClickHouseChain]]: