Add a useful error message on empty query result from ixmp4

RELEASE_NOTES.md

@@ -4,8 +4,8 @@
 - [#944](https://github.com/IAMconsortium/pyam/pull/944) Refactor to a `format_n()` function for nice log messages
 - [#943](https://github.com/IAMconsortium/pyam/pull/943) Improved handling for division by zero
 - [#941](https://github.com/IAMconsortium/pyam/pull/941) Add a `compute.share()` method
-- [#938](https://github.com/IAMconsortium/pyam/pull/938) Add a function to initialize an IamDataFrame from an *
-  *ixmp4.Run**
+- [#938](https://github.com/IAMconsortium/pyam/pull/938) Add a function to initialize an IamDataFrame from an **ixmp4.Run**
+- [#950](https://github.com/IAMconsortium/pyam/pull/950) Add a useful error message on empty query result from an **ixmp4.Platform**
 
 # Release v3.2.0
 

pyam/ixmp4.py

@@ -42,18 +42,24 @@ def read_ixmp4(
     meta_filters = dict(
         run=dict(default_only=default_only, model=model, scenario=scenario)
     )
-    iamc_filters = dict(
-        run=dict(default_only=default_only),
-        model=model,
-        scenario=scenario,
-        region=region,
-        variable=variable,
-        unit=unit,
-        year=year,
-    )
+    iamc_filters = dict(run=dict(default_only=default_only))
+    for key, value in (
+        ("model", model),
+        ("scenario", scenario),
+        ("region", region),
+        ("variable", variable),
+        ("unit", unit),
+        ("year", year),
+    ):
+        if value is not None:
+            iamc_filters[key] = value
+
     data = platform.iamc.tabulate(**iamc_filters)
     meta = platform.meta.tabulate(**meta_filters)
 
+    if data.empty:
+        raise ValueError("No scenario data with filters " + str(iamc_filters))
+
     # if default-only, simplify to standard IAMC index, add `version` as meta indicator
     if default_only:
         index = ["model", "scenario"]

tests/test_ixmp4.py

@@ -112,6 +112,11 @@ def test_ixmp4_reserved_columns(test_platform, test_df_year, drop_meta):
     assert_iamframe_equal(test_df_year, pyam.read_ixmp4(test_platform))
 
 
+def test_ixmp4_empty_result(test_platform):
+    with pytest.raises(ValueError, match=r"No scenario data with filters \{.*'foo'\}"):
+        read_ixmp4(test_platform, variable="foo")
+
+
 def test_ixmp4_read_run(test_platform, test_df):
     """Initialize an IamDataFrame from an ixmp4 run"""