Fix max recursion depth exceeded when concatenate (#599)

* Fix infinite recursion when concatenate

Author: sandorkertesz

docs/release_notes/version_0.12_updates.rst

@@ -1,6 +1,16 @@
 Version 0.12 Updates
 /////////////////////////
 
+
+Version 0.12.2
+===============
+
+Fixes
+++++++++
+
+- Fixed issue when failed to build a fieldlist with concatenation because maximum recursion depth exceeded (:pr:`599`)
+
+
 Version 0.12.1
 ===============
 

src/earthkit/data/core/index.py

@@ -633,11 +633,18 @@ def __repr__(self):
 
 class MultiIndex(Index):
     def __init__(self, indexes, *args, **kwargs):
-        self._indexes = list(indexes)
+        self._indexes = list(self._flatten(indexes))
         super().__init__(*args, **kwargs)
         # self.indexes = list(i for i in indexes if len(i))
         # TODO: propagate  index._init_args, index._init_order_by, index._init_kwargs, for each i in indexes?
 
+    def _flatten(self, indexes):
+        for i in indexes:
+            if isinstance(i, MultiIndex):
+                yield from self._flatten(i._indexes)
+            else:
+                yield i
+
     def sel(self, *args, **kwargs):
         if not args and not kwargs:
             return self

tests/grib/test_grib_concat.py

@@ -150,6 +150,17 @@ def test_grib_from_empty_3():
     _check_save_to_disk(ds3, 8, md)
 
 
+# See github issue #588
+def test_grib_concat_large():
+    ds_e = from_source("empty")
+    ds1 = from_source("file", earthkit_examples_file("test.grib"))
+
+    for _ in range(2000):
+        ds_e += ds1.sel(param="msl")
+
+    assert len(ds_e) == 2000
+
+
 if __name__ == "__main__":
     from earthkit.data.testing import main