wip: sql source and reader

Author: Oisin-M

src/earthkit/data/readers/sql.py

@@ -0,0 +1,31 @@
+from . import Reader
+
+
+class SQLReader(Reader):
+
+    def __init__(self, source, sql_engine, sql, **kwargs):
+        super().__init__(source, sql_engine)
+        self._sql_engine = sql_engine
+        self._sql = sql
+
+    def mutate_source(self):
+        return self
+
+    def to_pandas(self, **kwargs):
+        import pandas as pd
+
+        conn = self._sql_engine.connect()
+        try:
+            df = pd.read_sql(self._sql, conn, **kwargs)
+        finally:
+            conn.close()
+
+        return df
+
+    def to_xarray(self, **kwargs):
+
+        return self.to_pandas().to_xarray(**kwargs)
+
+
+def reader(source, path, *, magic=None, deeper_check=False, fwf=False, **kwargs):
+    return

src/earthkit/data/sources/sql.py

@@ -0,0 +1,29 @@
+from ..readers.sql import SQLReader
+from . import Source
+
+
+def create_sqlalchemy_engine(backend, kwargs):
+    from sqlalchemy import create_engine
+
+    if backend == "sqlite":
+        return create_engine(f"sqlite:///{kwargs['path']}")
+    else:
+        raise NotImplementedError(f"Backend: {backend} is not implemented.")
+
+
+class SQLSource(Source):
+
+    def __init__(self, backend, backend_kwargs, sql, **kwargs):
+        super().__init__(**kwargs)
+        sql_engine = create_sqlalchemy_engine(backend, backend_kwargs)
+        self._reader = SQLReader(self, sql_engine, sql, **kwargs)
+
+    def mutate(self):
+        source = self._reader.mutate_source()
+        if source not in (None, self):
+            source._parent = self
+            return source
+        return self
+
+
+source = SQLSource