Add aggregate function VARIANCE() for sqlite

pyfpdb/fpdb_db.py

@@ -30,11 +30,29 @@ except ImportError:
     logging.info("Not using sqlalchemy connection pool.")
     use_pool = False
 
+try:
+    from numpy import var
+    use_numpy = True
+except ImportError:
+    logging.info("Not using numpy to define variance in sqlite.")
+    use_numpy = False
 
 import fpdb_simple
 import FpdbSQLQueries
 import Configuration
 
+# Variance created as sqlite has a bunch of undefined aggregate functions.
+
+class VARIANCE:
+    def __init__(self):
+        self.store = []
+
+    def step(self, value):
+        self.store.append(value)
+
+    def finalize(self):
+        return float(var(self.store))
+
 class fpdb_db:
     MYSQL_INNODB = 2
     PGSQL = 3
@@ -130,6 +148,10 @@ class fpdb_db:
                                      , detect_types=sqlite3.PARSE_DECLTYPES )
             sqlite3.register_converter("bool", lambda x: bool(int(x)))
             sqlite3.register_adapter(bool, lambda x: "1" if x else "0")
+            if use_numpy:
+                self.db.create_aggregate("variance", 1, VARIANCE)
+            else:
+                logging.warning("Some database functions will not work without NumPy support")
         else:
             raise FpdbError("unrecognised database backend:"+backend)
         self.cursor = self.db.cursor()