minor tweaks to aggregation docs and tests

Author: dantownsend

docs/src/piccolo/query_types/select.rst

@@ -86,7 +86,7 @@ Aggregate functions
 Count
 ~~~~~
 
-Returns the number of rows which match the query.
+Returns the number of rows which match the query:
 
 .. code-block:: python
 
@@ -96,7 +96,7 @@ Returns the number of rows which match the query.
 Avg
 ~~~
 
-Returns average of numeric rows which match the query.
+Returns the average for a given column:
 
 .. code-block:: python
 
@@ -108,7 +108,7 @@ Returns average of numeric rows which match the query.
 Sum
 ~~~
 
-Returns sum of numeric rows which match the query.
+Returns the sum for a given column:
 
 .. code-block:: python
 
@@ -120,7 +120,7 @@ Returns sum of numeric rows which match the query.
 Max
 ~~~
 
-Returns maximum of rows which match the query.
+Returns the maximum for a given column:
 
 .. code-block:: python
 
@@ -132,7 +132,7 @@ Returns maximum of rows which match the query.
 Min
 ~~~
 
-Returns minimum of rows which match the query.
+Returns the minimum for a given column:
 
 .. code-block:: python
 
@@ -144,7 +144,7 @@ Returns minimum of rows which match the query.
 Additional features
 ~~~~~~~~~~~~~~~~~~~
 
-You also can chain multiple different aggregate functions in one query,
+You also can chain multiple different aggregate functions in one query:
 
 .. code-block:: python
 
@@ -153,7 +153,7 @@ You also can chain multiple different aggregate functions in one query,
     >>> response
     {"avg": 750.0, "sum": 1500}
 
-use aliases for aggregate functions like this
+And can use aliases for aggregate functions like this:
 
 .. code-block:: python
 
@@ -162,11 +162,7 @@ use aliases for aggregate functions like this
     >>> response["popularity_avg"]
     750.0
 
-or use ``as_alias`` method for aggregate functions like this.
-
-.. code-block:: python
-
-    >>> from piccolo.query import Avg
+    # Alternatively, you can use the `as_alias` method.
     >>> response = Band.select(Avg(Band.popularity).as_alias("popularity_avg")).first().run_sync()
     >>> response["popularity_avg"]
     750.0

piccolo/query/methods/select.py

@@ -25,6 +25,10 @@
     from piccolo.table import Table  # noqa
 
 
+def is_numeric_column(column: Column) -> bool:
+    return column.value_type in (int, decimal.Decimal, float)
+
+
 class Avg(Selectable):
     """
     AVG() SQL function. Column type must be numeric to run the query.
@@ -35,20 +39,18 @@ class Avg(Selectable):
     """
 
     def __init__(self, column: Column, alias: str = "avg"):
-        self.column = column
+        if is_numeric_column(column):
+            self.column = column
+        else:
+            raise ValueError("Column type must be numeric to run the query.")
         self.alias = alias
 
     def as_alias(self, alias: str) -> Avg:
         self.alias = alias
         return self
 
     def get_select_string(self, engine_type: str, just_alias=False) -> str:
-        if self.column.value_type in (int, decimal.Decimal, float):
-            column_name = self.column._meta.get_full_name(
-                just_alias=just_alias
-            )
-        else:
-            raise ValueError("Column type must be numeric to run the query.")
+        column_name = self.column._meta.get_full_name(just_alias=just_alias)
         return f"AVG({column_name}) AS {self.alias}"
 
 
@@ -128,20 +130,18 @@ class Sum(Selectable):
     """
 
     def __init__(self, column: Column, alias: str = "sum"):
-        self.column = column
+        if is_numeric_column(column):
+            self.column = column
+        else:
+            raise ValueError("Column type must be numeric to run the query.")
         self.alias = alias
 
     def as_alias(self, alias: str) -> Sum:
         self.alias = alias
         return self
 
     def get_select_string(self, engine_type: str, just_alias=False) -> str:
-        if self.column.value_type in (int, decimal.Decimal, float):
-            column_name = self.column._meta.get_full_name(
-                just_alias=just_alias
-            )
-        else:
-            raise ValueError("Column type must be numeric to run the query.")
+        column_name = self.column._meta.get_full_name(just_alias=just_alias)
         return f"SUM({column_name}) AS {self.alias}"
 
 

tests/table/test_select.py

@@ -735,11 +735,11 @@ def test_chain_different_functions_alias(self):
         self.assertTrue(response["popularity_sum"] == 3010)
 
     def test_avg_validation(self):
-        with self.assertRaises(Exception):
+        with self.assertRaises(ValueError):
             Band.select(Avg(Band.name)).run_sync()
 
     def test_sum_validation(self):
-        with self.assertRaises(Exception):
+        with self.assertRaises(ValueError):
             Band.select(Sum(Band.name)).run_sync()
 
     def test_columns(self):