Teach json how to serialize decimals

src/pgmon.py

@@ -27,6 +27,8 @@ from urllib.parse import urlparse, parse_qs
 import requests
 import re
 
+from decimal import Decimal
+
 VERSION = "1.0.3"
 
 # Configuration
@@ -391,6 +393,16 @@ def get_query(metric, version):
     raise MetricVersionError("Missing metric query for PostgreSQL {}".format(version))
 
 
+def json_encode_special(obj):
+    """
+    Encoder function to handle types the standard JSON package doesn't know what
+    to do with
+    """
+    if isinstance(obj, Decimal):
+        return float(obj)
+    raise TypeError(f'Cannot serialize object of {type(obj)}')
+
+
 def run_query_no_retry(pool, return_type, query, args):
     """
     Run the query with no explicit retry code
@@ -408,11 +420,11 @@ def run_query_no_retry(pool, return_type, query, args):
                 elif return_type == "row":
                     if len(res) == 0:
                         return "[]"
-                    return json.dumps(res[0])
+                    return json.dumps(res[0], default=json_encode_special)
                 elif return_type == "column":
                     if len(res) == 0:
                         return "[]"
-                    return json.dumps([list(r.values())[0] for r in res])
+                    return json.dumps([list(r.values())[0] for r in res], default=json_encode_special)
                 elif return_type == "set":
                     return json.dumps(res)
         except:

src/test_pgmon.py

@@ -5,6 +5,8 @@ import tempfile
 
 import logging
 
+from decimal import Decimal
+
 import pgmon
 
 # Silence most logging output
@@ -789,3 +791,17 @@ metrics:
         # Make sure we can pull the RSS file (we assume the 9.6 series won't be getting
         # any more updates)
         self.assertEqual(pgmon.get_latest_version(), 90624)
+
+    def test_json_encode_special(self):
+        # Confirm that we're getting the right type
+        self.assertFalse(isinstance(Decimal('0.5'), float))
+        self.assertTrue(isinstance(pgmon.json_encode_special(Decimal('0.5')), float))
+
+        # Make sure we get sane values
+        self.assertEqual(pgmon.json_encode_special(Decimal('0.5')), 0.5)
+        self.assertEqual(pgmon.json_encode_special(Decimal('12')), 12.0)
+
+        # Make sure we can still fail for other types
+        self.assertRaises(
+            TypeError, pgmon.json_encode_special, object
+        )