Identify tasks in configuration serialization errors

CHANGES

@@ -13,6 +13,8 @@ Changes
 0.38.0 (*unreleased*)
 ======================
 
+- fix #362: identify the task when ``config_changed`` cannot JSON-serialize
+  its configuration, preserving the original error as the exception cause.
 - site/docs: landing page is now generated by sphinx (``doc/index.rst`` using ``sphinx-design``),
   replacing the hand-written ``doc/index.html`` and its vendored
   bootstrap / font-awesome / owl-carousel assets.

doit/tools.py

@@ -73,7 +73,12 @@ def configure_task(self, task):
 
     def __call__(self, task, values):
         """return True if config values are UNCHANGED"""
-        self.config_digest = self._calc_digest()
+        try:
+            self.config_digest = self._calc_digest()
+        except TypeError as error:
+            msg = (f"Task '{task.name}': config_changed could not serialize "
+                   f"configuration: {error}")
+            raise TypeError(msg) from error
         last_success = values.get('_config_changed')
         if last_success is None:
             return False

tests/test_tools.py

@@ -1,6 +1,7 @@
 import os
 import datetime
 import json
+from pathlib import Path
 import operator
 import unittest
 from sys import executable
@@ -55,6 +56,20 @@ class NotValid(object): pass
         uptodate = tools.config_changed(NotValid())
         self.assertRaises(Exception, uptodate, None, None)
 
+    def test_unserializable_config_identifies_task(self):
+        for config in ({'source': Path('input')},
+                       {'nested': {'source': Path('input')}},
+                       {Path('input'): 'value'}):
+            with self.subTest(config=config):
+                check = tools.config_changed(config)
+                current_task = task.Task('build:assets', None, uptodate=[check])
+                with self.assertRaisesRegex(
+                        TypeError, "build:assets.*config_changed") as raised:
+                    check(current_task, {})
+                self.assertIsInstance(raised.exception.__cause__, TypeError)
+                self.assertIn('Path', str(raised.exception.__cause__))
+                self.assertIsNone(check.config_digest)
+
     def test_string(self):
         ua = tools.config_changed('a')
         ub = tools.config_changed('b')