POC doit modifications to handle non-serializable python-action results

dodo.py

'''
To use:

touch testfile
doit

Serialized .doit.db database (JSON format) will contain:

{
  "create_object": {
    "_values_:": {
      "obj": null
    },
    "result:": {
      "obj": null
    },
    "checker:": "MD5Checker",
    "deps:": [
      "testfile"
    ],
    "testfile": [
      1557929656.729679,
      0,
      "d41d8cd98f00b204e9800998ecf8427e"
    ]
  },
  "null": {
    "_values_:": {
      "null": null
    },
    "result:": {
      "null": null
    },
    "checker:": "MD5Checker",
    "deps:": []
  },
  "use_object": {
    "_values_:": {
      "_result:null": {
        "null": null
      },
      "_result:create_object": {
        "obj": null
      }
    },
    "checker:": "MD5Checker",
    "deps:": []
  }
}

'''

import random


def task_create_object():
    'Create an instance of Test()'

    class Test:
        a = random.randint(0, 100)

    def instantiate():
        return {'obj': Test()}

    # uptodate: this task can never be uptodate since the result is not serializable
    # and the cached value will always be null
    return {'actions': [instantiate], 'file_dep': ['testfile'], 'uptodate': [False]}


def task_use_object():
    'Display Test().a'

    def use_object(obj):
        assert obj is not None, 'Did not receive expected Test() instance'
        print(f'Value: {obj.a}')

    return {
        'actions': [use_object],
        'getargs': {'obj': ('create_object', 'obj')},
        'verbosity': 2,
    }

poc_modifications.diff

diff --git a/doit/dependency.py b/doit/dependency.py
index 8497314..0597137 100644
--- a/doit/dependency.py
+++ b/doit/dependency.py
@@ -18,6 +18,13 @@ import dbm as ddbm
 
 import json
 
+class SkipEncoder(json.JSONEncoder):
+    def default(self, obj):
+        if type(obj) not in [str, int, float, bool, None]:
+            print(f'Serializing {type(obj)} as None')
+            return None
+        else:
+            return obj
 
 class DatabaseException(Exception):
     """Exception class for whatever backend exception"""
@@ -80,7 +87,7 @@ class JsonDB(object):
         """save DB content in file"""
         try:
             db_file = open(self.name, 'w')
-            json.dump(self._db, db_file)
+            json.dump(self._db, db_file, cls=SkipEncoder)
         finally:
             db_file.close()
 
@@ -159,7 +166,8 @@ class DbmDB(object):
     def dump(self):
         """save/close DBM file"""
         for task_id in self.dirty:
-            self._dbm[task_id] = json.dumps(self._db[task_id])
+            self._dbm[task_id] = json.dumps(self._db[task_id], cls=SkipEncoder)
+            pass
         self._dbm.close()