无法将其他字典列表中的字典附加到字典列表中

Question

Been Googling hours again for answer but seems like my situation is unique, or my understanding of Py is...uh, unique.

I have a list of dictionaries (let's call it sqlListOfDict) built from MySQLdb.cursors.DictCursor query, Py print yields this:

[{'volYtday': 18083292,
  'volToday': 49395261,
  'time': datetime.timedelta(0, 34800),
  'priceChgPct': Decimal('1.2400'),
  'volDiffPct': 173.15414140301445,
  'priceNom': Decimal('6.5300'),
  'date': datetime.date(2013, 4, 30),
  'secCode': 'xoxoxo',
  'volDiff': 31311969}]

Then I have another list of dictionaries (let's call it myListOfDict), declared empty, Py print yields this:

[{'priceChgPct': '',
  'volYtday': '',
  'priceNom': '',
  'volDiffPct': '',
  'volToday': '',
  'time': '',
  'date': '',
  'secCode': '',
  'volDiff': ''}]

I need to append myListOfDict with a specific dictionary from sqlListOfDict by index number, the code:

myListOfDict.append(sqlListOfDict[0])

No joy, myListOfDict is still as declared (empty) and Py doesn't give any traceback.

I did some experiments:

listDictA = [dict.fromkeys(['secCode', 'volDiff'])]

listDictB = [{'secCode': 'valB1', 'volDiff': 1000},
             {'secCode': 'valB2', 'volDiff': 1000},
             {'secCode': 'valB3', 'volDiff': 1000},
             {'secCode': 'valB4', 'volDiff': 1000},
             {'secCode': 'valB5', 'volDiff': 1000},
             {'secCode': 'valA1', 'volDiff': 1000}]

Ran the same command:

listDictA.append(listDictB[0])

It worked! What did I do wrong for the first case? Thanks for helping.

Answer 1

It sounds like you're expecting that the first item in myListOfDict is filled up with values from the first entry in sqlListOfDict .

You say that myListOfDict is declared empty , but the fact is that you don't need that kind of declaration. In Python, you can append whatever objects you like to a list, without any declaration of what types go into it.

I'll simplify your example a bit, and hope that clears it up:

>>> myListOfDict = []  # create an empty list
>>> sqlListOfDict = [{'volYtday': 18083292,
                      'volToday': 49395261}]
>>> myListOfDict.append(sqlListOfDict[0])
>>> myListOfDict
[{'volYtday': 18083292, 'volToday': 49395261}]

Also, if you're dealing with two lists, you might want to consider extending the first list with the second, instead of append only the first item.

>>> myListOfDict.extend(sqlListOfDict)