Python List Comprehensions in Dict List

Question

I have a list that contains dicts like this:

[
    {'match_id': 1L, 'player_b_id': 2L, 'round_id': 1L, 'match_winner_id': 2L, 'match_date': datetime.date(2016, 3, 9), 'player_a_id': 1L, 'tournament_id': 1L}, 
    {'match_id': 2L, 'player_b_id': 4L, 'round_id': 1L, 'match_winner_id': 4L, 'match_date': datetime.date(2016, 3, 10), 'player_a_id': 3L, 'tournament_id': 1L}
]

and I'm wondering how can I modify each element's 'match_date' to a string date. I know this can be accomplished using a regular loop but I'm trying to learn about list comprehensions and got stuck in this part.

I tried something like this without success:

matchesData = [str(v) if k == 'match_date' else v for k, v in enumerate(matchesData)]

Answer 1

If you must use a list comprehension, then you must also rebuild each dictionary:

matchesData = [{k: str(v) if k == 'match_date' else v for k, v in d.items()} 
               for d in matchesData]

or use

matchesData = [dict(d, match_date=str(d['match_date']))
               for d in matchesData]

The first example uses a dictionary comprehension, that simply creates a new dictionary from the old by looping over each key-value pair. For the 'match_date' key the values is passed through the str() function. This means the match_date key is entirely optional.

The second version requires 'match_date' to exist, and creates a copy of the original dictionary with the dict() function , adding in an extra key (which will replace the original 'match_date' key-value pair from d ).

Demo:

>>> from pprint import pprint
>>> matchesData = [
...     {'match_id': 1L, 'player_b_id': 2L, 'round_id': 1L, 'match_winner_id': 2L, 'match_date': datetime.date(2016, 3, 9), 'player_a_id': 1L, 'tournament_id': 1L},
...     {'match_id': 2L, 'player_b_id': 4L, 'round_id': 1L, 'match_winner_id': 4L, 'match_date': datetime.date(2016, 3, 10), 'player_a_id': 3L, 'tournament_id': 1L}
... ]
>>> pprint([{k: str(v) if k == 'match_date' else v for k, v in d.items()} for d in matchesData])
[{'match_date': '2016-03-09',
  'match_id': 1L,
  'match_winner_id': 2L,
  'player_a_id': 1L,
  'player_b_id': 2L,
  'round_id': 1L,
  'tournament_id': 1L},
 {'match_date': '2016-03-10',
  'match_id': 2L,
  'match_winner_id': 4L,
  'player_a_id': 3L,
  'player_b_id': 4L,
  'round_id': 1L,
  'tournament_id': 1L}]
>>> pprint([dict(d, match_date=str(d['match_date'])) for d in matchesData])
[{'match_date': '2016-03-09',
  'match_id': 1L,
  'match_winner_id': 2L,
  'player_a_id': 1L,
  'player_b_id': 2L,
  'round_id': 1L,
  'tournament_id': 1L},
 {'match_date': '2016-03-10',
  'match_id': 2L,
  'match_winner_id': 4L,
  'player_a_id': 3L,
  'player_b_id': 4L,
  'round_id': 1L,
  'tournament_id': 1L}]