Python: How to query a list of objects?

Question

Let's say I have this list of objects:

listt = [{
      "CustomerId": "1",
      "Date": "2017-02-02",
      "Content": "AAAAAAAA",
      "Type": 2
    },
    {
      "CustomerId": "2",
      "Date": "2017-02-03",
      "Content": "BBBBBBBB",
      "Type": 1
    },
    {
      "CustomerId": "3",
      "Date": "2017-02-01",
      "Content": "CCCCCCCCC",
      "Type": 1
    },
    {
      "CustomerId": "4",
      "Date": "2017-02-12",
      "Content": "DDDDDDDDDD",
      "Type": 2
    }, ]

What's the cleanest way of finding answers to these?

1. Minimum Date where Type = 1.

=> 2017-02-1

2. Select Content where Type = 2 and Date = (Minimum Date among all objects with Type = 2)

=> AAAAAAAA

I'm reading about leveraging lambda and filters but I haven't been able to make any progress. What can I try next?

Answer 1

These are basic Python data structures. Rather than map and filter I would suggest using comprehensions. E.g.:

>>> listt = [{
...       "CustomerId": "1",
...       "Date": "2017-02-02",
...       "Content": "AAAAAAAA",
...       "Type": 2
...     },
...     {
...       "CustomerId": "2",
...       "Date": "2017-02-03",
...       "Content": "BBBBBBBB",
...       "Type": 1
...     },
...     {
...       "CustomerId": "3",
...       "Date": "2017-02-01",
...       "Content": "CCCCCCCCC",
...       "Type": 1
...     },
...     {
...       "CustomerId": "4",
...       "Date": "2017-02-12",
...       "Content": "DDDDDDDDDD",
...       "Type": 2
...     }, ]
>>> min(d['Date'] for d in listt if d['Type'] == 1)
'2017-02-01'
>>>

Or, for you second query:

>>> min_date = min(d['Date'] for d in listt if d['Type'] == 2)
>>> [d['Content'] for d in listt if d['Date'] == min_date]
['AAAAAAAA']
>>>

Trying to stick to comprehension constructs keeps things more readable, IMO, rather than using lambda, although, that also has its place and is rather a matter of style. However, list-comprehensions are faster in general than equivalent map with lambda. However, map can be faster with built-in functions.

Answer 2

For finding the minimum date with type=1, you may firstly filter the list on type=1 and then pass the filtered list to min function (with key as lambda x: x['Date'] to find element with minimum 'Date') as:

#                        performs `min` operation on `'Date'` v
>>> min([d for d in listt if d['Type'] ==1], key=lambda x: x['Date'])
{'CustomerId': '3', 'Type': 1, 'Content': 'CCCCCCCCC', 'Date': '2017-02-01'}

This is a dict object having the minimum Date in the list. Let's say it is stored as variable my_dict. In order to find the date, do:

my_dict['Date']

For finding the content associated to it, do:

my_dict['Content']

Note: For finding the content of Type=2, replace d['Type'] ==1 with d['Type'] ==2 in the min statement.