Iterate over a list of dictionaries and get value from previous dictionary

Question

I have a list of dictionaries:

history_changes = [
{'userName': 'recg', 'modificationDate': '2022-07-14T04:01:39+00:00', 'changeComment': 'more details about L2'},
{'userName': 'artf', 'modificationDate': '2022-07-15T04:01:39+00:00', 'changeComment': 'more details about L1'},
{'userName': 'zrt',  'modificationDate': '2022-07-16T04:01:39+00:00', 'changeComment': 'more details about L3'}]

I’m iterating over it and trying to insert data in a database.

for record in history_changes[1:]:
   comment = record['changeComment']
   to_time = record['modificationDate']

I also need the "start_time" information which is the ‘modificationDate’ from the previous dictionary.

Expected output:

 comment = 'more details about L1'
 start_time = '2022-07-14T04:01:39+00:00'
 to_time = '2022-07-15T04:01:39+00:00'

 comment = 'more details about L3'
 start_time = '2022-07-15T04:01:39+00:00'
 to_time = '2022-07-16T04:01:39+00:00'

How can I get the 'modificationDate' from the previous dictionary when iterating over the list?

Asked By: coLby

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Source

Answer 1

history_changes = [
{'userName': 'recg', 'modificationDate': '2022-07-14T04:01:39+00:00', 'changeComment': 'more details about L2'},
{'userName': 'artf', 'modificationDate': '2022-07-15T04:01:39+00:00', 'changeComment': 'more details about L1'},
{'username': 'zrt',  'modificationDate': '2022-07-16T04:01:39+00:00', 'changeComment': 'more details about L3'}]


for x in range(1,len(history_changes)):  # see here I am starting from 1 index 
    print(history_changes[x]['changeComment'])
    print(history_changes[x-1]['modificationDate'])
    print(history_changes[x]['modificationDate'])

#output
more details about L1
2022-07-14T04:01:39+00:00
2022-07-15T04:01:39+00:00
more details about L3
2022-07-15T04:01:39+00:00
2022-07-16T04:01:39+00:00

If you want to store these value then:

comment=[]
start_time=[]
to_time=[]
for x in range(1,len(history_changes)):
    comment.append(history_changes[x]['changeComment'])
    start_time.append(history_changes[x-1]['modificationDate'])
    to_time.append(history_changes[x]['modificationDate'])

Answered By: God Is One

Answer 2

In Python, when you have a list of n objects, say x, a common pattern to operate on the n-1 consecutive pairs of objects is:

[do_something(a, b) for a, b in zip(x, x[1:])]

If you were to look for the n-2 consecutive triplets, you would use:

[do_something(a, b, c) for a, b, c in zip(x, x[1:], x[2:])]

Adapted to your problem, you could use e.g.:

new_records = [
    dict(
        comment=record['changeComment'],
        start_time=prev['modificationDate'],
        to_time=record['modificationDate'],
    ) for prev, record in zip(history_changes, history_changes[1:])
]

>>> new_records
[{'comment': 'more details about L1',
  'start_time': '2022-07-14T04:01:39+00:00',
  'to_time': '2022-07-15T04:01:39+00:00'},
 {'comment': 'more details about L3',
  'start_time': '2022-07-15T04:01:39+00:00',
  'to_time': '2022-07-16T04:01:39+00:00'}]

Or, to operate directly on the augmented records (e.g. to insert them in a DB one by one instead of in batches):

for prev, record in zip(history_changes, history_changes[1:]):
    comment = record['changeComment']
    start_time = prev['modificationDate']
    to_time = record['modificationDate']
    out = dict(comment=comment, start_time=start_time, to_time=to_time)
    print('n'.join([f'{k:<10}= {v}' for k, v in out.items()]))

Which prints out:

comment   = more details about L1
start_time= 2022-07-14T04:01:39+00:00
to_time   = 2022-07-15T04:01:39+00:00
comment   = more details about L3
start_time= 2022-07-15T04:01:39+00:00
to_time   = 2022-07-16T04:01:39+00:00

Answered By: Pierre D

Iterate over a list of dictionaries and get value from previous dictionary

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