Update Dict in List Comprehensions

Question

I have some dictionaries that I need to fill recursively and for that I would like to use list comprehensions this way

list_dict = [{'a':'b'},{'a':'c'}]

list_dict = [dict.update({'b':dict['a']}) for dict in list_dict]

The goal was to work with datetime where I would add a value to each dictionary.

The problem is that when using Dict.update() it returns None instead of the dictionary value which returns more or less this:

[None,None]

I also tried to use a map() in this way:

list(map(lambda dict: dict.update({'novo_valor':100}),list_dict))

But I got the same return.

I tried some other things like using and inside the list comprehension but without success.

Does anyone have any ideas that might help.

Practical example:

MAX_DAYS_TO_INACTIVE = 15
MAX_DAYS_TO_EXPIRATION = 30
Entrada = {
        'UserName':'user.1',
        'AccessKeyId':'CCCCCCCCCCCCCCCCCCCC',
        'Status':'Inactive',
     'CreateDate':datetime.datetime(2019,9,10,20,12,45,tzinfo=tzutc())
    },
    {
        'UserName':'user.2',
        'AccessKeyId':'BBBBBBBBBBBBBBBBBBBB',
        'Status':'Active',
      'CreateDate':datetime.datetime(2020,4,16,1,26,18,tzinfo=tzutc())
    }
]

Function I created:

def add_date_paramters(complete_access_key_list):
    key_list = list()
    for acess_key in complete_access_key_list:
        acess_key.update(
            {
                'InvalidationDate':acess_key['CreateDate']+datetime.timedelta(days=MAX_DAYS_TO_INACTIVE),
                'ExpirationDate':acess_key['CreateDate']+datetime.timedelta(days=MAX_DAYS_TO_EXPIRATION),
            }
        )
        key_list.append(acess_key)
    return key_list

I hope I have something like this:

saida = [ {
        'UserName':'user.1',
        'AccessKeyId':'CCCCCCCCCCCCCCCCCCCC',
        'Status':'Inactive',
        'CreateDate':datetime.datetime(2019,9,10,20,12,45,tzinfo=tzutc()),
        'InvalidationDate':datetime.datetime(2019, 9, 25, 20, 12, 45, tzinfo=tzutc()),
        'ExpirationDate':datetime.datetime(2019, 10, 10, 20, 12, 45, tzinfo=tzutc())
    },
    {
        'UserName':'user.2',
        'AccessKeyId':'BBBBBBBBBBBBBBBBBBBB',
        'Status':'Active',
        'CreateDate':datetime.datetime(2020,4,16,1,26,18,tzinfo=tzutc()),
        'InvalidationDate':datetime.datetime(2020, 5, 1, 1, 26, 18, tzinfo=tzutc()),
        'ExpirationDate':datetime.datetime(2020, 5, 16, 1, 26, 18, tzinfo=tzutc())
    }, ]

Answer 1

Basic solution in which the list of dictionaries is updated in-place:

from dateutil.tz import tzutc
from datetime import timedelta, datetime

MAX_DAYS_TO_INACTIVE = 15
MAX_DAYS_TO_EXPIRATION = 30

list_dict = [
    {
        'UserName'   : 'user.1',
        'AccessKeyId': 'CCCCCCCCCCCCCCCCCCCC',
        'Status'     : 'Inactive',
        'CreateDate' : datetime(2019,9,10,20,12,45,tzinfo=tzutc())
    },
    {
        'UserName'   : 'user.2',
        'AccessKeyId': 'BBBBBBBBBBBBBBBBBBBB',
        'Status'     : 'Active',
        'CreateDate' : datetime(2020,4,16,1,26,18,tzinfo=tzutc())
    }
]

for dic in list_dict:
    dic['InvalidationDate'] = dic['CreateDate'] + timedelta(days=MAX_DAYS_TO_INACTIVE)
    dic['ExpirationDate'] = dic['CreateDate'] + timedelta(days=MAX_DAYS_TO_EXPIRATION)

print(list_dict)

Exit:

[
  {
    'UserName': 'user.1',
    'AccessKeyId': 'CCCCCCCCCCCCCCCCCCCC',
    'Status': 'Inactive',
    'CreateDate': datetime.datetime(2019, 9, 10, 20, 12, 45, tzinfo=tzutc()),
    'InvalidationDate': datetime.datetime(2019, 9, 25, 20, 12, 45, tzinfo=tzutc()),
    'ExpirationDate': datetime.datetime(2019, 10, 10, 20, 12, 45, tzinfo=tzutc())
  },
  {
    'UserName': 'user.2',
    'AccessKeyId': 'BBBBBBBBBBBBBBBBBBBB',
    'Status': 'Active',
    'CreateDate': datetime.datetime(2020, 4, 16, 1, 26, 18, tzinfo=tzutc()),
    'InvalidationDate': datetime.datetime(2020, 5, 1, 1, 26, 18, tzinfo=tzutc()),
    'ExpirationDate': datetime.datetime(2020, 5, 16, 1, 26, 18, tzinfo=tzutc())
  }
]

See working on Repl.it

Encapsulating in a function:

def add_date_parameters(lst):
    for dic in lst:
        dic.update({
            'InvalidationDate': dic['CreateDate'] + timedelta(days=MAX_DAYS_TO_INACTIVE),
            'ExpirationDate'  : dic['CreateDate'] + timedelta(days=MAX_DAYS_TO_EXPIRATION)
        })
    return lst

Answer 2

I called a friend and explained the case.

He suggested me use the operated or when the dict.update() return None the expression consider the return as what comes after:

solution:

def add_date_paramters(complete_access_key_list):
    return map( lambda acess_key: acess_key.update(
        {
            'InvalidationDate':acess_key['CreateDate']+datetime.timedelta(days=MAX_DAYS_TO_INACTIVE),
            'ExpirationDate':acess_key['CreateDate']+datetime.timedelta(days=MAX_DAYS_TO_EXPIRATION)
        }
    ) or acess_key ,complete_access_key_list)

My mistake was trying to use and which ended up being fake on the first check and didn’t return anything. Thank you for your attention.