python - list of dictionaries check key:value

Question

UPDATE: Just to be clear, I'd like to check the key-value of the 'name' and 'last' and add only if these are not already in the list.

I have:

lst = [{'name':'John', 'last':'Smith'.... .... (other key-values)... }, 
{'name':'Will', 'last':'Smith'... ... (other key-values)... }]

I want to append aa new dict into this list only if it is not the exact same as an existing dictionary.

In other words:

dict1 = {'name':'John', 'last':'Smith'} # ==> wouldn't be appended

but...

dict2 = {'name':'John', 'last':'Brown'} # ==> WOULD be appended

Could someone explain the simplest way to do this, as well as in English, what is happening in the solution. THANKS!

Reference: Python: Check if any list element is a key in a dictionary

Answer 1

Since you asked for a way to only check the two keys, even if the dicts have other keys in them:

name_pairs = set((i['name'], i['last']) for i in lst)
if (d['name'], d['last']) not in name_pairs:
    lst.append(d)

Answer 2

You can do it with this list comprehension just append everything to your list and run this:

lst.append(dict1)
lst.append(dict2)
[dict(y) for y in set(tuple(x.items()) for x in lst)]

The output is:

[
    {'last': 'Smith', 'name': 'John'},
    {'last': 'Brown', 'name': 'John'},
    {'last': 'Smith', 'name': 'Will'}
]

With this method you can add extra fields and it will still work.

Answer 3

You could also write a small method to do it and return the list

def update_if_not_exist(lst, val):
    if len([d for d in lst if (d['name'], d['last']) == (val['name'], val['last'])]) == 0:
        lst.append(val)
    return lst

lst = update_if_not_exist(lst, dict1)
lst = update_if_not_exist(lst, dict2)

It works by filtering the original list to matching the name and last keys and seeing if the result is empty.

Answer 4

>>> class Person(dict):
...     def __eq__(self, other):
...         return (self['first'] == other['first'] and
...                 self['second'] == other['second'])
...     def __hash__(self):
...         return hash((self['first'], self['second']))

>>> l = [{'first': 'John', 'second': 'Smith', 'age': 23},
...         {'first': 'John', 'second': 'Smith', 'age': 30},
...         {'first': 'Ann', 'second': 'Rice', 'age': 31}]

>>> l = set(map(Person, l))
>>> print l
set([{'first': 'Ann', 'second': 'Rice', 'age': 31},
    {'first': 'John', 'second': 'Smith', 'age': 23}])

Instance of the Person class can be used as simple dict.