Merge two list of dictionaries with same key:value in python

Question

Good eveneing, I want to merge two lists of dictonaries if they have the same key:value in it. Its like a join in SQL. I'm not allowed to import any modules for this problem.

Here an example:

Input:

>>> series = [
... {'s_id': 'bb', 'title': 'Breaking Bad'},
... {'s_id': 'bcs', 'title': 'Better Call Saul'}]

>>> series_characters = [
... {'c_id': 'ww', 's_id': 'bb'},
... {'c_id': 'sw', 's_id': 'bb'}, 
... {'c_id': 'sg', 's_id': 'bb'}
... {'c_id': 'sg', 's_id': 'bcs'}

Output should be a list of dicts with both infomration inside it:

out= [
{'s_id': 'bb', 'title': 'Breaking Bad', 'c_id': 'ww'},
{'s_id': 'bcs', 'title': 'Better Call Saul', 'c_id': 'sg'}]

I tried somthing like that but I think that my thoughts are to complicated and the code doesen't work:

def _join(tbl1, tbl2):
    """
    Helping function to merge two tables inform of a list

    Argurments:
        tbl1 (list): list of dict's containung a table
        tbl2 (list): list of dict's containung a table
    Returns:
        back_lst (list): list, containing wanted rows of the table

    """
    back_lst = []
    for element1 in tbl1:
        for element2 in tbl2:
            for key in tbl1[element1]:
                if tbl1[element1[key]] == tbl2[element2[key]]:
                    a = tbl1[element1].copy()
                    a.update(tbl2[element2])
                    back_lst.append(a)
    return back_lst

It would be nice to get some help here. Thanks in advance.

in series_charachter for s_id bb, the c_id is ww ,sw, sg then why did it only merge with sg not other? — harshil9968
– harshil9968, Commented Dec 5, 2016 at 18:39
@harshil9968 good point, also it should be noted what if there are more than one common keys e.g. in SQL where tbl1.name=tbl2.name and tbl1.id=tbl2.id. — sonus21
– sonus21, Commented Dec 5, 2016 at 18:47

AChampion · Accepted Answer · 2016-12-05 19:01:30Z

3

Given all of your keys are just strings you can do:

>>> [dict(a, **b) for a in series_characters for b in series if a['s_id'] == b['s_id']]
[{'c_id': 'ww', 's_id': 'bb', 'title': 'Breaking Bad'},
 {'c_id': 'sw', 's_id': 'bb', 'title': 'Breaking Bad'},
 {'c_id': 'sg', 's_id': 'bb', 'title': 'Breaking Bad'},
 {'c_id': 'sg', 's_id': 'bcs', 'title': 'Better Call Saul'}]

answered Dec 5, 2016 at 19:01

AChampion

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Comments

user3030010 · Accepted Answer · 2016-12-05 18:45:06Z

0

You don't want to be doing tbl1[element1], assuming that tbl1 is a list of dicts. element1 should already be the dict you want, so just do element1.copy(). Same for indexing tbl2.

answered Dec 5, 2016 at 18:45

user3030010

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sonus21 · Accepted Answer · 2016-12-05 19:07:51Z

# your code goes here

def joinTable( tbl1, tbl2 ):
    op = []
    if len( tbl1 )  > 0 and len( tbl2 ) > 0:
        keys = set( tbl1[0].keys()).intersection( tbl2[0].keys())
        opkeys = set( tbl1[0].keys()).union( tbl2[0].keys())
        for row1 in tbl1:
            key1 = set( row1.keys())
            for row2 in tbl2:
                key2 = set( row2.keys())
                assert key1.intersection( key2 ) == keys
                assert key1.union( key2 ) == opkeys
                match = True
                for key in keys:
                    match = row1[ key] == row2[key]
                if match:
                    d = dict()
                    for key in opkeys:
                        d[ key ] = row1[ key ]  if key in row1 else row2[key]
                    op.append( d )
    return op

print joinTable( [{'s_id': 'bb', 'title': 'Breaking Bad'},{'s_id': 'bcs', 'title': 'Better Call Saul'}],[
{'c_id': 'ww', 's_id': 'bb'},{'c_id': 'sw', 's_id': 'bb'}, {'c_id': 'sg', 's_id': 'bb'},{'c_id': 'sg', 's_id': 'bcs'}])

Ideone

It implements the join algorithm of SQL. Something similar to https://blogs.msdn.microsoft.com/craigfr/2006/08/03/merge-join/ .

You can further extends this to like particular key and restrict value as well.

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