Python: Merge two list of diferent objects by attribute

Question

I’m trying to find an efficient way to merge two list of python objects (classes) with diferent structures and merge them into a new list of new object. The code:

from datetime import datetime


class StructureOne(object):
    def __init__(self, date_time: datetime, name: str):
        self.date_time: datetime = date_time
        self.name: str = name
        self.value = None

    def set_value(self,value:float):
        self.value = value


class StructureTwo(object):
    def __init__(self, date_time: datetime, value: float):
        self.date_time = date_time
        self.value: float = value


def merge_lists(list_one: list[StructureOne], list_two: list[StructureTwo]) -> list[StructureOne]:
    for element_one in list_one:
        i = 0
        while i < len(list_two) and element_one.value is None:
            if element_one.date_time == list_two[i].date_time:
                element_one.set_value(value=list_two[i].value)
            i += 1
    return list_one


list_one: list[StructureOne] = [
    StructureOne(date_time=datetime(2022, 1, 1, 0), name='zero'),
    StructureOne(date_time=datetime(2022, 1, 1, 1), name='one'),
    StructureOne(date_time=datetime(2022, 1, 1, 2), name='two'),
    StructureOne(date_time=datetime(2022, 1, 1, 3), name='three'),
]

list_two: list[StructureTwo] = [
    StructureTwo(date_time=datetime(2022, 1, 1, 0), value=0),
    StructureTwo(date_time=datetime(2022, 1, 1, 1), value=1),
    StructureTwo(date_time=datetime(2022, 1, 1, 2), value=2),
    StructureTwo(date_time=datetime(2022, 1, 1, 3), value=3),
]


merged_list: list[StructureOne] = merge_lists(list_one=list_one, list_two=list_two)

The desired result is


list_one: list[StructureOne] = [
    StructureOne(date_time=datetime(2022, 1, 1, 0), name='zero', value=0),
    StructureOne(date_time=datetime(2022, 1, 1, 1), name='one', value=1),
    StructureOne(date_time=datetime(2022, 1, 1, 2), name='two', value=2),
    StructureOne(date_time=datetime(2022, 1, 1, 3), name='three', value=3),
]

We are trying to not use external libraries like py-linq.

Asked By: Anabel Jesús Lorenzo

||

Source

Answer 1

Have you tried py_linq or some other library?

Thanks

Answered By: Guillem Peñafiel i Ràmia

Answer 2

Assumptions:

The lists might not be sorted.
The date_time field is unique within each list.
It is okey if the output is sorted.
You want to populate the value of the objects in list_one with the value from the objects in list_two on matching date_time.
You only want to populate the value of an object in list_one if it is None.
In your code, you only populate the value if it is not None; I assume that you want to do the opposite.

def merge_lists(list_one: list[StructureOne], list_two: list[StructureTwo]) -> list[StructureOne]:

    # sort lists by date_time
    list_1 = sorted(list_one, key=lambda x: x.date_time)
    list_2 = sorted(list_two, key=lambda x: x.date_time)

    # merge sort
    i, j = 0, 0
    while i < len(list_1) and j < len(list_2):
        if list_1[i].date_time < list_2[j].date_time:
            i += 1
        elif list_1[i].date_time > list_2[j].date_time:
            j += 1
        else:
            # date_time matches
            # populate value from list_two into list_one if value is None
            if list_1[i].value is None:
                list_1[i].value = list_2[j].value
            i += 1
            j += 1

    return list_1

Answered By: Filip

Answer 3

If all you want is to change the value attribute of StructureOne to the value of StructureTwo in the order of each list then you can do this to modify the elements in list_one:

for elem_one, elem_two in zip(list_one,list_two):
    elem_one.value = elem_two.value

You don’t even need to have the value attribute in StructureOne, it will work the same:

class StructureOne(object):
    def __init__(self, date_time: datetime, name: str):
        self.date_time: datetime = date_time
        self.name: str = name

If by merging you mean to get rid of list_two you can delete the variable so it will get discarded.

del list_two

Printing the elements after the merge will have your value members updated:

for e in list_one:
    print(e.date_time,e.name,e.value)

# Output:

2022-01-01 00:00:00 zero 0
2022-01-01 01:00:00 one 1
2022-01-01 02:00:00 two 2
2022-01-01 03:00:00 three 3

Your __init__ function has a lot of duplication in names, that’s why it can be a good idea to use dataclasses, it will work the same, but it’s easier to read and maintain:

from dataclasses import dataclass

@dataclass
class StructureOne:
    date_time : datetime.datetime
    name : str

@dataclass
class StructureTwo:
    date_time : datetime.datetime
    value : float

Answered By: Gábor Fekete

Python: Merge two list of diferent objects by attribute

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