将元素插入字典列表

Question

I have declared an empty list and I want to append dictionaries through a for loop by using the extend() method, but I end with a list with the same content in all its positions (the same dictionary)

This is the code:

def get_tasks(self, actor, task_state):
    tasks = Task.objects.filter(actor=actor.id, state=task_state)
    tasks_to_return = []
    task_data = {}

    for task in tasks:
        task_data['name'] = getattr(task, 'name')
        task_data['description'] = getattr(task, 'description')
        task_data['start_date'] = getattr(task, 'start_date')
        task_data['finish_date'] = getattr(task, 'finish_date')
        task_data['situation'] = task.get_situation()
        tasks_to_return.extend(task_data)

    return tasks_to_return

If I change extend() for append() , the result is even worst.

Answer 1

That is because you each time add the same dictionary to the list: indeed you only change the key/values of that dictionary, and add that dictionary a second time, etc.

Furthermore by using extend , you actually add a iterable of elements to a list. A dictionary is an iterable : you iterate over the keys.

So what you should do is (1) construct a new dictionary each time, and (2) use append instead of extend:

for task in tasks:
    
    task_data['name'] = getattr(task, 'name')
    task_data['description'] = getattr(task, 'description')
    task_data['start_date'] = getattr(task, 'start_date')
    task_data['finish_date'] = getattr(task, 'finish_date')
    task_data['situation'] = task.get_situation()
    tasks_to_return.(task_data)

We can also improve the code, for instance instead of setting the key-value pairs, we can do this at the construction of the dictionary. You use getattr(..) , but this is not necessary here: we can use task.name instead of getattr(task, 'name') , so a more elegant approach is probably:

tasks_to_return = []
task_data = {}

for task in tasks:
    
    tasks_to_return.append(task_data)

Now since the list is initially empty, we can also use list comprehension :

[ {
        'name': task.name,
        'description': task.description,
        'start_date': task.start_date,
        'finish_date': task.finish_date,
        'situation': task.situation
    } 

Answer 2

Your extend or append adds the original reference to task_data , whose contents you keep changing. Instead, either make a new empty object for task_data , or append a copy of the one you reuse:

import copy
...
    tasks_to_return.append(copy.copy(task_data))