Python 字典列表到新字典或列表

Question

Pardon the noob question, I am new to Python.

I am working with a list of dictionaries:

dataset = [
    {'id':1,'dateCollected':'2021-03-02','orders':7},
    {'id':2,'dateCollected':'2021-03-03','orders':8},
]

.... this goes on for 50 records....

I would like to a make a for loop that iterates through every dictionary in the list and adds certain key-value pairs to a new dictionary.

For Example:

match_history_data = {}

for i in dataset:
    match_history_data['DateCollected'] = i['dateCollected']
    match_history_data['Orders'] = i['orders']

print(match_history_data)

results in:

{'DateCollected': '2021-03-03', 'Orders': 8}

I would like the result to be:

{'DateCollected':'2021-03-02','Orders':7},
{'DateCollected':'2021-03-03','Orders':8}

This works how I want it to, but only for the first iteration in the for loop. How do I get around this so that it goes thru all the records in 'dataset', and creates the new list or dictionary?

Answer 1

You're creating a new dictionary, match_history_data and then just setting the same entries 'DateCollected' and 'Orders' over and over again. Each iteration of your for loop is just overwriting the same entries.

What you want is a list of new dictionaries:

match_history_data = []

for item in dataset:
    match_history_data.append(
            {"DateCollected" : item["dateCollected"],
             "Orders" : item["orders"]})

You can achieve this in a neater fashion using a list comprehension:

match_history_data = [{"DateCollected" : item["dateCollected"], "Orders" : item["orders"]} for item in dataset]

Answer 2

How about this

match_history_data = [{"DateCollected":x["dateCollected"], "Orders":x["orders"]} for x in dataset]

Answer 3

What you have right now overwrites a single dictionary as your loop through the original list of dictionaries, since keys are unique in dictionaries. What you need to is another list to which you can append the new dictionaries during each iteration to a separate list.

For example

new_list = []

for i in dataset:
    match_history_data = {}
    match_history_data['DateCollected'] = i['dateCollected']
    match_history_data['Orders'] = i['orders']
    new_list.append(match_history_data)

If you don't want to preserve the original list, you can simply drop the id key in each dictionary.