How to omit key and value when creating list of dictionaries using comprehension

Question

I am creating a list of dictionaries from a separate list of dictionaries and transforming the content. I would like to conditionally exclude keys during the comprehension, but I'm not sure how. Here's an example of what I'm trying to do:

old_dict_list = [
    { 'old_key_1': 'value1a', 'old_key_2': 'value2a', 'old_key_3': 'value3a'},
    { 'old_key_1': 'value1b', 'old_key_2': 'value2b', 'old_key_3': 'value3b'},
]

new_dict_list = [
    {
        'new_key1': old_dict['old_key_1'],
        'new_key2': old_dict['old_key_2'], # exclude this key entirely if some condition is met
        'new_key3': old_dict['old_key_3'],
    }
    for old_dict in old_dict_list
]

print(new_dict_list)

I don't know of any way to conditionally exclude a key entirely when using comprehension like this. The only idea I've had is to use a fixed key value like None when the condition is met, then make a separate pass through the list of dictionaries and remove those None keys.

Is there any way to do this in just the one comprehension pass?

Answer 1

I wouldn't get hung up on the fact you are making your new list in a comprehension.

Clearly if you can transform a dictionary from the old format to the new format this is just

new = [transform(o) for o in old]

Your question then is "can I write a function to transform", which is "yes" if there is any way to describe what you are trying to do (your example is far from complete):

def transform(old):
    return {new(k): v for k, v in old.items() if condition(k, v)}

def new(key):
    return {'old_key_1': 'new_key1', ...}[key]

def condition(key, value):
    if key == 'old_key_2' and value == 'value2b':
         return False
    return True