How to add another attribute in dictionary inside a one line for loop [duplicate]

Question

I have a list of dictionary and a string. I want to add a selected attribute in each dictionary inside the list. I am wondering if this is possible using a one liner.

Here are my inputs:

saved_fields = "apple|cherry|banana".split('|')
fields = [
    {
        'name' : 'cherry'
    }, 
    {
        'name' : 'apple'
    }, 
    {
        'name' : 'orange'
    }
]

This is my expected output:

[
    {
        'name' : 'cherry',
        'selected' : True
    }, 
    {
        'name' : 'apple',
        'selected' : True
    }, 
    {
        'name' : 'orange',
        'selected' : False
    }
]

I tried this:

new_fields = [item [item['selected'] if item['name'] in saved_fields] for item in fields]

Answer 1

I don't necessarily think "one line way" is the best way.

s = set(saved_fields)  # set lookup is more efficient 
for d in fields:
    d['status'] = d['name'] in s

fields
# [{'name': 'cherry', 'status': True},
#  {'name': 'apple', 'status': True},
#  {'name': 'orange', 'status': False}]

Simple. Explicit. Obvious.

This updates your dictionary in-place, which is better if you have a lot of records or other keys besides "name" and "status" that you haven't told us about.

---

If you insist on a one-liner, this is one preserves other keys:

[{**d, 'status': d['name'] in s} for d in fields]  
# [{'name': 'cherry', 'status': True},
#  {'name': 'apple', 'status': True},
#  {'name': 'orange', 'status': False}]

This is list comprehension syntax and creates a new list of dictionaries, leaving the original untouched.

The {**d, ...} portion is necessary to preserve keys that are not otherwise modified. I didn't see any other answers doing this, so thought it was worth calling out.

The extended unpacking syntax works for python3.5+ only, for older versions, change {**d, 'status': d['name'] in s} to dict(d, **{'status': d['name'] in s}).