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

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.

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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}).

You could update the dictionairy with the selected key

for x in fields: x.update({'selected': x['name'] in saved_fields})

print(fields)

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

result = [
    {"name": fruit['name'],
     "selected": fruit['name'] in saved_fields } 
    for fruit in fields
]

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

And as a one-liner:

result = [{"name": fruit['name'], "selected": fruit['name'] in saved_fields} for fruit in fields]