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]

Are you sure you copied your code correctly? That looks like it would be an error. — kubatucka
– kubatucka, Commented Jul 7, 2020 at 8:19
"I tried this" - and how did that go? What error or output did you get? How did you try to fix it? Some people (myself partially included) appreciate seeing an attempt just for the sake of it, but really the point should be to iterate upon it and break it down and debug it and maybe that leads you to asking a different question, or maybe it just leads you to the answer. — Bernhard Barker
– Bernhard Barker, Commented Jul 7, 2020 at 17:50
Can you do it in multiple lines? Starting there and then converting that to a single line would likely be much easier than trying to come up with a single line directly. Although what's with the "in one line" questions? Trying to force something into one line just makes for unreadable code. — Bernhard Barker
– Bernhard Barker, Commented Jul 7, 2020 at 17:55
A "one line for-loop" is a dict comprehension, in your case. (Or list comprehension, or set comprehension). Please read about dict comprehensions. — smci
– smci, Commented Jul 7, 2020 at 20:06

cs95 · Accepted Answer · 2020-07-07 21:06:14Z

15

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}).

edited Jul 7, 2020 at 21:06

answered Jul 7, 2020 at 8:23

cs95

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13 Comments

smci Over a year ago

You should explain to the the OP that this "one line for-loop" is a dict comprehension.

cs95 Over a year ago

@smci It's not a dict comprehension though? It's a list comprehension which creates a dictionary each iteration. Although the focal point of my answer is that it isn't always necessary you use one.

cs95 Over a year ago

@smci no, a dict comprehension would look like {key : value for something in something_else}. {**d, 'a': 'b', 'c': 'd'} just creates a dictionary.

smci Over a year ago

cs95: No, I didn't downvote it! Not so paranoid, my friend. I don't believe I've ever downvoted any of your excellent content on SO, well maybe only one answer once. And certainly not this one.

smci Over a year ago

"The second answer is just a herring to satisfy the OP's "one line" requirement, like I said not the focal point of the answer". I actually prefer it! I like it a lot. Also, people coming from functional languages will like it too.

|

borisdonchev · Accepted Answer · 2020-07-07 08:22:38Z

5

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]

answered Jul 7, 2020 at 8:22

borisdonchev

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Comments

renatodamas · Accepted Answer · 2021-03-29 08:57:29Z

5

You could update the dictionary 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}]

edited Mar 29, 2021 at 8:57

renatodamas

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answered Jul 7, 2020 at 8:24

ScootCork

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2 Comments

cs95 Over a year ago

I strongly do not recommend using list comprehensions for side effects, this is a well-known antipattern. You are generating a result of [None, None, None] and throwing it away.

ScootCork Over a year ago

Good point, updated to different one liner as requested by OP.

cs95 · Accepted Answer · 2020-07-07 18:18:03Z

4

The proposed solutions will work even if there is more than one entry in the dicts.

Taking your inputs :

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

Using the Dict.update():

>>> [item.update({'selected': item['name'] in saved_fields}) for item in fields]
[None, None, None]

Will return [None, None, None] but modifies the fields variable inplace.

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

note that while this is a one-liner, this is not always recommended.

If you want a new list without modifying fields. It can be done using ** operator on Dict cf as shown in @cs95 answer. ** explanation:

>>> new_fields = [{**item, 'selected': item['name'] in saved_fields} for item in fields]
>>> new_fields
[{'name': 'cherry', 'selected': True},
 {'name': 'apple', 'selected': True},
 {'name': 'orange', 'selected': False}]

>>> fields
[{'name': 'cherry'}, {'name': 'apple'}, {'name': 'orange'}]

edited Jul 7, 2020 at 18:18

cs95

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answered Jul 7, 2020 at 9:30

Quentin Possamaï

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1 Comment

Barmar Over a year ago

Using a list comprehension just for side effects is a Python anti-pattern

Alexander Lekontsev · Accepted Answer · 2020-07-07 08:23:39Z

2

[{'name': item['name'], 'selected': item['name'] in saved_fields} for item in fields]

edited Jul 7, 2020 at 8:23

answered Jul 7, 2020 at 8:21

Alexander Lekontsev

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