Update JSON element in Python List Comprehension

Question

Trying to update/add to a JSON element using if/then statement in python list comprehension. The first part where I'm setting the JSON key 'first_seen' is failing. Any ideas why?

now = datetime.datetime.now().strftime("%B, %d, %Y")
[obj["last_seen"] = now for obj in ref_db if obj['user']==user else add_new(user, ext_source, source, first_seen, now)]

the error is:

[obj["last_seen"] = now for obj in ref_db if obj['user']==user else add_new(user, ext_source, source, first_seen, now)]
                  ^
SyntaxError: invalid syntax

I understand from the error my syntax is wrong, but I can't figure out why it's wrong. Can you not use an equals (=) sign in a list comprehension?

Thanks for the help.

It's unclear what result you're expecting. Are you expecting obj["last_seen"] to be the resulting list? — jonrsharpe
– jonrsharpe, Commented Nov 4, 2018 at 13:57
You need a plain if/else statement inside a for loop instead of a list comprehension — Carlos Mermingas
– Carlos Mermingas, Commented Nov 4, 2018 at 13:58
Please include a minimal reproducible example that we can actually run. — timgeb
– timgeb, Commented Nov 4, 2018 at 13:59
Possible duplicate of How can I do assignments in a list comprehension? — Carlos Mermingas
– Carlos Mermingas, Commented Nov 4, 2018 at 14:00

Daniel · Accepted Answer · 2018-11-04 14:00:08Z

4

List-Comprehensions are for creating lists. You just want to use a for-loop:

now = datetime.datetime.now().strftime("%B, %d, %Y")
for obj in ref_db:
    if obj['user'] == user:
        obj["last_seen"] = now
    else:
        add_new(user, ext_source, source, first_seen, now)

answered Nov 4, 2018 at 14:00

Daniel

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