JSON string and .format() in python3

Question

I'm trying to generate a JSON string using .format(). I tried the following:

TODO_JSON = '{"id": {0},"title": {1},"completed:" {2}}'
print(TODO_JSON.format(42, 'Some Task', False))

which raises

File "path/to/file", line 2, in <module>
    print(TODO_JSON.format(42, 'Some Task', False))
KeyError: '"id"'

Why is this error occurring ? Why is 'id' getting interpreted as a key and not as part of a string ?

This is fine for learning, but I'd use the json module for practical work as it'll escape things for you. — Nick T
– Nick T, Commented May 21, 2017 at 3:05
@NickT I was looking for a good reason to use the json module instead of a simple string – you provide a good one. — Theo
– Theo, Commented May 21, 2017 at 3:23

akuiper · Accepted Answer · 2017-05-21 03:05:12Z

6

{} has special meaning in str.format (place holder and variable name), if you need literal {} with format, you can use {{ and }}:

TODO_JSON = '{{"id": {0},"title": {1},"completed:" {2}}}'
print(TODO_JSON.format(42, 'Some Task', False))
# {"id": 42,"title": Some Task,"completed:" False}

edited May 21, 2017 at 3:05

answered May 21, 2017 at 3:03

akuiper

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1 Comment

Theo Over a year ago

Thanks for your answer.

Taku · Accepted Answer · 2017-05-21 03:19:54Z

1

You can use % formatting style.

TODO_JSON = '{"id": %i,"title": %s,"completed:" %s}'
print(TODO_JSON % (42, 'Some Task', False))

answered May 21, 2017 at 3:19

Taku

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Comments

Francois Mockers · Accepted Answer · 2017-05-21 03:05:26Z

0

Because it's trying to parse to outer {} that are part of the json formatting as something that should be formatted by format

But you should try the json module

import json
todo = {'id': 42, 'title': 'Some Task', 'completed': False}
TODO_JSON = json.dumps(todo)

answered May 21, 2017 at 3:05

Francois Mockers

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1 Comment

Theo Over a year ago

My data comes in as a tuple from the database, so it's quire simple to just pass it in the string. What is the benefit of creating a dict and parsing it ?

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