How to use str.format inside a string of json format?

Question

Python Version 3.5

I'm trying to make an API call to configure a device using json as the format. Some of the json will vary depending on the desired naming, so I need to call a variable in the string. I am able to accomplish this using the old style %s... % (variable), but not with the new style {}... .format(variable).

Failed EX:

(Testing with {"fvAp":{"attributes":{"name":(variable)}}})

a = "\"app-name\""

app_config = ''' { "fvAp": { "attributes": { "name": {} }, "children": [ { "fvAEPg": { "attributes": { "name": "app" }, "children": [ { "fvRsBd": { "attributes": { "tnFvBDName": "default" }, } } ] } }, { "fvAEPg": { "attributes": { "name": "db" }, "children": [ { "fvRsBd": { "attributes": { "tnFvBDName": "default" }, } } ] } } ] } } '''.format(a)

print(app_config)

Traceback (most recent call last): File "C:/..., line 49, in '''.format('a') KeyError: '\n "fvAp"'

Working EX:

a = "\"app-name\""

app_config = ''' { "fvAp": { "attributes": { "name": %s }, "children": [ { "fvAEPg": { "attributes": { "name": "app" }, "children": [ { "fvRsBd": { "attributes": { "tnFvBDName": "default" }, } } ] } }, { "fvAEPg": { "attributes": { "name": "db" }, "children": [ { "fvRsBd": { "attributes": { "tnFvBDName": "default" }, } } ] } } ] } } ''' % a

print(app_config)

How do I get this to work using str.format method?

Answer 1

Format String Syntax section says:

Format strings contain “replacement fields” surrounded by curly braces {}. Anything that is not contained in braces is considered literal text, which is copied unchanged to the output. If you need to include a brace character in the literal text, it can be escaped by doubling: {{ and }}.

So if you want to use .format method, you need to escape all JSON curly braces in your template string:

>>> '{{"fvAp": {{"attributes": {{"name": {}}}}}}}'.format('"app-name"')
'{"fvAp": {"attributes": {"name": "app-name"}}}'

That looks really bad.

There's a better way to do that with string.Template:

>>> from string import Template
>>> t = Template('{"fvAp": {"attributes": {"name": "${name}"}}')
>>> t.substitute(name='StackOverflow')
'{"fvAp": {"attributes": {"name": "StackOverflow"}}'

Though I suggest abandoning the idea of generating configs this way altogether and using a factory function and json.dumps instead:

>>> import json
>>> def make_config(name):
...     return {'fvAp': {'attributes': {'name': name}}}
>>> app_config = make_config('StackOverflow')
>>> json.dumps(app_config)
'{"fvAp": {"attributes": {"name": "StackOverflow"}}}'