How to use f-string formatting around a string of a dictionary

Question

The following code causes an Invalid Format Specifier when the string is converted to an f-string. I can't pinpoint what the problem is as my quotations look okay.

expected_document = f'{"name":"tenders","value":"chicken","key":"{key}"}'

causes:

>       expected_document = f'{"name":"tenders","value":"chicken","key":"{key}"}'
E       ValueError: Invalid format specifier

while removing the f:

expected_document = '{"name":"tenders","value":"chicken","key":"{key}"}'

works fine.

Why are you using an f-string at all? Does the "works fine" example give you the result you want? Or is that `key` the only thing you want interpolated? — Ryan Haining, Aug 01 '19 at 17:42
Also when using string formatting, double up the brackets to escape them. Ex: `{` should be `{{` if you want it escaped. — Error - Syntactical Remorse, Aug 01 '19 at 17:43
I need expected_document to be a string where {key} is a variable surrounded by double quotes. — tenders27, Aug 01 '19 at 17:49

Bilesh Ganguly · Answer 1 · 2019-08-02T19:21:18.337

Why use f-string at all?

The following code works.

key = 'test'

expected_document = { "name": "tenders", "value": "chicken", "key": key }

print(expected_document)

Output:

{'name': 'tenders', 'value': 'chicken', 'key': 'test'}

Update #1: If you want it as a string and don't want to do type conversion, then...

key = 'test'

expected_document_1 = '{"name":"tenders","value":"chicken","key":"' + key + '"}'  # old fashioned way

print(expected_document_1)

expected_document_2 = f'{{"name":"tenders","value":"chicken","key":"{key}"}}'  # using f-string

print(expected_document_2)

Output:

{"name":"tenders","value":"chicken","key":"test"}
{"name":"tenders","value":"chicken","key":"test"}

Update #2: @Error - Syntactical Remorse had already suggested the second option of escaping the braces in one of the comments.

Because I need expected_document to be a string, I'm asserting it equals another string where key could change. Maybe it's possible I could change types of that string to a dictionary or something? — tenders27, Aug 01 '19 at 17:52

score 0 · Answer 2 · answered Aug 01 '19 at 17:41

0

I think you can just put the f inside the dict like this:

key = 'test'

expected_document = {"name":"tenders","value":"chicken","key":f"{key}"}

answered Aug 01 '19 at 17:41

bbd108

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I do need expected_document to be a string, thanks for replying! – tenders27 Aug 01 '19 at 17:47
just put str(expected_document) afterwards – bbd108 Aug 01 '19 at 17:57
You don't need to use f-string in your example. `expected_document = {"name": "tenders", "value": "chicken", "key": key}` will work just fine. – Bilesh Ganguly Aug 01 '19 at 18:02

score 0 · Answer 3 · answered Aug 01 '19 at 17:53

You could compile the dictionary without using interpolation and then convert it to a string.

temporary_variable = {"name": "tenders", "value": "chicken", "key": key}
expected_document = str(temporary_variable)

Or you could even put this in one line.

expected_document = str({"name": "tenders", "value": "chicken", "key": key})

I am using Python 3.6.3 - I don't know how other versions would handle this. A potential drawback of this is that you shouldn't rely on dictionaries to maintain their order, which could cause this to break.

How to use f-string formatting around a string of a dictionary

3 Answers3