Format on python string gives error

Question

In python, I'm trying to create a string by format()

json_data_resp = '{"error":false,"code":"{0}","mac":"{1}","message":"Device configured successfully"}'.format(activation_code, macaddress)

When I execute this code, it gives me an error like so:

KeyError: '"error"'

What is it that I'm doing incorrectly?

`format` looks for `{}` to replace for the values. You have one big `{}` with some small `{]`s inside. Thats not valid syntax — rafaelc, Aug 10 '18 at 03:25
You shouldn't be using `str.format` to create JSON strings in the first place. — abarnert, Aug 10 '18 at 03:31
Due to memory limitations of micropython, its recommended I use format : https://docs.micropython.org/en/latest/esp8266/reference/constrained.html — Ouroboros, Aug 10 '18 at 04:04

blhsing · Accepted Answer · 2018-08-10T03:29:57.373

3

You should escape literal braces by doubling them:

json_data_resp = '{{"error":false,"code":"{0}","mac":"{1}","message":"Device configured successfully"}}'.format(activation_code, macaddress)

Excerpt from Format String Syntax:

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 }}.

edited Aug 10 '18 at 03:29

answered Aug 10 '18 at 03:28

blhsing

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Since I'm using micropython, the format is what is recommended instead of concatenation. This is exactly what I was unable to catch. Thanks – Ouroboros Aug 10 '18 at 04:03

score 0 · Answer 2 · answered Aug 10 '18 at 03:28

0

You can do '%' for string formatting:

json_data_resp = '{"error":false,"code":"%s","mac":"%s","message":"Device configured successfully"}'%(activation_code, macaddress)

answered Aug 10 '18 at 03:28

U13-Forward

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score 0 · Answer 3 · answered Aug 10 '18 at 03:28

0

It is trying to insert a variable inside of the curly braces you put around the entire string. If you want to have curly braces in a formatted string, you have to use two.

string = "{{test_number: {0}}}".format(37)

answered Aug 10 '18 at 03:28

Aeolus

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score 0 · Answer 4 · answered Aug 10 '18 at 03:30

Format strings have fields inside curly braces. Each field has an optional number, name, or expression, and optional :spec, and an optional !conversion.

So, when you use this as a format string:

'{"error":false,"code":"{0}","mac":"{1}","message":"Device configured successfully"}'

You have a field named "error", with spec false,"code":"{0}","mac":"{1}","message":"Device configured successfully".

To format that field, you need a keyword parameter named "error", and of course you don't have one.

Obviously, you didn't want that whole thing to be a field. But that means you need to escape the braces by doubling them:

'{{"error": false,"code":"{0}","mac":"{1}","message":"Device configured successfully"}}'

Or, better… why are you trying to create a JSON string by str.format in the first place? Why not just create a dict and serialize it?

json_data_resp = json.dumps({
    "error": False, "code": activation_code, "mac": macaddress,
    "message": "Device configured successfully"})

Format on python string gives error

4 Answers4