str.format fails with JSON?

[carlopires at gmail.com]

Hi,

When I run this piece of code:

'From {"value": 1}, value={value}'.format(value=1)

Python complains about the missing "value" parameter (2.7.12 and 3.6.x):

Traceback (most recent call last):
  File "test_format.py", line 1, in <module>
    'From {"value": 1}, value={value}'.format(value=1)
KeyError: '"value"

But according to the format string syntax (https://docs.python.org/2/library/string.html): 

replacement_field ::=  "{" [field_name] ["!" conversion] [":" format_spec] "}"
field_name        ::=  arg_name ("." attribute_name | "[" element_index "]")*
arg_name          ::=  [identifier | integer]
attribute_name    ::=  identifier
element_index     ::=  integer | index_string
index_string      ::=  <any source character except "]"> +
conversion        ::=  "r" | "s"
format_spec       ::=  <described in the next section>

The replacement_field, which in this case, is composed by an identifier, shouldn't have quotation marks. Here is the lexical definition for an identifier (according to the documentation):

identifier ::=  (letter|"_") (letter | digit | "_")*
letter     ::=  lowercase | uppercase
lowercase  ::=  "a"..."z"
uppercase  ::=  "A"..."Z"
digit      ::=  "0"..."9"

So according to the specification, {value} should be recognized as a valid format string identifier and {"value"} should be ignored.

Python seems to not follow the specification in the documentation. Anything inside the keys is accepted as identifier.