Why is Python interpreting this string as a dictionary when formatting?

Question

I am having problems using format with a string which looks like a Python dictionary.

I want to generate the following string: {"one":1} If I try to do it as:

'{"one":{}}'.format(1)

the interpreter throws a KeyError:

>>> a = '{"one":{}}'.format(1)
Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
KeyError: '"one"'

I understand that the issue probably revolves around the fact that the string contains {, which can be confused with format's {}. Why is this happening exactly and how can it be solved?

I know of percentage formatting, but I'd like to find a solution that doesn't involve ditching format().

Keep in mind than percentage formatting is faster: format=0.7228269569986878, %=0.03355357599866693 ( result of `timeit.timeit` on your example ) — Yaroslav Surzhikov, Sep 11 '17 at 15:02
@YaroslavSurzhikov Unless you're generating vast amounts of strings I don't think that really matters. — JAB, Sep 11 '17 at 18:54
If you want to make a string of a dictionary, consider `str({"one":1})`. — marcelm, Sep 11 '17 at 20:27
Note: Even if the question isn't identical, if an answer explains the reason for something, [it still qualifies](https://stackoverflow.com/questions/41983180/is-the-empty-tuple-in-python-a-constant#comment71144721_41983180). — cs95, Sep 11 '17 at 20:46
@cᴏʟᴅsᴘᴇᴇᴅ in this specific Q the issue isn't only how to solve this but why it is interpreted as a dictionary (which I address). Do you know any other answers that might answer the second part? If so please add it to the list of dupes. I'm leaning towards re-opening but I'm pretty sure there's an answer for the second issue too (Which I didn't find with a *very basic* search :-). — Dimitris Fasarakis Hilliard, Sep 11 '17 at 20:55
@JimFasarakisHilliard Found one... https://stackoverflow.com/questions/31859757/format-keyerror-when-using-curly-brackets-in-strings — cs95, Sep 11 '17 at 20:58

Dimitris Fasarakis Hilliard · Accepted Answer · 2017-09-16T13:33:54.883

The formatting of '{"one": {}}' is using an identifier as the field_name and will essentially try to look for a keyword argument that's been supplied to .format and has the name '"one"'.

As the docs state:

The field_name itself begins with an arg_name that is either a number or a keyword. If it’s a number, it refers to a positional argument, and if it’s a keyword, it refers to a named keyword argument.

_{(emphasis mine)}

That's why you get the KeyError exception; it tries to look for a key in the mapping of keyword arguments provided to format. (which, in this case, is empty, hence the error).

As a solution, just escape the outer curly braces:

>>> '{{"one":{}}}'.format(1)
'{"one":1}'

The same remedy applies if you decide on using f-strings in the future:

>>> f'{{"one": {1}}}'
'{"one": 1}'

Ah! I knew escaping was the problem, but I was using `\` to escape, not double curly braces. Thanks! — user2891462, Sep 11 '17 at 14:26

score 13 · Answer 2 · edited Sep 11 '17 at 21:03

You need doubled curly braces {{ }} to escape curly braces in string formatting.

a= '{{"one":{}}}'.format(1)

from doc:

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 }}.

If you do not escape braces, str.format() will look for the value of key '"one"' to format the string. For example:

b = '{"one"} foo'.format(**{'"one"':1})
print(b) # 1 foo

score 2 · Answer 3 · edited Sep 11 '17 at 16:17

2

Braces can be escaped by using double braces, use:

'{{"one":{}}}'.format(1)

edited Sep 11 '17 at 16:17

Dimitris Fasarakis Hilliard

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answered Sep 11 '17 at 14:26

v.coder

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Why is Python interpreting this string as a dictionary when formatting?

3 Answers3