How to check if string has format arguments in Python?

Question

I am formatting strings using named arguments with .format(). How can I obtain a list of arguments?

For example:

>>> my_string = 'I live in {city}, {state}, {country}.'
>>> get_format_args(my_string)
# ['city', 'state', 'country']

Note that order does not matter. I have dug a fair amount into the string.Formatter documentation to no avail. I am sure you could write regex to do this, but there must bet a more elegant way.

Answer 1

It looks like you can get field names using the Formatter.parse method:

>>> import string
>>> my_string = 'I live in {city}, {state}, {country}.'
>>> [tup[1] for tup in string.Formatter().parse(my_string) if tup[1] is not None]
['city', 'state', 'country']

This will also return non-named arguments. Example: "{foo}{1}{}" will return ['foo', '1', '']. But if necessary you can filter out the latter two by using str.isdigit() and comparing against the empty string respectively.

Answer 2

Regex can solve your problem.

>>> import re 
>>> re.findall(r'{(.*?)}', 'I live in {city}, {state}, {country}.')
['city', 'state', 'country']

---

Edit:

To avoid matching escaped placeholders like '{{city}}' you should change your pattern to something like:

(?<=(?<!\{)\{)[^{}]*(?=\}(?!\}))

Explanation:

(?<=      # Assert that the following can be matched before the current position
 (?<!\{)  #  (only if the preceding character isn't a {)
\{        #  a {
)         # End of lookbehind
[^{}]*    # Match any number of characters except braces
(?=       # Assert that it's possible to match...
 \}       #  a }
 (?!\})   #  (only if there is not another } that follows)
)         # End of lookahead