How can I get the list of names used in a formatting string?

Question

Given a formatting string:

x = "hello %(foo)s  there %(bar)s"

Is there a way to get the names of the formatting variables? (Without directly parsing them myself).

Using a Regex wouldn't be too tough but I was wondering if there was a more direct way to get these.

Answer 1

Use a dict subclass with overridden __missing__ method and from there you can collect all the missing format variables:

class StringFormatVarsCollector(dict):
    def __init__(self, *args, **kwargs):
        self.format_vars = []

    def __missing__(self, k):
        self.format_vars.append(k)
...         
def get_format_vars(s):
    d = StringFormatVarsCollector()     
    s % d                    
    return d.format_vars
... 
>>> get_format_vars("hello %(foo)s  there %(bar)s")
['foo', 'bar']

Answer 2

If you don't want to parse the string, you can use this little function:

def find_format_vars(string):
    vars= {}
    while True:
        try:
            string%vars
            break
        except KeyError as e:
            vars[e.message]= ''
    return vars.keys()

>>> print find_format_vars("hello %(foo)s there %(bar)s") ['foo', 'bar']