Extract substrings in python
Question: I want to parse a string to extract all the substrings in curly braces:

    
    
    'The value of x is {x}, and the list is {y} of len {}'
    

should produce:

    
    
    (x, y)
    

Then I want to format the string to print the initial string with the values:

    
    
    str.format('The value of x is {x}, and the list is {y} of len {}', x, y, len(y))
    

How can I do that?

    
    
    Example usage:
    def somefunc():
        x = 123
        y = ['a', 'b']
        MyFormat('The value of x is {x}, and the list is {y} of len {}',len(y))
    
    output:
        The value of x is 123, and the list is ['a', 'b'] of len 2
    


Answer: You can use
[string.Formatter.parse](https://docs.python.org/2/library/string.html#string.Formatter.parse):

> Loop over the format_string and return an iterable of tuples (literal_text,
> field_name, format_spec, conversion). This is used by vformat() to break the
> string into either literal text, or replacement fields.
>
> The values in the tuple conceptually represent a span of literal text
> followed by a single replacement field. If there is no literal text (which
> can happen if two replacement fields occur consecutively), then literal_text
> will be a zero-length string. If there is no replacement field, then the
> values of field_name, format_spec and conversion will be None.
    
    
    from string import Formatter
    
    s = 'The value of x is {x}, and the list is {y} of len {}'
    
    print([t[1] for t in Formatter().parse(s) if t[1]])
    ['x', 'y']
    

Not sure how that really helps what you are trying to do as you can just pass
x and y to str.format in your function or use **locals:

    
    
    def somefunc():
        x = 123
        y = ['a', 'b']
        print('The value of x is {x}, and the list is {y} of len {}'.format(len(y),**locals()))
    

If you wanted to print the named args you could add the Formatter output:

    
    
    def somefunc():
        x = 123
        y = ['a', 'b']
        print("The named args are {}".format( [t[1] for t in Formatter().parse(s) if t[1]]))
        print('The value of x is {x}, and the list is {y} of len {}'.format(len(y), **locals()))
    

Which would output:

    
    
    The named args are ['x', 'y']
    The value of x is 123, and the list is ['a', 'b'] of len 2
    

