字符串格式的参数个数

Question

I would like to know the number of arguments required for a given format string. For example, "%s is my name, and %s is my hair color" would have two arguments required. I could find the number of times % shows up in the string, but that wouldn't work if I really wanted a % in my string and it had %% to signify that. It also doesn't seem at all elegant. Is there a better way?

Answer 1

The simplest way I could come up with is this:

my_string = "%s is my name, and %s is my hair color"
my_string.count('%') - 2 * my_string.count('%%')

Answer 2

Well, you could use the formatter object for this, since this needs this function for its own formatting purposes. However you have to change your place holders.

import string

fmt = string.Formatter()
my_string = "Hello, {0:s} is my name and {1:s} is my hair color. I have 30% blue eyes"
parts = fmt.parse(my_string)
print list(parts)

This gives:

[('Hello, ', '0', 's', None), (' is my name and ', '1', 's', None), (' is my hair color. I have 30% blue eyes', None, None, None)]

Now it is a matter of filtering the right parts out, namely where the 3rd item in every tuple is not None.

Everything can be changed to a one-liner like this:

len([p for p in fmt.parse(my_string) if p[2] is not None]) # == 2

Answer 3

Not exactly efficient - and not entirely serious :) ...

for n in xrange(10, -1, -1):
    try:
        s % (('',)*n)
    except TypeError:
        continue
    print n
    break