Slicing strings in str.format

Question

I want to achieve the following with str.format:

x,y = 1234,5678
print str(x)[2:] + str(y)[:2]

The only way I was able to do it was:

print '{0}{1}'.format(str(x)[2:],str(y)[:2])

Now, this an example and what I really have is a long and messy string, and so I want to put slicing inside the {}. I've studied the docs, but I can't figure out the correct syntax. My question is: is it possible to slice strings inside a replacement field?

Answer 1

No, you can't apply slicing to strings inside a the replacement field.

You'll need to refer to the Format Specification Mini-Language; it defines what is possible. This mini language defines how you format the referenced value (the part after the : in the replacement field syntax).

Answer 2

You could do something like this.

NOTE
This is a rough example and should not be considered complete and tested. But I think it shows you a way to start getting where you want to be.

import string

class SliceFormatter(string.Formatter):

    def get_value(self, key, args, kwds):
        if '|' in key:
            try:
                key, indexes = key.split('|')
                indexes = map(int, indexes.split(','))
                if key.isdigit():
                    return args[int(key)][slice(*indexes)]
                return kwds[key][slice(*indexes)]
            except KeyError:
                return kwds.get(key, 'Missing')
        return super(SliceFormatter, self).get_value(key, args, kwds)


phrase = "Hello {name|0,5}, nice to meet you.  I am {name|6,9}.  That is {0|0,4}."
fmt = SliceFormatter()
print fmt.format(phrase, "JeffJeffJeff", name="Larry Bob")

OUTPUT

Hello Larry, nice to meet you.  I am Bob.  That is Jeff.

NOTE 2
There is no support for slicing like [:5] or [6:], but I think that would be easy enough to implement as well. Also there is no error checking for slice indexes out of range, etc.