Print string left aligned with fixed width and suffix

Using Pythons string formatting, is there a nice way to add a suffix to a left aligned string that is padded to a fixed size?

I want to print a list of key-value-pairs in the following formatting:

a_key:        23
another_key:  42
...

The problem is the ':'. The best solution I found so far is to append the ':' to the key name:

print "{:<20}  {}".format(key+':', value)

But I think this is a rather ugly solution, as it diminishes the separation of formatting and values. Is it possible to achieve this directly in the format specification?

What I am looking for is something like this:

print "{do something here}  {}".format(key, value)

You cannot change "".format() as it is a built-in but if it is acceptable to provide the string and parameters to a method:

print(kf.format("{:t{}}  {}", key, ':', value))

you can do so by subclassing string.Formatter to allow empty format fields and provide a special type handler t:

from string import Formatter
import sys

if sys.version_info < (3,):
    int_type = (int, long)
else:
    int_type = (int)

class TrailingFormatter(Formatter):

    def vformat(self, *args):
        self._automatic = None
        return super(TrailingFormatter, self).vformat(*args)

    def get_value(self, key, args, kwargs):
        if key == '':
            if self._automatic is None:
                self._automatic = 0
            elif self._automatic == -1:
                raise ValueError("cannot switch from manual field specification "
                                 "to automatic field numbering")
            key = self._automatic
            self._automatic += 1
        elif isinstance(key, int_type):
            if self._automatic is None:
                self._automatic = -1
            elif self._automatic != -1:
                raise ValueError("cannot switch from automatic field numbering "
                                 "to manual field specification")
        return super(TrailingFormatter, self).get_value(key, args, kwargs)

    def format_field(self, value, spec):
        if len(spec) > 1 and spec[0] == 't':
            value = str(value) + spec[1]  # append the extra character
            spec = spec[2:]
        return super(TrailingFormatter, self).format_field(value, spec)

kf = TrailingFormatter()
w = 20
ch = ':'
x = dict(a_key=23, another_key=42)

for k in sorted(x):
    v = x[k]
    print(kf.format('{:t{}<{}} {}', k, ch, w, v))

gives you:

 a_key:               23
 another_key:         42

You can of course hardcode the ch, and w values:

    print(kf.format('{:t:<20} {}', k, v))

for better readability, but less flexibility.

---

A backport of the Python 3.4 string.Formatter() that includes a bugfix for versions (at least) up to 3.5.0rc1, that includes this code is now available on PyPI