Print string left aligned with fixed width and suffix

Question

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)

have you seen this http://stackoverflow.com/a/10837046/1984350? — Adeeb, Feb 25 '15 at 08:41
@Adeeb: That question does not cover the suffix I want to add — luator, Feb 25 '15 at 08:49
do you want the suffix to be immediately after the key text or do you want it to be after the remaining space if the key is smaller than the allocated space? which option between these http://paste.ubuntu.com/10404650/ — Adeeb, Feb 25 '15 at 08:55
@Adeeb: I want it immediately after the key (see my example), so the padding should be added _after_ the suffix. — luator, Feb 25 '15 at 08:56

Anthon · Accepted Answer · 2015-09-09T12:04:28.710

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

Much more complicated than I hoped, but it works, so thanks :). — luator, Sep 08 '15 at 14:03
Yes it would be much nicer if you could just patch that all on the built-in string and then call "....".altformat() but unfortunately you can't. — Anthon, Sep 08 '15 at 14:05

martineau · Answer 2 · 2015-02-25T10:13:43.160

0

Here's one way:

d = {'a_key': 23, 'another_key': 42}

for key, value in d.items():
    print '{}: {:<{width}} {}'.format(key, '', value, width=20-len(str(key)))

Output:

another_key:           42
a_key:                 23

Another alternative would be:

    print '{}:{}{}'.format(key, ' '*(20-len(str(key))), value)

edited Feb 25 '15 at 10:13

answered Feb 25 '15 at 10:04

martineau

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This is working, but it still has the flaw that formatting and data are mixed. I hoped there is some way to achieve this without having to add something in the `.format(...)` part. – luator Feb 25 '15 at 16:19

Print string left aligned with fixed width and suffix

2 Answers2

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