Python fixed width string format using vars or dict

Question

I'm working on a Python project and I'm hoping to use a few shortcuts to help format class data in a string. More specifically, I would like to be able to use something similar to '{a}{b}{c}'.format(**vars(self), [strlen, strlen, strlen]) and specify the string length of each attribute that is displayed. For example:

class Dummy(object):
    def __init__(self):
        self.value1 = 'A VALUE'
        self.value2 = 'ANOTHER VALUE'
        self.value3 = 'THIRD VALUE'

    def to_s(self):
        # want value1 to be 20 chars
        # value2 to be 8 chars
        # value3 to be 10 chars
        # is something similar to this possible
        return '{value1},{value2},{value3}'.format(**vars(self), [20, 8, 10])


    def to_s2(self):
        # or will I have to reference each explicitly and specify the either padding or slicing?
        return '{},{},{}'.format(self.value1.ljust(20), self.value2[:8], self.value3[:10])

I know it's a long shot, but a couple of these classes have 30 or 40 attribs and it would make life so much easier if this is doable.

Why not do this in __str__() method, if you want it to be the default string-formatting for that class? — smci
– smci, Commented Jan 9, 2024 at 21:31

PM 2Ring · Accepted Answer · 2016-10-25 17:21:51Z

3

You can nest {} fields inside {} fields, but only one level of nesting is permitted. Fortunately, only one level of nesting is actually needed. :)

From Format String Syntax:

A format_spec field can also include nested replacement fields within it. These nested replacement fields may contain a field name, conversion flag and format specification, but deeper nesting is not allowed. The replacement fields within the format_spec are substituted before the format_spec string is interpreted. This allows the formatting of a value to be dynamically specified.

class Dummy(object):
    def __init__(self):
        self.value1 = 'A VALUE'
        self.value2 = 'ANOTHER VALUE'
        self.value3 = 'THIRD VALUE'

    def __str__(self):
        # want value1 to be 20 chars
        # value2 to be 8 chars
        # value3 to be 10 chars
        return '{value1:{0}},{value2:{1}},{value3:{2}}'.format(*[20, 8, 10], **vars(self))

print(Dummy())

output

A VALUE             ,ANOTHER VALUE,THIRD VALUE

answered Oct 25, 2016 at 17:21

PM 2Ring

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1 Comment

PM 2Ring Over a year ago

Note that this also applies to formatted string literals, aka f-strings.

Andy S · Accepted Answer · 2016-10-25 17:27:33Z

Something like this might work:

class Dummy(object):
    def __init__(self):
        self.value1 = 'A VALUE'
        self.value2 = 'ANOTHER VALUE'
        self.value3 = 'THIRD VALUE'

    def to_s(self):
        return '{0.value1:<20},{0.value2:8},{0.value3:10}'.format(self)

Se more details about formatting in https://docs.python.org/2/library/string.html#formatstrings. In case you want longer list of attributes and more dynamic formatting you can also construct format string dynamically, e.g. (untested):

    field_formats = [('value1', '<20'),
                     ('value2', '8'),
                     ('value3', '>10'))  # etc.

    def to_s(self):
        fmt = ','.join('{0.%s:%s}' % fld for fld in field_formats)
        return fmt.format(self)

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