Python3 - Use a variables inside string formatter arguments

Question

I have some formatted columns that I'm printing. I would like to use the following variables to set the lengths in my .format arguments

number_length = 5
name_length = 24
viewers_length = 9

I have

print('{0:<5}{1:<24}{2:<9}'.format(' #','channel','viewers'), end = '')

Ideally I would like something like

print('{0:<number_length}{1:<name_length}{2:<viewers_length}'.format(
     ' #','channel','viewers'), end = '')

But this gives me an invalid string formatter error.

I have tried with % before the variables and parenthesis, but have had no luck.

Can you share what the desired output would be?

MrAlexBailey
– MrAlexBailey

2015-02-04 20:22:44 +00:00
Commented Feb 4, 2015 at 20:22 — MrAlexBailey
– MrAlexBailey, Commented Feb 4, 2015 at 20:22

jonrsharpe · Accepted Answer · 2015-02-04 20:31:25Z

8

You need to:

Wrap the names in braces, too; and
Pass the widths as keyword arguments to str.format.

For example:

>>> print("{0:>{number_length}}".format(1, number_length=8))
       1

You can also use dictionary unpacking:

>>> widths = {'number_length': 8}
>>> print("{0:>{number_length}}".format(1, **widths))
       1

str.format won't look in the local scope for appropriate names; they must be passed explicitly.

For your example, this could work like:

>>> widths = {'number_length': 5,
              'name_length': 24,
              'viewers_length': 9}
>>> template= '{0:<{number_length}}{1:<{name_length}}{2:<{viewers_length}}'
>>> print(template.format('#', 'channel', 'visitors', end='', **widths))
#    channel                 visitors

(Note that end, and any other explicit keyword arguments, must come before **widths.)

edited Feb 4, 2015 at 20:31

answered Feb 4, 2015 at 20:25

jonrsharpe

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Comments

vaultah · Accepted Answer · 2015-02-07 09:40:55Z

2

I'd do it like this

In [11]: widths = dict(number=5, name=24, viewers=9)

In [12]: data = ('#', 'channel', 'viewers')

In [13]: '{:<{number}}{:<{name}}{:<{viewers}}'.format(*data, **widths)
Out[13]: '#    channel                 viewers  '

Of course, it's also possible to generate the format string, without actually getting it formatted:

In [14]: '{{:<{number}}}{{:<{name}}}{{:<{viewers}}}'.format(**widths)
Out[14]: '{:<5}{:<24}{:<9}'

edited Feb 7, 2015 at 9:40

answered Feb 4, 2015 at 20:23

vaultah

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1 Comment

jonrsharpe Over a year ago

The extra step with doubled braces is potentially superfluous; you can have nested single braces and pass the widths and values to be formatted at the same time.

Cory Kramer · Accepted Answer · 2015-02-04 20:23:00Z

1

You can build your format string first

f = '{0:<%d}{1:<%d}{2:<%d}' % (number_length, name_length, viewers_length)
#produces
'{0:<5}{1:<24}{2:<9}'

Then use that in your other format call

print(f.format(' #','channel','viewers'), end = ''))

answered Feb 4, 2015 at 20:23

Cory Kramer

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2 Comments

jonrsharpe Over a year ago

It seems rather odd to mix two different string formatting styles!

Spencer Wood Over a year ago

Perfect, thank you. Didn't know you could do this. I also like this because I have to use this formatting more than once, so building it a single time is preferable.

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