python - 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.

Answer 1

You need to:

1. Wrap the names in braces, too; and
2. 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.)