Using the f-string format becomes very easy nowadays.

If you were using

print(f'{token:10}')

And you want the 10 to be another variable (for example the max length of all the tokens), you would write

print(f'{token:{maxTokenLength}}')

In other words, enclose the variable within {}

In your particular case, all you need is this.

head = 'eggs', 'bacon', 'spam'  
w1, w2, w3 = 8, 7, 10  # column widths  

print(f'  {head[0]:>{w1}}  {head[1]:>{w2}}  {head[2]:>{w3}}')
print(f'  {"="*w1:>{w1}}  {"="*w2:>{w2}}  {"="*w3:>{w3}}')

Which produces

      eggs    bacon        spam
  ========  =======  ==========

If you are using it in a formatted string with the format() method which is preferred over the older style ''% formatting

>>> 'One hundred and twenty three with three leading zeros {0:06}.'.format(123)
'One hundred and twenty three with three leading zeros 000123.'

See
http://docs.python.org/library/stdtypes.html#str.format
http://docs.python.org/library/string.html#formatstrings

Here is an example with variable width

>>> '{num:0{width}}'.format(num=123, width=6)
'000123'

You can even specify the fill char as a variable

>>> '{num:{fill}{width}}'.format(num=123, fill='0', width=6)
'000123'

In this statement print(f"|{students[i+j]:^{CELL_WIDTH}}", end='') the data type of variable CELL_WIDTH is float which is supposed to be int ..

Change the statement CELL_WIDTH = (LINE_WIDTH - STUDENTS_PER_ROW - 1) / STUDENTS_PER_ROW with CELL_WIDTH = (LINE_WIDTH - STUDENTS_PER_ROW - 1) // STUDENTS_PER_ROW which will make CELL_WIDTH datatype as int..

You can do this using the str.format() method.

>>> width = 20
>>> print("{:>{width}} : {:>{width}}".format("Python", "Very Good", width=width))
              Python :            Very Good

Starting from Python 3.6 you can use f-string to do this:

In [579]: lang = 'Python'

In [580]: adj = 'Very Good'

In [581]: width = 20

In [582]: f'{lang:>{width}}: {adj:>{width}}'
Out[582]: '              Python:            Very Good'

Encase SHOWLEN in brackets

"{varname:<{SHOWLEN}.{SHOWLEN}f}".format(varname=34.54, SHOWLEN=8)

This is evident from the following quote from the Format String Syntax documentation:

A format_spec field can also include nested replacement fields within it. These nested replacement fields can contain only a field name; conversion flags and format specifications are 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.

I find using str.format much more elegant:

>>> '{0: <5}'.format('s')
's    '
>>> '{0: <5}'.format('ss')
'ss   '
>>> '{0: <5}'.format('sss')
'sss  '
>>> '{0: <5}'.format('ssss')
'ssss '
>>> '{0: <5}'.format('sssss')
'sssss'

In case you want to align the string to the right use > instead of <:

>>> '{0: >5}'.format('ss')
'   ss'

Edit 1: As mentioned in the comments: the 0 in '{0: <5}' indicates the argument’s index passed to str.format().

Edit 2: In python3 one could use also f-strings:

sub_str='s'
for i in range(1,6):
    s = sub_str*i
    print(f'{s:>5}')
    
'    s'
'   ss'
'  sss'
' ssss'
'sssss'

or:

for i in range(1,5):
    s = sub_str*i
    print(f'{s:<5}')
's    '
'ss   '
'sss  '
'ssss '
'sssss'

of note, in some places above, ' ' (single quotation marks) were added to emphasize the width of the printed strings.

Per the Python Format Specification Mini-Language, alignment specifiers (e.g. <) must precede the width specifier (e.g. 15). With this criteria in mind, the correct formulation for your format string is {:<15}. However, left-alignment is inferred by default for strings, so you can write this simply as {:15}.

>>> print(f'{"string":<15}|')
string         |
>>> print(f'{"string":15}|')
string         |

You can do a one-liner with strings using the following format:

print(f"{'One':>10}")
#        One

See the docs here. Note the relevant section:

> Forces the field to be right-aligned within the available space (this is the default for numbers).

(Emphasis added)

This is why you don't need the > when formatting numbers.