How to format a string using variables instead of numbers within curly braces?

I'm using Python 3.7 and I have stored a value inside a variable. This variable holds the value of padding which I want to use within curly braces for string formatting. The code explains what I am trying to do.

def print_formatted(number):
    for i in range(1, number + 1):
        binum = bin(i).replace('0b', '')
        ocnum = oct(i).replace('0o', '')
        hexnum = hex(i).replace('0x', '')

        length = len(bin(number).replace('0b', ''))
        print('{0:>length} {1:>length} {2:>length} {3:>length}'.format(i, ocnum, hexnum, binum)) # Error here

This is the code that I have been trying to run. What I am trying to do is to right align the numbers by padding it by the value of the length of the last binary number.

> ValueError: Invalid format specifier

This is the error I get. What am I doing wrong?

You can use f-strings and also format specifiers to avoid use of the hex, oct and bin builtins and then string slicing and use int.bit_length() instead of taking the length of the binary string, eg:

def print_formatted(number):
    # get number of bits required to store number
    w = number.bit_length()
    for n in range(1, number + 1):
        # print each number as decimal, then octal, then hex, then binary with padding
        print(f'{n:>{w}} {n:>{w}o} {n:>{w}x} {n:>{w}b}')

Running print_formatted(20) will give you:

    1     1     1     1
    2     2     2    10
    3     3     3    11
    4     4     4   100
    5     5     5   101
    6     6     6   110
    7     7     7   111
    8    10     8  1000
    9    11     9  1001
   10    12     a  1010
   11    13     b  1011
   12    14     c  1100
   13    15     d  1101
   14    16     e  1110
   15    17     f  1111
   16    20    10 10000
   17    21    11 10001
   18    22    12 10010
   19    23    13 10011
   20    24    14 10100

You must first incorporate the value of the variable length into your format string and, strangely, using .format() is the best way to do it.

Change the following line

print('{0:>length} {1:>length} {2:>length} {3:>length}'.format(i, ocnum, hexnum, binum))

to

print('{{0:>{length}}} {{1:>{length}}} {{2:>{length}}} {{3:>{length}}}'.format(length=length).format(i, ocnum, hexnum, binum))

You can use f-strings.

def print_formatted(number):
    length = len(bin(number)) - 2
    for i in range(1, number + 1):
        ocnum = oct(i)[2:]
        hexnum = hex(i)[2:]
        binum = bin(i)[2:]
        print(' '.join(f'{n:>{length}}' for n in (i, ocnum, hexnum, binum)))

>>> print_formatted(20)
    1     1     1     1
    2     2     2    10
    3     3     3    11
    4     4     4   100
    5     5     5   101
    6     6     6   110
    7     7     7   111
    8    10     8  1000
    9    11     9  1001
   10    12     a  1010
   11    13     b  1011
   12    14     c  1100
   13    15     d  1101
   14    16     e  1110
   15    17     f  1111
   16    20    10 10000
   17    21    11 10001
   18    22    12 10010
   19    23    13 10011
   20    24    14 10100

I usually do it in two steps, creating a format string first. It works by escaping the braces - {{,}} - that need to remain in the format string.

>>> length = 4
>>> f'{{:>{length}}} {{:>{length}}} {{:>{length}}} {{:>{length}}}'
'{:>4} {:>4} {:>4} {:>4}'
>>> 


fmt = f'{{:>{length}}} {{:>{length}}} {{:>{length}}} {{:>{length}}}'
print(fmt.format(i, ocnum, hexnum, binum))