what is "{:<20}" format_string meaning in python? [duplicate]

Question

What does {:<20} {:>4} mean in format_string？

Beginner

popularity = [["Language", 2017, 2012, 2007, 2002, 1997, 1992, 1987], 
          ["Java", 1, 2, 1, 1, 15, 0, 0],
          ["C", 2, 1, 2, 2, 1, 1, 1],
          ["C++", 3, 3, 3, 3, 2, 2, 5],
          ["C#", 4, 4, 7, 13, 0, 0, 0],
          ["Python", 5, 7, 6, 11, 27, 0, 0],
          ["Visual Basic .NET", 6, 17, 0, 0, 0, 0, 0],
          ["PHP", 7, 6, 4, 5, 0, 0, 0],
          ["JavaScript", 8, 9, 8, 7, 23, 0, 0],
          ["Perl", 9, 8, 5, 4, 4, 10, 0]]

format_string = "{:<20}  {:>4}  {:>4}  {:>4}  {:>4}  {:>4}  {:>4}  {:>4}"

Answer 1

Doc here:

format_spec ::= [[fill]align][sign][#][0][width][grouping_option][.precision][type]

'<': Forces the field to be left-aligned within the available space (this is the default for most objects).

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

So {:<20} means left-aligned with width 20. Similarly, {:>4} means right-aligned with width 4.

To see how it works:

for l in popularity: print(format_string.format(*l))

Output:

Language              2017  2012  2007  2002  1997  1992  1987
Java                     1     2     1     1    15     0     0
C                        2     1     2     2     1     1     1
C++                      3     3     3     3     2     2     5
C#                       4     4     7    13     0     0     0
Python                   5     7     6    11    27     0     0
Visual Basic .NET        6    17     0     0     0     0     0
PHP                      7     6     4     5     0     0     0
JavaScript               8     9     8     7    23     0     0
Perl                     9     8     5     4     4    10     0

By the way, Python ranked Top 1 in 2017, and stays on top in 2018.