python的format打印格式'{0:2d} @ {1:2d} {2}{0:<2d}'的含义

# BEGIN BISECT_DEMO
import bisect
import sys

HAYSTACK = [1, 4, 5, 6, 8, 12, 15, 20, 21, 23, 23, 26, 29, 30]
NEEDLES = [0, 1, 2, 5, 8, 10, 22, 23, 29, 30, 31]

ROW_FMT = '{0:2d} @ {1:2d}    {2}{0:<2d}'

def demo(bisect_fn):
    for needle in reversed(NEEDLES):
        position = bisect_fn(HAYSTACK, needle)  # <1>
        offset = position * '  |'  # <2>
        print(ROW_FMT.format(needle, position, offset))  # <3>

if __name__ == '__main__':

    if sys.argv[-1] == 'left':    # <4>
        bisect_fn = bisect.bisect_left
    else:
        bisect_fn = bisect.bisect

    print('DEMO:', bisect_fn.__name__)  # <5>
    print('haystack ->', ' '.join('%2d' % n for n in HAYSTACK))
    demo(bisect_fn)

这段代码，有个地方不太理解为啥，打印的时候传参传了3个，打印的值实际只有3个，后面百度了下format的用法才理解

ROW_FMT = '{0:2d} @ {1:2d}    {2}{0:<2d}'

1、{0:2d}表示的是取第一个值，d表示数字，2表示2位数
2、{0:<2d}同上，区别是<表示的左对齐