In [32]:  a=[1,2,3]; i=3
In [33]: '{a}'.format(**locals())
Out[33]: '[1, 2, 3, 4]'
In [34]: '{a[i]}'.format(**locals())
TypeError: list indices must be integers, not str
In [35]: '{a[{i}]}'.format(**locals())
TypeError: list indices must be integers, not str
In [36]: '{a[{0}]}'.format(3, **locals())
TypeError: list indices must be integers, not str


In [37]: '{a[0]}'.format(**locals())
Out[37]: '1'