Some built-in to pad a list in python

a += [''] * (N - len(a))

or if you don't want to change a in place

new_a = a + [''] * (N - len(a))

you can always create a subclass of list and call the method whatever you please

class MyList(list):
    def ljust(self, n, fillvalue=''):
        return self + [fillvalue] * (n - len(self))

a = MyList(['1'])
b = a.ljust(5, '')

I think this approach is more visual and pythonic.

a = (a + N * [''])[:N]

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