A list comprehension is a convenient way of writing nested for
loops. The first for
expression is the outmost for
so you need to swap the order in which you have written your for
expressions. As an example:
>>> [ a+b for a in 'san' for b in '123' ]
['s1', 's2', 's3', 'a1', 'a2', 'a3', 'n1', 'n2', 'n3']
>>> for a in 'san':
... for b in '123':
... print a+b,
...
s1 s2 s3 a1 a2 a3 n1 n2 n3