Join string before, between, and after

Suppose I have this list:

lis = ['a','b','c','d']

If I do 'x'.join(lis) the result is:

'axbxcxd'

What would be a clean, simple way to get this output?

'xaxbxcxdx'

I could write a helper function:

def joiner(s, it):
    return s+s.join(it)+s

and call it like joiner('x',lis) which returns xaxbxcxdx, but it doesn't look as clean as it could be. Is there a better way to get this result?

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