queue implemented with a pair of stacks

staq.py

class staq:
    def __init__(self, stack = []):
        self.enqstk = stack
        self.deqstk = []
    def _swap(self):
        if len(self.deqstk) == 0:
            while len(self.enqstk) > 0:
                _pus(self.deqstk, _pop(self.enqstk))
    def enq(self, item):
        _pus(self.enqstk, item)
    def deq(self):
        self._swap()
        return _pop(self.deqstk)
    def __str__(self):
        return "%s -> %s" % (self.enqstk,self.deqstk)
    def __repr__(self):
        return str(self)
def _pop(stack):
    out = stack[-1]
    stack[:] = stack[:-1]
    return out
def _pus(stack, item):
    stack += [item]

if __name__ == "__main__":
    q = staq([1,2,3])
    print("q = staq([1,2,3]) : %s" % (q,))
    print("q.deq() -> %s : %s" % (q.deq(), q))
    q.enq(4)
    print("q.enq(4)     : %s" % (q,))
    print("q.deq() -> %s : %s" % (q.deq(), q))
    print("q.deq() -> %s : %s" % (q.deq(), q))
    print("q.deq() -> %s : %s" % (q.deq(), q))