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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def get_linearoperator(shape, A, timer=None):
 """Enhances aslinearoperator if A is None."""

    ret = None
    import scipy.sparse.linalg as scipylinalg
    if isinstance(A, LinearOperator):
        ret = A
    elif A is None:
        ret = IdentityLinearOperator(shape)
    elif isinstance(A, numpy.ndarray) or isspmatrix(A):
        ret = MatrixLinearOperator(A)
    elif isinstance(A, numpy.matrix):
        ret = MatrixLinearOperator(numpy.atleast_2d(numpy.asarray(A)))
    elif isinstance(A, scipylinalg.LinearOperator):
        if not hasattr(A, 'dtype'):
            raise ArgumentError('scipy LinearOperator has no dtype.')
        ret = LinearOperator(A.shape, dot=A.matvec, dot_adj=A.rmatvec,
                             dtype=A.dtype)
    else:
        raise TypeError('type not understood')

    # set up timer if requested
    if A is not None and not isinstance(A, IdentityLinearOperator) \
            and timer is not None:
        ret = TimedLinearOperator(ret, timer)

    # check shape
    if shape != ret.shape:
        raise LinearOperatorError('shape mismatch')

    return ret