from .unique import unique
from .unique import NotUnique


class NotAMatrix(Exception):
    """marker exception for matrix conformity"""


def transpose(A):
    """transpose a 2D matrix, `A`"""

    if not A:
        # empty matrix
        return A

    # Asset that all rows have the same length
    try:
        length = unique([len(row) for row in A])
    except NotUnique as e:
        raise NotAMatrix("Multiple number of columns detected: {}".format(sorted(e.multiples)))

    # perform the transpose
    transposed = [[None for i in range(len(A))]
                  for j in range(length)]
    for i, row in enumerate(A):
        for j, col in enumerate(row):
            transposed[j][i] = col
    return transposed