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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 diff_sizes(a, b, progressbar=None):
 """Return list of tuples where sizes differ. Tuple structure: (identifier, size in a, size in b) Assumes list of identifiers in a and b are identical. :param a: first :class:`dtoolcore.DataSet` :param b: second :class:`dtoolcore.DataSet` :returns: list of tuples for all items with different sizes """

    difference = []

    for i in a.identifiers:
        a_size = a.item_properties(i)["size_in_bytes"]
        b_size = b.item_properties(i)["size_in_bytes"]
        if a_size != b_size:
            difference.append((i, a_size, b_size))
        if progressbar:
            progressbar.update(1)

    return difference