def bookkeep_reactant(mol):
    """Bookkeep bonds in the reactant.

    Parameters
    ----------
    mol : rdkit.Chem.rdchem.Mol
        RDKit molecule instance for reactants.

    Returns
    -------
    pair_to_bond_type : dict
        Mapping 2-tuples of atoms to bond type. 1, 2, 3, 1.5 are
        separately for single, double, triple and aromatic bond.
    """
    pair_to_bond_type = dict()
    for bond in mol.GetBonds():
        atom1, atom2 = bond.GetBeginAtomIdx(), bond.GetEndAtomIdx()
        atom1, atom2 = min(atom1, atom2), max(atom1, atom2)
        type_val = bond.GetBondTypeAsDouble()
        pair_to_bond_type[(atom1, atom2)] = type_val

    return pair_to_bond_type