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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 _create_rpmmacros(runas='root'):
    '''
    Create the .rpmmacros file in user's home directory
    '''
    home = os.path.expanduser('~')
    rpmbuilddir = os.path.join(home, 'rpmbuild')
    if not os.path.isdir(rpmbuilddir):
        __salt__['file.makedirs_perms'](name=rpmbuilddir, user=runas, group='mock')

    mockdir = os.path.join(home, 'mock')
    if not os.path.isdir(mockdir):
        __salt__['file.makedirs_perms'](name=mockdir, user=runas, group='mock')

    rpmmacros = os.path.join(home, '.rpmmacros')
    with salt.utils.files.fopen(rpmmacros, 'w') as afile:
        afile.write(
            salt.utils.stringutils.to_str('%_topdir {0}\n'.format(rpmbuilddir))
        )
        afile.write('%signature gpg\n')
        afile.write('%_source_filedigest_algorithm 8\n')
        afile.write('%_binary_filedigest_algorithm 8\n')
        afile.write('%_gpg_name packaging@saltstack.com\n')