dfdict['a'] = [1, 2, 3, 4]
dfdict['b'] = [5, 6, 7, 8]
dfdict['c'] = [9, 10, 11, 12]
df = pandas.DataFrame(dfdict)
df.to_csv('dfTest.txt', '\t', header=True, cols=['b', 'a', 'c'])
Сохранение порядка столбцов в Python Pandas DataFrame