In a bash terminal, obtained result: 

$python > Normal.txt 2> Error.txt
import a
$cat Normal.txt 
$cat Error.txt 
Python 2.7.12 (default, Dec  4 2017, 14:50:18) 
[GCC 5.4.0 20160609] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ImportError: No module named a
>>> 
$


That i was expected  :

$python > Normal.txt 2> Error.txt
import a
$cat Normal.txt 

Python 2.7.12 (default, Dec  4 2017, 14:50:18) 
[GCC 5.4.0 20160609] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> $cat Error.txt 
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ImportError: No module named a
$