Python new test framework should be used (Refer this for more detail)

Python: Random tests are vulnerable to input modification

Asking to encode the string with multiple keys instead of 1 is still a duplicate.

Please use snake_case naming convention:

In order to avoid invalidation of solution, please use this snippet:

try:
  from solution import columnar_cipher
except ImportError:
  from solution import columnarCipher as columnar_cipher

Other than that replace the same in all assertion and change the initial solution accordingly.