When trying to submit a solution in python, an error pops up

Traceback (most recent call last):
File "/workspace/default/tests.py", line 4, in <module>
    from solution import Molecule, InvalidBond, UnlockedMolecule, LockedMolecule, EmptyMolecule   
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
ImportError: cannot import name 'InvalidBond' from 'solution' (/workspace/default/solution.py)

as well as in the sample tests. When I replaced line 2 in the sample tests with from solution import Molecule, the error dissapeared, but the problem remains when trying to submit a solution in the main tests

When I was trying to submit a python solution, it passed all the fixed tests, but when it got to random I got an error

Traceback (most recent call last):
  File "/workspace/default/tests.py", line 139, in <module>
    for i in range(20): test_fnc(*test_gen(RN(5)+20,RN(6)+7,2))
                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/workspace/default/tests.py", line 136, in test_fnc
    test.assert_equals(blast_sequence(nx,ny),ref_funk(nx,ny))
                                             ^^^^^^^^^^^^^^^
  File "/workspace/default/tests.py", line 53, in ref_funk
    mv = zv + y
         ~~~^~~
TypeError: can only concatenate list (not "int") to list

My solution seems to work with the random test cases, but somehow it still gives the error