Incorrect highlighting because of flaws in ObjectAssignmentParser #359

When checking whether an object is correctly created and the exercise only contains a single = operator in addition to other 'object-mutating' operations that don't use =, highlighting trips up.

Reproducible example (in words)

# solution
x = []
for i in range(3):
    x.append(i)

# sct
Ex().check_object('x').has_equal_value()

If the student submits:

# student
x = []
for i in range(3):
    x.append(2 * i)

The x = [] will be highlighted, even though the mistake is in the x.append() step. This is extremely confusing for students! You can see it in action here

Reproducible example (in code)

from pythonwhat.local import setup_state
from pythonwhat.Test import TestFail
sol_code = "x = []\nfor i in range(3): x.append(i)"
stu_code = "x = []\nfor i in range(3): x.append(2 * i)"
s = setup_state(sol_code, stu_code)
try:
    s.check_object('x').has_equal_value()
except TestFail as e:
    line_info = e.feedback.line_info

line_info

This will print out:

{'column_start': 0, 'line_start': 1, 'line_end': 1, 'column_end': 6}

While it should have a line_start on line_end equal to 2, or just not highlight anything at all.