HackerRank issue or just me?

brace_match.py

"""
Go to the following link and paste the below code: https://www.hackerrank.com/challenges/ctci-balanced-brackets/problem

Issue: An empty deque (or list) "magically" has uninserted values present when initialized.

Steps to reproduce:
1) Run this code to see it passes the test cases.
2) Comment out the __init__() method in the stack, and replace "contents = None" with "contents = deque()" or "contents = []"
3) Run to observe it fails the test cases.
4) Beneath "s = stack()" in is_matched(), create a new line and put "s.dump()" on it.
5) Re-run the sample test cases. Observe that the third case prints "['{', '[']"

Expected output:
On step 5, the third test case should print "[]"
"""

from collections import *

openers = ['(','[','{']
closers = [')',']', '}']
parens = ['(', ')']
squares = ['[', ']']
curlys = ['{', '}']


class stack:
    contents = None
    
    def __init__(self):
        self.contents = deque()
    
    def dump(self):
        print(self.contents)
    
    def size(self):
        return len(self.contents)
    
    def push(self, val):
        self.contents.append(val)
    
    def pop(self):
        if len(self.contents) == 0:
            return ''
        return self.contents.pop()
        

# test_type(): determine if two chars are same type of brace
def test_type(a, b):
    are_parens = (a in parens) and (b in parens)
    are_curlys = (a in curlys) and (b in curlys)
    are_squares = (a in squares) and (b in squares)
    return (are_parens or are_curlys or are_squares)


# is_matched(): determine if a string is a balanced bracket string
def is_matched(expression):
    s = stack()
    for char in expression:
        if char in openers:
            s.push(char)
        else:
            recent_opener = s.pop()
            if not test_type(recent_opener, char):
                return False
    
    # Catch unmatched left braces
    if (s.size() > 0):
        return False
    else:
        return True    

t = int(input().strip())
for a0 in range(t):
    expression = input().strip()
    if is_matched(expression) == True:
        print("YES")
    else:
        print("NO")