Buggy python submission (verdict: Runtime Error):
def zero_origin(s: str):
  return int(s) - 1


class UnionFindTree:
  """Union Find Tree for disjoint-set data structure.

  Args:
    n (int): number of the vertices.
  """
  def __init__(self, n: int):
    self.parent = list(range(n))
    self.rank = [0] * n
    self.size = [1] * n

  def find(self, i: int) -> int:
    if self.parent[i] != i:
      self.parent[i] = self.find(self.parent[i])
    return self.parent[i]

  def unite(self, i: int, j: int):
    ri = self.find(i)
    rj = self.find(j)
    size = self.size[ri] + self.size[rj]
    self.size[ri] = size
    self.size[rj] = size
    if ri != rj:
      if self.rank[ri] < self.rank[rj]:
        self.parent[ri] = rj
      else:
        self.parent[rj] = ri
        if self.rank[ri] == self.rank[rj]:
          self.rank[ri] += 1

  def is_same(self, i: int, j: int) -> bool:
    return self.find(i) == self.find(j)


n, m = map(int, input().split())
uft = UnionFindTree(n)
for _ in range(m):
  a, b = map(zero_origin, input().split())
  uft.unite(a, b)
print(max(utf.size))

Fixed submission:
def zero_origin(s: str):
  return int(s) - 1


class UnionFindTree:
  """Union Find Tree for disjoint-set data structure.

  Args:
    n (int): number of the vertices.
  """
  def __init__(self, n: int):
    self.parent = list(range(n))
    self.rank = [0] * n
    self.size = [1] * n

  def find(self, i: int) -> int:
    if self.parent[i] != i:
      self.parent[i] = self.find(self.parent[i])
    return self.parent[i]

  def unite(self, i: int, j: int):
    ri = self.find(i)
    rj = self.find(j)
    if ri != rj:
      if self.rank[ri] < self.rank[rj]:
        self.parent[ri] = rj
        self.size[rj] += self.size[ri]
      else:
        self.parent[rj] = ri
        self.size[ri] += self.size[rj]
        if self.rank[ri] == self.rank[rj]:
          self.rank[ri] += 1

  def is_same(self, i: int, j: int) -> bool:
    return self.find(i) == self.find(j)


n, m = map(int, input().split())
uft = UnionFindTree(n)
for _ in range(m):
  a, b = map(zero_origin, input().split())
  uft.unite(a, b)
print(max(uft.size))

Buggy python submission (verdict: Runtime Error):
def zero_origin(s: str):
  return int(s) - 1


class UnionFindTree:
  """Union Find Tree for disjoint-set data structure.

  Args:
    n (int): number of the vertices.
  """
  def __init__(self, n: int):
    self.parent = list(range(n))
    self.rank = [0] * n
    self.size = [1] * n

  def find(self, i: int) -> int:
    if self.parent[i] != i:
      self.parent[i] = self.find(self.parent[i])
    return self.parent[i]

  def unite(self, i: int, j: int):
    ri = self.find(i)
    rj = self.find(j)
    size = self.size[ri] + self.size[rj]
    self.size[ri] = size
    self.size[rj] = size
    if ri != rj:
      if self.rank[ri] < self.rank[rj]:
        self.parent[ri] = rj
      else:
        self.parent[rj] = ri
        if self.rank[ri] == self.rank[rj]:
          self.rank[ri] += 1

  def is_same(self, i: int, j: int) -> bool:
    return self.find(i) == self.find(j)


n, m = map(int, input().split())
uft = UnionFindTree(n)
for _ in range(m):
  a, b = map(zero_origin, input().split())
  uft.unite(a, b)
print(max(utf.size))

Fixed submission:
def zero_origin(s: str):
  return int(s) - 1


class UnionFindTree:
  """Union Find Tree for disjoint-set data structure.

  Args:
    n (int): number of the vertices.
  """
  def __init__(self, n: int):
    self.parent = list(range(n))
    self.rank = [0] * n
    self.size = [1] * n

  def find(self, i: int) -> int:
    if self.parent[i] != i:
      self.parent[i] = self.find(self.parent[i])
    return self.parent[i]

  def unite(self, i: int, j: int):
    ri = self.find(i)
    rj = self.find(j)
    if ri != rj:
      if self.rank[ri] < self.rank[rj]:
        self.parent[ri] = rj
        self.size[rj] += self.size[ri]
      else:
        self.parent[rj] = ri
        self.size[ri] += self.size[rj]
        if self.rank[ri] == self.rank[rj]:
          self.rank[ri] += 1

  def is_same(self, i: int, j: int) -> bool:
    return self.find(i) == self.find(j)


n, m = map(int, input().split())
uft = UnionFindTree(n)
for _ in range(m):
  a, b = map(zero_origin, input().split())
  uft.unite(a, b)
print(max(uft.size))