Buggy python submission (verdict: Time Limit Exceeded):
from collections import defaultdict

class UnionFind():
  def __init__(self, n):
    self.n = n
    self.parents = [-1] * n
    
  def find(self, x):
    if self.parents[x] < 0:
      return x
    else:
      self.parents[x] = self.find(self.parents[x])
      return self.parents[x]

  def union(self, x, y):
    x = self.find(x)
    y = self.find(y)

    if x == y:
        return

    if self.parents[x] > self.parents[y]:
        x, y = y, x

    self.parents[x] += self.parents[y]
    self.parents[y] = x

  def size(self, x):
    return -self.parents[self.find(x)]

  def same(self, x, y):
    return self.find(x) == self.find(y)

  def members(self, x):
    root = self.find(x)
    return [i for i in range(self.n) if self.find(i) == root]

  def roots(self):
    return [i for i, x in enumerate(self.parents) if x < 0]

  def group_count(self):
    return len(self.roots())

  def all_group_members(self):
    return {r: self.members(r) for r in self.roots()}

  def __str__(self):
    return '\n'.join('{}: {}'.format(r, self.members(r)) for r in self.roots())

n, m = map(int, input().split())
AB = [list(map(int, input().split())) for _ in range(m)]
  
uf = UnionFind(n)
for a, b in AB:
  a -= 1
  b -= 1
  uf.union(a, b)

ans = 0
for _, v in uf.all_group_members().items():
  ans = max(ans, len(v))
  
print(ans)

Fixed submission:
from collections import defaultdict

class UnionFind():
  def __init__(self, n):
    self.n = n
    self.parents = [-1] * n
    
  def find(self, x):
    if self.parents[x] < 0:
      return x
    else:
      self.parents[x] = self.find(self.parents[x])
      return self.parents[x]

  def union(self, x, y):
    x = self.find(x)
    y = self.find(y)

    if x == y:
        return

    if self.parents[x] > self.parents[y]:
        x, y = y, x

    self.parents[x] += self.parents[y]
    self.parents[y] = x

  def size(self, x):
    return -self.parents[self.find(x)]

  def same(self, x, y):
    return self.find(x) == self.find(y)

  def members(self, x):
    root = self.find(x)
    return [i for i in range(self.n) if self.find(i) == root]

  def roots(self):
    return [i for i, x in enumerate(self.parents) if x < 0]

  def group_count(self):
    return len(self.roots())

  def all_group_members(self):
    return {r: self.members(r) for r in self.roots()}

  def __str__(self):
    return '\n'.join('{}: {}'.format(r, self.members(r)) for r in self.roots())

n, m = map(int, input().split())
AB = [list(map(int, input().split())) for _ in range(m)]
  
uf = UnionFind(n)
for a, b in AB:
  a -= 1
  b -= 1
  uf.union(a, b)

ans = 0
for v in uf.roots():
  ans = max(ans, uf.size(v))
  
print(ans)

Buggy python submission (verdict: Time Limit Exceeded):
from collections import defaultdict

class UnionFind():
  def __init__(self, n):
    self.n = n
    self.parents = [-1] * n
    
  def find(self, x):
    if self.parents[x] < 0:
      return x
    else:
      self.parents[x] = self.find(self.parents[x])
      return self.parents[x]

  def union(self, x, y):
    x = self.find(x)
    y = self.find(y)

    if x == y:
        return

    if self.parents[x] > self.parents[y]:
        x, y = y, x

    self.parents[x] += self.parents[y]
    self.parents[y] = x

  def size(self, x):
    return -self.parents[self.find(x)]

  def same(self, x, y):
    return self.find(x) == self.find(y)

  def members(self, x):
    root = self.find(x)
    return [i for i in range(self.n) if self.find(i) == root]

  def roots(self):
    return [i for i, x in enumerate(self.parents) if x < 0]

  def group_count(self):
    return len(self.roots())

  def all_group_members(self):
    return {r: self.members(r) for r in self.roots()}

  def __str__(self):
    return '\n'.join('{}: {}'.format(r, self.members(r)) for r in self.roots())

n, m = map(int, input().split())
AB = [list(map(int, input().split())) for _ in range(m)]
  
uf = UnionFind(n)
for a, b in AB:
  a -= 1
  b -= 1
  uf.union(a, b)

ans = 0
for _, v in uf.all_group_members().items():
  ans = max(ans, len(v))
  
print(ans)

Fixed submission:
from collections import defaultdict

class UnionFind():
  def __init__(self, n):
    self.n = n
    self.parents = [-1] * n
    
  def find(self, x):
    if self.parents[x] < 0:
      return x
    else:
      self.parents[x] = self.find(self.parents[x])
      return self.parents[x]

  def union(self, x, y):
    x = self.find(x)
    y = self.find(y)

    if x == y:
        return

    if self.parents[x] > self.parents[y]:
        x, y = y, x

    self.parents[x] += self.parents[y]
    self.parents[y] = x

  def size(self, x):
    return -self.parents[self.find(x)]

  def same(self, x, y):
    return self.find(x) == self.find(y)

  def members(self, x):
    root = self.find(x)
    return [i for i in range(self.n) if self.find(i) == root]

  def roots(self):
    return [i for i, x in enumerate(self.parents) if x < 0]

  def group_count(self):
    return len(self.roots())

  def all_group_members(self):
    return {r: self.members(r) for r in self.roots()}

  def __str__(self):
    return '\n'.join('{}: {}'.format(r, self.members(r)) for r in self.roots())

n, m = map(int, input().split())
AB = [list(map(int, input().split())) for _ in range(m)]
  
uf = UnionFind(n)
for a, b in AB:
  a -= 1
  b -= 1
  uf.union(a, b)

ans = 0
for v in uf.roots():
  ans = max(ans, uf.size(v))
  
print(ans)