Buggy python submission (verdict: Runtime Error):
# -*- coding: utf-8 -*-

n, m = map(int, input().split())
friend_list = []
friend_list2 = []
for i in range(m):
  a, b = map(int, input().split())
  friend_list.append([a, b])
while friend_list:
  friend = friend_list.pop()
  if not friend_list2:
    friend_list2.append(friend)
  else:
    temp_list = friend_list2.copy()
    for i in temp_list:
      if friend[0] in i and friend[1] not in i:
        i.append(friend[1])
        break
      elif friend[1] in i and friend[0] not in i:
        i.append(friend[0])
        break
      elif friend[0] in i and friend[1] in i:
        break
      else:
        friend_list2.append(friend)

print(max(map(len, friend_list2)))

Fixed submission:
# -*- coding: utf-8 -*-
import numpy
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())
uf = UnionFind(n)
for i in range(m):
    a, b = map(int, input().split())
    uf.union(a-1, b-1)
answer = []
for i in range(n):
    answer.append(uf.size(i))
print(max(answer))

Buggy python submission (verdict: Runtime Error):
# -*- coding: utf-8 -*-

n, m = map(int, input().split())
friend_list = []
friend_list2 = []
for i in range(m):
  a, b = map(int, input().split())
  friend_list.append([a, b])
while friend_list:
  friend = friend_list.pop()
  if not friend_list2:
    friend_list2.append(friend)
  else:
    temp_list = friend_list2.copy()
    for i in temp_list:
      if friend[0] in i:
        i.append(friend[1])
        break
      elif friend[1] in i:
        i.append(friend[0])
        break
      else:
        friend_list2.append(friend)
friend_list3 = []
for i in friend_list2:
  friend_list3.append(set(i))
friend_list2 = []
for i in friend_list3:
  friend_list2.append(len(i))
print(max(friend_list2))

Fixed submission:
# -*- coding: utf-8 -*-
import numpy
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())
uf = UnionFind(n)
for i in range(m):
    a, b = map(int, input().split())
    uf.union(a-1, b-1)
answer = []
for i in range(n):
    answer.append(uf.size(i))
print(max(answer))