Buggy python submission (verdict: Runtime Error):
n,m = (int(x) for x in input().split())
friendlist = [0]*(n+1)
classes = 0
for i in range(m):
    a,b = (int(x)-1 for x in input().split())
    if friendlist[a]!=0:
        friendlist[b] = friendlist[a]
    elif friendlist[b]!=0:
        friendlist[a] = friendlist[b]
    else:
        classes += 1
        friendlist[a] = classes
        friendlist[b] = classes

count = [0]*(classes+1)
for i in range(n):
    count[friendlist[i]] += 1
print(max(count[1:]))


Fixed submission:
import os

n,m = (int(x) for x in input().split())

#Union Find

#xの根を求める
def find(x):
    if par[x] < 0:
        return x
    else:
        par[x] = find(par[x])
        return par[x]
#xとyの属する集合の併合
def unite(x,y):
    x = find(x)
    y = find(y)

    if x == y:
        return False
    else:
        #sizeの大きいほうがx
        if par[x] > par[y]:
            x,y = y,x
        par[x] += par[y]
        par[y] = x
        return True

#xとyが同じ集合に属するかの判定
def same(x,y):
    return find(x) == find(y)

#xが属する集合の個数
def size(x):
    return -par[find(x)]

#初期化
#根なら-size,子なら親の頂点
par = [-1]*n

for i in range(m):
    a,b = (int(x)-1 for x in input().split())
    unite(a,b)

ans = 0
for i in range(0,n):
    ans = max([ans,size(i)])

print(ans)

Buggy python submission (verdict: Runtime Error):
n,m = (int(x) for x in input().split())
friendlist = [0]*n
classes = 1
for i in range(m):
    a,b = (int(x)-1 for x in input().split())
    if friendlist[a]!=0:
        friendlist[b] = friendlist[a]
    elif friendlist[b]!=0:
        friendlist[a] = friendlist[b]
    else:
        friendlist[a] = classes
        friendlist[b] = classes
        classes += 1

count = [0]*classes
for i in range(n):
    count[friendlist[i]] += 1

print(max(count[1:]))

Fixed submission:
import os

n,m = (int(x) for x in input().split())

#Union Find

#xの根を求める
def find(x):
    if par[x] < 0:
        return x
    else:
        par[x] = find(par[x])
        return par[x]
#xとyの属する集合の併合
def unite(x,y):
    x = find(x)
    y = find(y)

    if x == y:
        return False
    else:
        #sizeの大きいほうがx
        if par[x] > par[y]:
            x,y = y,x
        par[x] += par[y]
        par[y] = x
        return True

#xとyが同じ集合に属するかの判定
def same(x,y):
    return find(x) == find(y)

#xが属する集合の個数
def size(x):
    return -par[find(x)]

#初期化
#根なら-size,子なら親の頂点
par = [-1]*n

for i in range(m):
    a,b = (int(x)-1 for x in input().split())
    unite(a,b)

ans = 0
for i in range(0,n):
    ans = max([ans,size(i)])

print(ans)

Buggy python submission (verdict: Runtime Error):
n,m = (int(x) for x in input().split())
friendlist = [0]*n
classes = 1
for i in range(m):
    a,b = (int(x)-1 for x in input().split())
    if friendlist[a]!=0:
        friendlist[b] = friendlist[a]
    elif friendlist[b]!=0:
        friendlist[a] = friendlist[b]
    else:
        friendlist[a] = classes
        friendlist[b] = classes
        classes += 1

count = [0]*classes
for i in range(n):
    count[friendlist[i]] += 1

print(count)
print(max(count[1:]))


Fixed submission:
import os

n,m = (int(x) for x in input().split())

#Union Find

#xの根を求める
def find(x):
    if par[x] < 0:
        return x
    else:
        par[x] = find(par[x])
        return par[x]
#xとyの属する集合の併合
def unite(x,y):
    x = find(x)
    y = find(y)

    if x == y:
        return False
    else:
        #sizeの大きいほうがx
        if par[x] > par[y]:
            x,y = y,x
        par[x] += par[y]
        par[y] = x
        return True

#xとyが同じ集合に属するかの判定
def same(x,y):
    return find(x) == find(y)

#xが属する集合の個数
def size(x):
    return -par[find(x)]

#初期化
#根なら-size,子なら親の頂点
par = [-1]*n

for i in range(m):
    a,b = (int(x)-1 for x in input().split())
    unite(a,b)

ans = 0
for i in range(0,n):
    ans = max([ans,size(i)])

print(ans)