Buggy python submission (verdict: Runtime Error):
import sys
input = sys.stdin.readline

def main():
    n , m = map( int, input().split() )
    Sets = []

    Dicto = [ -1 for i in range(n + 1)]

    for i in range( m ):
        a , b = map( int , input().split() )
        if Dicto[a] == -1 and Dicto[b] == -1:
            Sets.append( set( [a,b] ) )
            x_i = len(Sets)
            Dicto[ a ] = x_i - 1
            Dicto[ b ] = x_i - 1
        elif Dicto[ a ] != -1 and Dicto[ b ] == -1:
            Sets[ Dicto[a] ].add( b )
            Dicto[ b ] = Dicto[a]
        elif Dicto[ b ] != -1 and Dicto[ a ] == -1:
            Sets[ Dicto[b] ].add( a )
            Dicto[ a ] = Dicto[b]
        elif Dicto[ b ] != -1 and Dicto[ a ] != -1 and Dicto[ a ] != Dicto[ b ]:
            if Dicto[ b ] > Dicto[ a ]:
                set_x = Sets.pop( Dicto[b] )
                Sets[ Dicto[a] ] |= ( set_x )
                l_set_x = list( set_x )
                for j in l_set_x:
                    Dicto[j] = Dicto[a]

            else:
                set_x = Sets.pop( Dicto[a] )
                Sets[ Dicto[b] ] |= ( set_x )
                l_set_x = list( set_x )
                for j in l_set_x:
                    Dicto[j] = Dicto[b]

    #print(Sets)
    #print(Dicto)

    max_len = 1

    for i in range( len(Sets) ):
        if max_len < len( Sets[i] ):
            max_len = len( Sets[i] )

    print(max_len)


main()

Fixed submission:
import sys
input = sys.stdin.readline

def main():
    n , m = map( int, input().split() )
    Sets = []

    Dicto = [ -1 for i in range(n + 1)]

    for i in range( m ):
        a , b = map( int , input().split() )
        if Dicto[a] == -1 and Dicto[b] == -1:
            Sets.append( set( [a,b] ) )
            x_i = len(Sets)
            Dicto[ a ] = x_i - 1
            Dicto[ b ] = x_i - 1
        elif Dicto[ a ] != -1 and Dicto[ b ] == -1:
            Sets[ Dicto[a] ].add( b )
            Dicto[ b ] = Dicto[a]
        elif Dicto[ b ] != -1 and Dicto[ a ] == -1:
            Sets[ Dicto[b] ].add( a )
            Dicto[ a ] = Dicto[b]
        elif Dicto[ b ] != -1 and Dicto[ a ] != -1 and Dicto[ a ] != Dicto[ b ]:
            if Dicto[ b ] > Dicto[ a ]:
                set_x = Sets[Dicto[b]]
                Sets[ Dicto[a] ] |= ( set_x )
                l_set_x = list( set_x )
                for j in l_set_x:
                    Dicto[j] = Dicto[a]

            else:
                set_x = Sets[Dicto[a]]
                Sets[ Dicto[b] ] |= ( set_x )
                l_set_x = list( set_x )
                for j in l_set_x:
                    Dicto[j] = Dicto[b]

    #print(Sets)
    #print(Dicto)

    max_len = 1

    for i in range( len(Sets) ):
        if max_len < len( Sets[i] ):
            max_len = len( Sets[i] )

    print(max_len)


main()

Buggy python submission (verdict: Runtime Error):
import sys
input = sys.stdin.readline

def main():
    n , m = map( int, input().split() )
    Sets = []

    Dicto = [ -1 for i in range(n + 1)]

    for i in range( m ):
        a , b = map( int , input().split() )
        if Dicto[a] == -1 and Dicto[b] == -1:
            Sets.append( set( [a,b] ) )
            x_i = len(Sets)
            Dicto[ a ] = x_i - 1
            Dicto[ b ] = x_i - 1
        elif Dicto[ a ] != -1 and Dicto[ b ] == -1:
            Sets[ Dicto[a] ].add( b )
            Dicto[ b ] = Dicto[a]
        elif Dicto[ b ] != -1 and Dicto[ a ] == -1:
            Sets[ Dicto[b] ].add( a )
            Dicto[ a ] = Dicto[b]
        elif Dicto[ b ] != -1 and Dicto[ a ] != -1 and Dicto[ a ] != Dicto[ b ]:
            print('!!' , Dicto[ a ] , Dicto[ b ])
            if Dicto[ b ] > Dicto[ a ]:
                set_x = Sets.pop( Dicto[b] )
                Sets[ Dicto[a] ] |= ( set_x )
                l_set_x = list( set_x )
                for j in l_set_x:
                    Dicto[j] = Dicto[a]

            else:
                set_x = Sets.pop( Dicto[a] )
                Sets[ Dicto[b] ] |= ( set_x )
                l_set_x = list( set_x )
                for j in l_set_x:
                    Dicto[j] = Dicto[b]

    #print(Sets)
    #print(Dicto)

    max_len = 1

    for i in range( len(Sets) ):
        if max_len < len( Sets[i] ):
            max_len = len( Sets[i] )

    print(max_len)


main()

Fixed submission:
import sys
input = sys.stdin.readline

def main():
    n , m = map( int, input().split() )
    Sets = []

    Dicto = [ -1 for i in range(n + 1)]

    for i in range( m ):
        a , b = map( int , input().split() )
        if Dicto[a] == -1 and Dicto[b] == -1:
            Sets.append( set( [a,b] ) )
            x_i = len(Sets)
            Dicto[ a ] = x_i - 1
            Dicto[ b ] = x_i - 1
        elif Dicto[ a ] != -1 and Dicto[ b ] == -1:
            Sets[ Dicto[a] ].add( b )
            Dicto[ b ] = Dicto[a]
        elif Dicto[ b ] != -1 and Dicto[ a ] == -1:
            Sets[ Dicto[b] ].add( a )
            Dicto[ a ] = Dicto[b]
        elif Dicto[ b ] != -1 and Dicto[ a ] != -1 and Dicto[ a ] != Dicto[ b ]:
            if Dicto[ b ] > Dicto[ a ]:
                set_x = Sets[Dicto[b]]
                Sets[ Dicto[a] ] |= ( set_x )
                l_set_x = list( set_x )
                for j in l_set_x:
                    Dicto[j] = Dicto[a]

            else:
                set_x = Sets[Dicto[a]]
                Sets[ Dicto[b] ] |= ( set_x )
                l_set_x = list( set_x )
                for j in l_set_x:
                    Dicto[j] = Dicto[b]

    #print(Sets)
    #print(Dicto)

    max_len = 1

    for i in range( len(Sets) ):
        if max_len < len( Sets[i] ):
            max_len = len( Sets[i] )

    print(max_len)


main()

Buggy python submission (verdict: Runtime Error):
import sys
input = sys.stdin.readline

def main():
    n , m = map( int, input().split() )
    Sets = []

    Dicto = [ -1 for i in range(n + 1)]

    for i in range( m ):
        a , b = map( int , input().split() )
        if Dicto[a] == -1 and Dicto[b] == -1:
            Sets.append( set( [a,b] ) )
            x_i = len(Sets)
            Dicto[ a ] = x_i - 1
            Dicto[ b ] = x_i - 1
        elif Dicto[ a ] != -1 and Dicto[ b ] == -1:
            Sets[ Dicto[a] ].add( b )
            Dicto[ b ] = Dicto[a]
        elif Dicto[ b ] != -1 and Dicto[ a ] == -1:
            Sets[ Dicto[b] ].add( a )
            Dicto[ a ] = Dicto[b]
        elif Dicto[ b ] != -1 and Dicto[ a ] != -1:
            if Dicto[ b ] > Dicto[ a ]:
                set_x = Sets.pop( Dicto[b] )
                Sets[ Dicto[a] ] |= ( set_x )
                l_set_x = list( set_x )
                for j in l_set_x:
                    Dicto[j] = Dicto[a]

            else:
                set_x = Sets.pop( Dicto[a] )
                Sets[ Dicto[b] ] |= ( set_x )
                Dicto[a] = Dicto[b]

    #print(Sets)
    #print(Dicto)

    max_len = 1

    for i in range( len(Sets) ):
        if max_len < len( Sets[i] ):
            max_len = len( Sets[i] )

    print(max_len)


main()

Fixed submission:
import sys
input = sys.stdin.readline

def main():
    n , m = map( int, input().split() )
    Sets = []

    Dicto = [ -1 for i in range(n + 1)]

    for i in range( m ):
        a , b = map( int , input().split() )
        if Dicto[a] == -1 and Dicto[b] == -1:
            Sets.append( set( [a,b] ) )
            x_i = len(Sets)
            Dicto[ a ] = x_i - 1
            Dicto[ b ] = x_i - 1
        elif Dicto[ a ] != -1 and Dicto[ b ] == -1:
            Sets[ Dicto[a] ].add( b )
            Dicto[ b ] = Dicto[a]
        elif Dicto[ b ] != -1 and Dicto[ a ] == -1:
            Sets[ Dicto[b] ].add( a )
            Dicto[ a ] = Dicto[b]
        elif Dicto[ b ] != -1 and Dicto[ a ] != -1 and Dicto[ a ] != Dicto[ b ]:
            if Dicto[ b ] > Dicto[ a ]:
                set_x = Sets[Dicto[b]]
                Sets[ Dicto[a] ] |= ( set_x )
                l_set_x = list( set_x )
                for j in l_set_x:
                    Dicto[j] = Dicto[a]

            else:
                set_x = Sets[Dicto[a]]
                Sets[ Dicto[b] ] |= ( set_x )
                l_set_x = list( set_x )
                for j in l_set_x:
                    Dicto[j] = Dicto[b]

    #print(Sets)
    #print(Dicto)

    max_len = 1

    for i in range( len(Sets) ):
        if max_len < len( Sets[i] ):
            max_len = len( Sets[i] )

    print(max_len)


main()