Buggy python submission (verdict: Runtime Error):
#!/usr/bin/env python3
import sys

def solve_iter(N: int, M: int, A: "List[int]", B: "List[int]"):
    ic = N * (N - 1) // 2
    uf = UnionFind(N+1)
    for a, b in zip(A[::-1], B[::-1]):
        yield ic
        if uf.same(a, b):
            continue
        ic -= uf.size(a) * uf.size(b)
        uf.union(a, b)

def solve(N: int, M: int, A: "List[int]", B: "List[int]"):
    for a in reversed(list(solve_iter(N, M, A, B))):
        print(a)

# Generated by 1.1.7.1 https://github.com/kyuridenamida/atcoder-tools
def main():
    def iterate_tokens():
        for line in sys.stdin:
            for word in line.split():
                yield word
    tokens = iterate_tokens()
    N = int(next(tokens))  # type: int
    M = int(next(tokens))  # type: int
    A = [int()] * (M)  # type: "List[int]"
    B = [int()] * (M)  # type: "List[int]"
    for i in range(M):
        A[i] = int(next(tokens))
        B[i] = int(next(tokens))
    solve(N, M, A, B)

# https://note.nkmk.me/python-union-find/
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())


def test():
    import doctest
    doctest.testmod()

if __name__ == '__main__':
    #test()
    main()


Fixed submission:
#!/usr/bin/env python3
import sys

def solve_iter(N: int, M: int, A: "List[int]", B: "List[int]"):
    ic = N * (N - 1) // 2
    uf = UnionFind(N+1)
    for a, b in zip(A[::-1], B[::-1]):
        yield ic
        if uf.same(a, b):
            continue
        ic -= uf.size(a) * uf.size(b)
        uf.union(a, b)

def solve(N: int, M: int, A: "List[int]", B: "List[int]"):
    for a in reversed(list(solve_iter(N, M, A, B))):
        print(a)

# Generated by 1.1.7.1 https://github.com/kyuridenamida/atcoder-tools
def main():
    def iterate_tokens():
        for line in sys.stdin:
            for word in line.split():
                yield word
    tokens = iterate_tokens()
    N = int(next(tokens))  # type: int
    M = int(next(tokens))  # type: int
    A = [int()] * (M)  # type: "List[int]"
    B = [int()] * (M)  # type: "List[int]"
    for i in range(M):
        A[i] = int(next(tokens))
        B[i] = int(next(tokens))
    solve(N, M, A, B)

# https://note.nkmk.me/python-union-find/
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())


def test():
    import doctest
    doctest.testmod()

if __name__ == '__main__':
    #test()
    main()

Buggy python submission (verdict: Runtime Error):
#!/usr/bin/env python3
import sys
{ % if prediction_success % }
{ % endif % }
{ % if mod or yes_str or no_str % }

{ % endif % }
{ % if mod % }
MOD = None  # type: int
{ % endif % }
{ % if yes_str % }
YES = "None"  # type: str
{ % endif % }
{ % if no_str % }
NO = "None"  # type: str
{ % endif % }
{ % if prediction_success % }


def solve(N: int, M: int, A: "List[int]", B: "List[int]"):
    return
{ % endif % }


# Generated by 1.1.6 https://github.com/kyuridenamida/atcoder-tools  (tips: You use the default template now. You can remove this line by using your custom template)
def main():
    { % if prediction_success % }

    def iterate_tokens():
        for line in sys.stdin:
            for word in line.split():
                yield word
    tokens = iterate_tokens()
    N = int(next(tokens))  # type: int
    M = int(next(tokens))  # type: int
    A = [int()] * (M)  # type: "List[int]"
    B = [int()] * (M)  # type: "List[int]"
    for i in range(M):
        A[i] = int(next(tokens))
        B[i] = int(next(tokens))
    solve(N, M, A, B)
    { % else % }
    # Failed to predict input format
    pass
    { % endif % }


if __name__ == '__main__':
    main()


Fixed submission:
#!/usr/bin/env python3
import sys

def solve_iter(N: int, M: int, A: "List[int]", B: "List[int]"):
    ic = N * (N - 1) // 2
    uf = UnionFind(N+1)
    for a, b in zip(A[::-1], B[::-1]):
        yield ic
        if uf.same(a, b):
            continue
        ic -= uf.size(a) * uf.size(b)
        uf.union(a, b)

def solve(N: int, M: int, A: "List[int]", B: "List[int]"):
    for a in reversed(list(solve_iter(N, M, A, B))):
        print(a)

# Generated by 1.1.7.1 https://github.com/kyuridenamida/atcoder-tools
def main():
    def iterate_tokens():
        for line in sys.stdin:
            for word in line.split():
                yield word
    tokens = iterate_tokens()
    N = int(next(tokens))  # type: int
    M = int(next(tokens))  # type: int
    A = [int()] * (M)  # type: "List[int]"
    B = [int()] * (M)  # type: "List[int]"
    for i in range(M):
        A[i] = int(next(tokens))
        B[i] = int(next(tokens))
    solve(N, M, A, B)

# https://note.nkmk.me/python-union-find/
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())


def test():
    import doctest
    doctest.testmod()

if __name__ == '__main__':
    #test()
    main()