Buggy python submission (verdict: Time Limit Exceeded):
def order(c):
    return 1 << (ord(c) - 97)

def count(n):
    return bin(n).count("1")

def merge(x, y):
    return x | y

def main():
    N = int(input())
    S = [x for x in input()]
    Q = int(input())
    L = 2**22
    Seg = [0]*(L*2)

    def update(k, x):
        k += L 
        Seg[k] = x
        while k > 0:
            k = (k - 1) // 2
            Seg[k] = merge(Seg[2*k+1], Seg[2*k+2])
        Seg[0] = merge(Seg[1], Seg[2])

    def find(a, b, k, l, r):
        if r <= a or b <= l:
            return 0
        if a <= l and r <= b:
            #print("find*", (a,b,k,l,r), "=",count(Seg[k]))
            return Seg[k]
        vl = find(a, b, 2*k+1, l, (l+r)//2)
        vr = find(a, b, 2*k+2, (l+r)//2, r)
        #print("find_", (a,b,k,l,r), "=",count(merge(vl, vr)))
        return merge(vl, vr)

    # read S, and update tree
    for i in range(N):
        update(i, order(S[i]))
        #print("Seg", *[count(x) for x in Seg])
    #print("Seg=", [count(x) for x in Seg])

    for _ in range(Q):
        q, i, c = input().split()
        if q == '1':
            i = int(i)-1
            if S[i] == c:
                continue
            S[i] = c
            update(i, order(S[i]))
            #print("Seg=", [count(x) for x in Seg])
        else:
            l, r = int(i), int(c)
            #print((l,r))
            print(count(find(l, r+1, 0, 0, L)))
main()


Fixed submission:
import numpy as np

def count(n):
    return bin(n).count("1")

def merge(x, y):
    return x | y

def update(Seg, k, x):
    k += len(Seg)//2
    Seg[k] = x
    while k > 0:
        k = (k - 1) // 2
        Seg[k] = Seg[k*2+1]|Seg[k*2+2]

d = {c:(1 << (ord(c)-97)) for c in 'abcdefghijklmnopqrstuvwxyz'}

class SegmentTree:
    def __init__(self, A):
        N = len(A)
        n = (N-1).bit_length()
        self.N = 1 << n
        self.Seg = [0 for i in range(2*self.N)]
        #for i in range(N): self.update(i, A[i])
        if True:
            for i in range(N):
                self.Seg[self.N+i-1] = d[A[i]]
            for i in range(0,self.N-2)[::-1]:
                self.Seg[i] = self.Seg[i*2+1] | self.Seg[i*2+2]

    def update(self, k, x):
        i = self.N+k-1
        self.Seg[i] = d[x]
        while i > 0:
            i = (i-1)//2
            self.Seg[i] = self.Seg[i*2+1]|self.Seg[i*2+2]
    def merge(self, x, y):
        return x | y

    def query(self, a, b):
        return self.query_(a, b, 0, 0, self.N)

    def query_(self, a, b, k, l, r):
        if r <= a or b <= l:
            return 0
        if a <= l and r <= b:
            return self.Seg[k]
        vl = self.query_(a, b, 2*k+1, l, (l+r)//2)
        vr = self.query_(a, b, 2*k+2, (l+r)//2, r)
        return merge(vl, vr)

def main():
    import sys
    input = sys.stdin.readline
    N = int(input())
    S = input()[:-1]
    Q = int(input())
    Seg = SegmentTree(S)

    for _ in range(Q):
        q, i, c = input().split()
        if q == '1':
            i = int(i)-1
            Seg.update(i, c)
        else:
            l, r = int(i), int(c)
            print(count(Seg.query(l-1, r)))
main()

Buggy python submission (verdict: Runtime Error):
import math
A = ord('a')
N = int(input())
L = 2**int(math.ceil(math.log(N, 2)))
S = [x for x in input()]
Q = int(input())

Seg = [0]*(L*2-1)

def order(c):
    return 1 << (ord(c) - A)

def count(n):
    return bin(n).count("1")

def merge(x, y):
    return x | y

def update(k, x):
    k += L - 1
    Seg[k] = x
    while k > 0:
        k = (k - 1) // 2
        Seg[k] = merge(Seg[2*k+1], Seg[2*k+2])
    Seg[0] = merge(Seg[1], Seg[2])

def find(a, b, k, l, r):
    if r <= a or b <= l:
        return 0
    if a <= l and r <= b:
        return Seg[k]
    vl = find(a, b, 2*k+1, l, (l+r)//2)
    vr = find(a, b, 2*k+2, (l+r)//2, r)

    return merge(vl, vr)


# read S, and update tree
for i in range(N):
    c = order(S[i])
    update(i, c)
    #print("Seg", *[count(x) for x in Seg])

for _ in range(Q):
    q, i, c = input().split()
    if q == '1':
        i = int(i)-1
        if S[i] == c:
            continue
        S[i] = c
        update(i, order(S[i]))
    else:
        l, r = int(i)-1, int(c)
        print(count(find(l, r, 0, 0, L)))



Fixed submission:
import numpy as np

def count(n):
    return bin(n).count("1")

def merge(x, y):
    return x | y

def update(Seg, k, x):
    k += len(Seg)//2
    Seg[k] = x
    while k > 0:
        k = (k - 1) // 2
        Seg[k] = Seg[k*2+1]|Seg[k*2+2]

d = {c:(1 << (ord(c)-97)) for c in 'abcdefghijklmnopqrstuvwxyz'}

class SegmentTree:
    def __init__(self, A):
        N = len(A)
        n = (N-1).bit_length()
        self.N = 1 << n
        self.Seg = [0 for i in range(2*self.N)]
        #for i in range(N): self.update(i, A[i])
        if True:
            for i in range(N):
                self.Seg[self.N+i-1] = d[A[i]]
            for i in range(0,self.N-2)[::-1]:
                self.Seg[i] = self.Seg[i*2+1] | self.Seg[i*2+2]

    def update(self, k, x):
        i = self.N+k-1
        self.Seg[i] = d[x]
        while i > 0:
            i = (i-1)//2
            self.Seg[i] = self.Seg[i*2+1]|self.Seg[i*2+2]
    def merge(self, x, y):
        return x | y

    def query(self, a, b):
        return self.query_(a, b, 0, 0, self.N)

    def query_(self, a, b, k, l, r):
        if r <= a or b <= l:
            return 0
        if a <= l and r <= b:
            return self.Seg[k]
        vl = self.query_(a, b, 2*k+1, l, (l+r)//2)
        vr = self.query_(a, b, 2*k+2, (l+r)//2, r)
        return merge(vl, vr)

def main():
    import sys
    input = sys.stdin.readline
    N = int(input())
    S = input()[:-1]
    Q = int(input())
    Seg = SegmentTree(S)

    for _ in range(Q):
        q, i, c = input().split()
        if q == '1':
            i = int(i)-1
            Seg.update(i, c)
        else:
            l, r = int(i), int(c)
            print(count(Seg.query(l-1, r)))
main()

Buggy python submission (verdict: Runtime Error):
import math
A = ord('a')
N = int(input())
L = 2**int(math.ceil(math.log(N, 2)))
S = [x for x in input()]
Q = int(input())

Seg = [0]*(L*2-1)

def order(c):
    return 1 << (ord(c) - A)

def count(n):
    return bin(n).count("1")

def merge(x, y):
    return x | y

def update(k, x):
    k += L - 1
    Seg[k] = x
    while k > 0:
        k = (k - 1) // 2
        Seg[k] = merge(Seg[2*k+1], Seg[2*k+2])
    Seg[0] = merge(Seg[1], Seg[2])

def find(a, b, k, l, r):
    if r <= a or b <= l:
        return 0
    if a <= l and r <= b:
        return Seg[k]
    vl = find(a, b, 2*k+1, l, (l+r)//2)
    vr = find(a, b, 2*k+2, (l+r)//2, r)

    return merge(vl, vr)


# read S, and update tree
for i in range(N):
    c = order(S[i])
    update(i, order(S[i]))
    #print("Seg", *[count(x) for x in Seg])

for _ in range(1000):
    s = input()
    if not s:
        break
    q, i, c = s.split()
    if q == '1':
        i = int(i)-1
        if S[i] == c:
            continue
        S[i] = c
        update(i, order(S[i]))
    else:
        l, r = int(i)-1, int(c)
        #print(count(find(l, r, 0, 0, L)), (l, r))
        print(count(find(l, r, 0, 0, L)))



Fixed submission:
import numpy as np

def count(n):
    return bin(n).count("1")

def merge(x, y):
    return x | y

def update(Seg, k, x):
    k += len(Seg)//2
    Seg[k] = x
    while k > 0:
        k = (k - 1) // 2
        Seg[k] = Seg[k*2+1]|Seg[k*2+2]

d = {c:(1 << (ord(c)-97)) for c in 'abcdefghijklmnopqrstuvwxyz'}

class SegmentTree:
    def __init__(self, A):
        N = len(A)
        n = (N-1).bit_length()
        self.N = 1 << n
        self.Seg = [0 for i in range(2*self.N)]
        #for i in range(N): self.update(i, A[i])
        if True:
            for i in range(N):
                self.Seg[self.N+i-1] = d[A[i]]
            for i in range(0,self.N-2)[::-1]:
                self.Seg[i] = self.Seg[i*2+1] | self.Seg[i*2+2]

    def update(self, k, x):
        i = self.N+k-1
        self.Seg[i] = d[x]
        while i > 0:
            i = (i-1)//2
            self.Seg[i] = self.Seg[i*2+1]|self.Seg[i*2+2]
    def merge(self, x, y):
        return x | y

    def query(self, a, b):
        return self.query_(a, b, 0, 0, self.N)

    def query_(self, a, b, k, l, r):
        if r <= a or b <= l:
            return 0
        if a <= l and r <= b:
            return self.Seg[k]
        vl = self.query_(a, b, 2*k+1, l, (l+r)//2)
        vr = self.query_(a, b, 2*k+2, (l+r)//2, r)
        return merge(vl, vr)

def main():
    import sys
    input = sys.stdin.readline
    N = int(input())
    S = input()[:-1]
    Q = int(input())
    Seg = SegmentTree(S)

    for _ in range(Q):
        q, i, c = input().split()
        if q == '1':
            i = int(i)-1
            Seg.update(i, c)
        else:
            l, r = int(i), int(c)
            print(count(Seg.query(l-1, r)))
main()