class HCN:
def __init__(self, chieu_dai, chieu_rong):
self.chieu_dai = chieu_dai
self.chieu_rong = chieu_rong

def dien_tich(self):
return self.chieu_dai * self.chieu_rong

def chu_vi(self):
return (self.chieu_dai + self.chieu_rong) * 2

def to_string(self):
print(f"chieu dai: {self.chieu_dai} \n"
f"chieu rong: {self.chieu_rong} \n"
f"chu vi: {self.chu_vi()} \n"
f"dien tich: {self.dien_tich()}")

def write_to_string(self):
open_file = open("hcn.txt", "r")
open_file.write(self.to_string())
open_file.close()

chieu_dai = int(input("cd: "))
chieu_rong = int(input("cr: "))
chu_nhat = HCN(chieu_dai, chieu_rong)
chu_nhat.to_string()
chu_nhat.write_to_string()

lỗi em gặp ạ : open_file.write(self.to_string())
TypeError: write() argument must be str, not None