帮我查查这段代码的错误

这段代码中存在一些错误，主要集中在以下几个地方：

1. 在 `get_gdp_statistics()` 方法中，变量 `year` 没有被定义。您需要在该方法中定义或传入 `year` 变量，以便计算 GDP 增长率。

2. 在 `add_selected_city()` 方法中，如果用户已经选择了 10 个城市，您需要在添加新城市之前检查选择的城市数，而不是在添加新城市后检查。

3. 在 `get_gdp_statistics()` 方法中，如果城市没有足够的 GDP 数据（至少需要 3 年），则在计算 GDP 增长率时会出现除以零的错误。

以下是修改后的代码，已经修复了上述错误：

import pandas as pd
import numpy as np

# 定义城市类
class City:
    def __init__(self, name, province):
        self.name = name
        self.province = province
        self.gdp_data = {}

    def add_gdp_data(self, year, gdp):
        self.gdp_data[year] = gdp

    def get_gdp_data(self):
        return self.gdp_data

    def get_gdp_statistics(self, year):
        gdp_values = list(self.gdp_data.values())
        if len(gdp_values) < 3:
            return None

        min_gdp = min(gdp_values)
        max_gdp = max(gdp_values)
        avg_gdp = np.mean(gdp_values)
        growth_rate = (self.gdp_data[year + 2] - self.gdp_data[year]) / self.gdp_data[year + 2]

        return min_gdp, max_gdp, avg_gdp, growth_rate

# 定义用户类
class User:
    def __init__(self, name, phone):
        self.name = name
        self.phone = phone
        self.selected_cities = []

    def add_selected_city(self, city):
        if len(self.selected_cities) >= 10:
            print("You have already selected 10 cities. Please remove a city before adding a new one.")
        else:
            self.selected_cities.append(city)
            print(f"{city.name} added to your selected cities.")

    def remove_selected_city(self, city):
        if city in self.selected_cities:
            self.selected_cities.remove(city)
            print(f"{city.name} removed from your selected cities.")
        else:
            print(f"{city.name} is not in your selected cities.")

    def get_selected_cities(self):
        return self.selected_cities

# 创建城市实例
city1 = City("Shanghai", "Shanghai")
city2 = City("Beijing", "Beijing")

# 创建用户实例
user1 = User("Alice", "123456789")

# 添加城市到用户的选择列表中
user1.add_selected_city(city1)
user1.add_selected_city(city2)

# 获取用户选择的城市列表
print(user1.get_selected_cities())

# 从选择列表中删除一个城市
user1.remove_selected_city(city1)

# 获取用户选择的城市列表
print(user1.get_selected_cities())

# 在城市实例中添加GDP数据
city1.add_gdp_data(2020, 100)
city1.add_gdp_data(2019, 90)
city1.add_gdp_data(2018, 80)

# 获取城市GDP数据
print(city1.get_gdp_data())

# 获取城市GDP数据统计信息
gdp_stats = city1.get_gdp_statistics(2019)
if gdp_stats is not None:
    min_gdp, max_gdp, avg_gdp, growth_rate = gdp_stats
    print(f"min GDP: {min_gdp}, max GDP: {max_gdp}, avg GDP: {avg_gdp}, growth rate: {growth_rate}")
else:
    print("Cannot calculate GDP statistics due to insufficient data.")