今天在写以下Python代码时出现了问题。该段代码根据一些URL下载视频：
#coding=utf-8def save_case_infos(case_urls): for case_url in case_urls: # some download code ... # 打印已下载的视频个数 # 希望访问全局变量 case_total_num case_total_num += 1 print("current count: " + str(case_total_num))# 全局变量，表示已下载的视频个数case_total_num = 0# 下载视频文件urls = [r'http://cynhard.com/live/001', r'http://cynhard.com/live/002', r'http://cynhard.com/live/003']save_case_infos(urls)但是在运行时报以下错误：
Traceback (most recent call last): File "G:projectspythonglobaltest.py", line 21, in <module> save_case_infos(urls) File "G:projectspythonglobaltest.py", line 10, in save_case_infos case_total_num += 1UnboundLocalError: local variable 'case_total_num' referenced before assignment意思是说case_total_num为局部变量，在使用它之前没有被赋值。Python并没有按照我的意图将case_total_num当成全部变量。看来在函数内无法直接使用全局变量。
有什么办法能在函数内使用全局变量呢？根据官方文档，可以用global语句：
The global statement is a declaration which holds for the entire current code block. It means that the listed identifiers are to be interpreted as globals. It would be impossible to assign to a global variable without global.
意思是说global语句可以声明一个或多个变量为全局变量。该声明仅在当前代码块中有效。除此之外，没办法访问全局变量。
于是，在上面的代码中加入global声明语句就可以达到目的了：
#coding=utf-8def save_case_infos(case_urls): global case_total_num # global声明 for case_url in case_urls: # some download code ... # 打印已下载的视频个数 # 希望访问全局变量 case_total_num case_total_num += 1 print("current count: " + str(case_total_num))# 全局变量，表示已下载的视频个数case_total_num = 0# 下载视频文件urls = [r'http://cynhard.com/live/001', r'http://cynhard.com/live/002', r'http://cynhard.com/live/003']save_case_infos(urls)用global声明多个变量需要用逗号分隔：
#coding=utf-8def save_case_infos(case_urls): global succeeded_total_num, failed_total_num # 多个变量用逗号分隔 for case_url in case_urls: ok = False # some download code ... if ok: succeeded_total_num += 1 else: failed_total_num += 1 print("succeeded: " + str(succeeded_total_num) + " failed: " + str(failed_total_num))# 全局变量，下载成功和失败的视频个数succeeded_total_num = 0failed_total_num = 0# 下载视频文件urls = [r'http://cynhard.com/live/001', r'http://cynhard.com/live/002', r'http://cynhard.com/live/003']save_case_infos(urls)在使用全局变量的场合，也可用类变量代替：
#coding=utf-8def save_case_infos(case_urls): for case_url in case_urls: ok = False # some download code ... if ok: G.succeeded_total_num += 1 else: G.failed_total_num += 1 print("succeeded: " + str(G.succeeded_total_num) + " failed: " + str(G.failed_total_num))# 将全局使用的变量定义在类中 class G: succeeded_total_num = 0 failed_total_num = 0# 下载视频文件urls = [r'http://cynhard.com/live/001', r'http://cynhard.com/live/002', r'http://cynhard.com/live/003']save_case_infos(urls)这样便去掉了global声明，但是每一次用到类变量的时候都需要冠以类名。