循环字典进行操作时出现：RuntimeError: dictionary changed size during iteration的解决方案

在做对员工信息增删改查这个作业时，有一个需求是通过用户输入的id删除用户信息。我把用户信息从文件提取出来储存在了字典里，其中key是用户id,value是用户的其他信息。在循环字典的时候，当用户id和字典里的key相等时，会删除这条信息，当时删除时报错RuntimeError: dictionary changed size during iteration。

for key in staff_info:
    if user_id == key:
        print(key)
        staff_info.pop(key)
        id_exist = True

参考：https://www.python.org/dev/peps/pep-0234/#dictionary-iterators

在官网看到解释：

Dictionaries implement a tp_iter slot that returns an efficient iterator that iterates over the keys of the dictionary. During such an iteration, the dictionary should not be modified, except that setting the value for an existing key is allowed (deletions or additions are not, nor is the update() method). This means that we can write

for k in dict: ...

which is equivalent to, but much faster than

for k in dict.keys(): ...

as long as the restriction on modifications to the dictionary (either by the loop or by another thread) are not violated.

意思是多字典在被循环的时候不能被循环删除和更新，除了给一个已经存在的key设置value。还说道 for kin dict: ...和for k in dict.keys(): ...效果是一样的，但是前者速度更快。

那就来试验一下：

a = {'a': 1, 'b': 0, 'c': 1, 'd': 0}

for key in a:
    print(key, a[key])

for key in a.keys():
    print(key, a[key])

# 两个循环效果一样

print(a.keys())  # dict_keys(['a', 'b', 'c', 'd'])
print(a)  # {'a': 1, 'b': 0, 'c': 1, 'd': 0}

除了a.keys是dict_keys类型，它们的效果是一样的。

那么解决方案就出来了，转换成列表形式就OK了。

for key in list(staff_info.keys()):  
    if user_id == key:
        print(key)
        staff_info.pop(key)
        id_exist = True

有道词典

for k in dict: ...

详细X

k的dict类型:…