python3下迭代修改字典报错：dictionary changed size during iteration原因及解决

Python3:

>>> def a(para):
>>>     for k in para.keys():
>>>         para.pop(k) # 或者del para[k]
>>>         print(k)
>>> a({'a':1})
---------------------------------------------------------------------------
RuntimeError                              Traceback (most recent call last)
<ipython-input-71-9a5ca586de9e> in <module>
----> 1 a({'a':1})

<ipython-input-69-66ab94e5931e> in a(para)
      1 def a(para):
----> 2     for k in para.keys():
      3         para.pop(k)
      4         print(k)

RuntimeError: dictionary changed size during iteration

Python2:

>>> def a(para):
>>>     for k in para.keys():
>>>         para.pop(k) # 或者del para[k]
>>>         print(k)
>>> a({'a':1})
a

正常

以上问题出现的原因是在Python2中字典调用keys方法会返回keys的列表拷贝

>>> a({'a':1})
>>> a = {}
>>> a.keys()
[]

而Python3中则返回的是一个迭代器

>>> a = {}
>>> a.keys()
dict_keys([])

在迭代过程中修改字典结构会导致迭代器失效，如果必须要在迭代过程中修改字典结构则可以将迭代器转化为列表：

>>> def a(para):
>>>     for k in list(para.keys()):
>>>         para.pop(k) # 或者del para[k]
>>>         print(k)
>>> a({'a':1})
a

参考：https://stackoverflow.com/a/11941855