python - 迭代时将项目添加到字典中

Question

在迭代字典时向字典添加项目有多安全？乍一看还不错，但我想知道是否存在不安全的情况。键是字符串 0、1...等。

Answer 1

迭代时无法更改字典的大小:

>>> foo = {'spam': 'egg'}
>>> for i in foo:
...  foo['egg'] = 'spam'
... 
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
RuntimeError: dictionary changed size during iteration

您必须首先制作副本:

>>> foo = {'spam': 'egg'}
>>> for i in dict(foo):
...  foo['egg'] = 'spam'
... 
>>> foo
{'spam': 'egg', 'egg': 'spam'}