Currently dict iterating is guarded against changing dict's size. However when dict changed during iteration so that it's size left unchanged, this modification left unnoticed.

>>> d = dict.fromkeys('abcd')
>>> for i in d:
...     print(i)
...     d[i + 'x'] = None
...     del d[i]
... 
d
a
dx
dxx
ax
c
b

In general iterating over mutating dict considered logical error. It is good detect it as early as possible.

The proposed patch introduces a counter which changed every time when added or removed key. If an iterator detects that this counter is changed, it raises runtime error.