I deleted my dict, but my dict_keys don't mind, why is that?

Question

Since dict.keys (and dict.items and dict.values alike) returns a view of the dictionary object, I would assume that deleting the dictionary they were produced from to actually have an effect on the dict_keys we got them from.

With a simple dictionary and it's keys:

d = {i:i for i in range(20)}
k = d.keys()  # similarly with d.values and d.items

Apparently, this isn't the case:

del d
i = iter(k)
list(i) # works fine and prints the keys

Does anyone know why this is the case?

Answer 1

del d deletes the variable d, but the object that it refers to will continue to exist if there are other references. You don't need a dictionary view to observe this:

>>> d = {i:i for i in range(5)}
>>> dd = d
>>> d['x'] = 'x'
>>> del d
>>> d
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'd' is not defined
>>> dd
{0: 0, 1: 1, 2: 2, 3: 3, 4: 4, 'x': 'x'}

(The d['x']='x' line demonstrates that dd=d does not copy the dictionary itself, it only makes an additional reference.)

Also contrast the behavior of clear, which modifies the actual object:

>>> d = {i:i for i in range(5)}
>>> k = d.keys()
>>> del d
>>> list(k)
[0, 1, 2, 3, 4]

>>> d = {i:i for i in range(5)}
>>> k = d.keys()
>>> d.clear()
>>> list(k)
[]