Some strange behavior Python list and dict

Question

Can anyone explain why this happened with list and how to clean list after appending to another list?

>>> t = {}
>>> t["m"] = []
>>> t
{'m': []}
>>> t["m"].append('qweasdasd aweter')
>>> t["m"].append('asdasdaf ghghdhj')
>>> t
{'m': ['qweasdasd aweter', 'asdasdaf ghghdhj']}
>>> r = []
>>> r.append(t)
>>> r
[{'m': ['qweasdasd aweter', 'asdasdaf ghghdhj']}]
>>> t["m"] = []
>>> r
[{'m': []}]

Answer 1

That's a normal behaviour. Python uses references to store elements. When you do r.append(t) python will store t in r. If you modify t later, t in r will be also modified because it's the same object.

If you want to make t independant from the value stored in r you have to copy it. Look at the copy module.