Some Strange Behavior Python List And Dict

Can anyone explain why this happened with list and how to clean list after appending to another list? >>> t = {} >>> t['m'] = [] >>> t {'m': []} >>

Solution 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.

Solution 2:

When you call r.append(t) you are saying "Store a reference to the value stored in t at the end of the list r". The dictionary t references is not copied, so when you modify t you modify the data referenced by r.

This behaviour can be seen more clearly here:

>>>x = []>>>y = x>>>x.append(1)>>>y
[1]

This sets y to the value referenced by x so when x changes it appears that y changes as well.

Solution 3:

Dictionaries are mutable in Python, take a copy instead

>>> import copy
>>> r.append(copy.copy(t))
>>> t
{'m': ['qweasdasd aweter', 'asdasdaf ghghdhj']}
>>> r
[{'m': ['qweasdasd aweter', 'asdasdaf ghghdhj']}]
>>> t["m"]=None>>> r
[{'m': ['qweasdasd aweter', 'asdasdaf ghghdhj']}]

Solution 4:

You can try to do:

importcopy
r.append(copy.deepcopy(t))

and that should work.

Cheers

Solution 5:

This is normal. When you assign a object to a variable, python won't copy that object to your variable. It will simply assign the reference of original object to your new object.

In [1]: a = [1,2]   
In [2]: b = a
In [3]: a.remove(1)
In [4]: b
Out[4]: [2]

Here b will only hold the reference of original list a. If you want to override this behavior, you can use the copy module.

In [7]: import copy
In [8]: a = [1,2]
In [9]: b = copy.deepcopy(a)
In [10]: a.remove(1)
In [11]: b
Out[11]: [1, 2]

deepcopy duplicates everything including individual elements.