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': []}] 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.
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.
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']}] You can try to do:
import copy r.append(copy.deepcopy(t)) and that should work.
Cheers
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.
appendto r, In almost every case Python doesn't do anything unless you've explicitly told it to.