New submission from Andy Maier <[email protected]>:
Unpickling an object of a user class that derives from list seems to miss
calling the user class's __init__() method:
Consider this script, which defines a derived class of the built-in list for
the purpose of creating a case insensitive list. The real example has all
methods of list implemented, but this subset example also shows the issue:
-----------------
import pickle
def _lc_list(lst):
result = list()
for value in lst:
result.append(value.lower())
return result
class NocaseList(list):
#def __new__(cls, iterable=()):
# self = super(NocaseList, cls).__new__(cls, iterable)
# print("Debug: __new__() for self={}".format(id(self)))
# return self
def __init__(self, iterable=()):
print("Debug: __init__() for self={}".format(id(self)))
super(NocaseList, self).__init__(iterable)
self.lc_list = _lc_list(self)
def append(self, value):
super(NocaseList, self).append(value)
self.lc_list.append(value.lower())
def extend(self, iterable):
super(NocaseList, self).extend(iterable)
for value in iterable:
self.lc_list.append(value.lower())
ncl = NocaseList(['A', 'b'])
pkl = pickle.dumps(ncl)
nclx = pickle.loads(pkl)
-----------------
$ python ./pickle_extend.py
Debug: __init__() for self=4498704880
Traceback (most recent call last):
File "./pickle_extend.py", line 32, in <module>
nclx = pickle.loads(pkl)
File "./pickle_extend.py", line 28, in extend
self.lc_list.append(value.lower())
AttributeError: 'NocaseList' object has no attribute 'lc_list'
Uncommenting the __new__() method in the script shows that __new__() is called
but not __init__():
$ python ./pickle_extend.py
Debug: __new__() for self=4359683024
Debug: __init__() for self=4359683024
Debug: __new__() for self=4360134304
Traceback (most recent call last):
File "./pickle_extend.py", line 32, in <module>
nclx = pickle.loads(pkl)
File "./pickle_extend.py", line 28, in extend
self.lc_list.append(value.lower())
AttributeError: 'NocaseList' object has no attribute 'lc_list'
----------
messages: 375909
nosy: andymaier
priority: normal
severity: normal
status: open
title: Unpickling derived class of list does not call __init__()
versions: Python 3.8
_______________________________________
Python tracker <[email protected]>
<https://bugs.python.org/issue41639>
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