“ collections.deque”对象的“ maxlen”属性不可写

Question

class TailDeque(collections.deque):
    '''Implementation of deque with limited maxlen support for Python 2.5.'''
    def __init__(self, iterable=None, maxlen=-1):
        super(TailDeque, self).__init__([])
        self.maxlen = maxlen
        if iterable is not None:
            self.extend(iterable)

    def extend(self, iterable):
        for item in iterable:
            self.append(item)

    def extendleft(self, iterable):
        for item in iterable:
            self.appendleft(item)

    def appendleft(self, item):
        if len(self) == self.maxlen:
            self.pop()
        super(TailDeque, self).appendleft(item)    

    def append(self, item):
        if len(self) == self.maxlen:
            self.popleft()
        super(TailDeque, self).append(item)
logQueue = TailDeque(maxlen=20)

Can someone might explain and show me how to do this for python 3.4?

Im always getting

AttributeError: attribute 'maxlen' of 'collections.deque' objects is not writable

https://docs.python.org/2/library/collections.html#deque-objects

Answer 1

The maxlen parameter was added to collections.deque in 2.6 and 3.0. (The read-only maxlen attribute, set from the argument, was added in 2.7 and 3.1.) The code you posted is intended for 2.5 as a partial substitute. For 2.6+ and 3.1+ you should just use collections.deque and not bother with the partial backport. If you are writing code to support 2.5 to 2.7, you should replace collections.deque with its wrapper when running on 2.5. Something like

import sys

major = sys.version[0:3]

if major >= '2.6':
    from collections import deque
elif major == '2.5':
    from myutils import TailDeque as deque
else:
    raise MyException('This app requires 2.5 or later.')