How to clear a list (3 ways).

[francois.petitjean at bureauveritas.com]

Consider the following code :
#!/usr/bin/env python
# -*- coding: latin_1 -*-

"""
container.py  how to clear a container
"""

class Container(object):
    def __init__(self):
        self.elts = {}
        self.parts = []
    def clear(self):
        self.elts.clear()  #  for a dictionary it's clear :-)
        self.parts = []       #  (1)
     #  self.parts[:] = []    #  (2)
     #  del self.parts[:]     #  (3)
    def __len__(self):
        return len(self.parts)
    def add_predicate(self, part):
        """return True if part should be added (to override)"""
        return True
    def add(self, part):
        """return True if part is added"""
        res = self.add_predicate(part)
        if res:
            self.parts.append(part)
            self.elts[str(part)] = part # or wahtever
        return res
    def replace_parts(self, values):
        """return True if all values are added/replaced"""
        acceptable = all(map(self.add_predicate, values))  # FIXME
itertools.imap ?
        if not acceptable:
            return acceptable
        self.parts[:] = values
        # TODO elts
        return acceptable

Container1 = Container

class Container2(Container):
    def clear(self):
        self.elts.clear()  #  for a dictionary it's clear :-)
        self.parts[:] = []    #  (2)

class Container3(Container):
    def clear(self):
        self.elts.clear()  #  for a dictionary it's clear :-)
        del self.parts[:]     #  (3)


Solution (1) is somewhat broken  (see the test_container.rst hereafter) but
often used. For instance in configobj we have
    def clear(self):
        """
        A version of clear that also affects scalars/sections
        Also clears comments and configspec.

        Leaves other attributes alone :
            depth/main/parent are not affected
        """
        dict.clear(self)
        self.scalars = []
        self.sections = []
        self.comments = {}
        self.inline_comments = {}
        self.configspec = {}

What idiom do you use (and prefer) ?


test_container.rst  can be used like this :
>>> import doctest
>>> doctest.testfile('test_container.rst', encoding='latin_1')
(nfails, ntests) is printed.
===================
test_container.rst
===================

>>> from container import Container1, Container2, Container3
>>> cont1 = Container1()
>>> cont1.add(1)
True
>>> cont1.add(2)
True
>>> cont1.add(3)
True
>>> parts = cont1.parts
>>> parts
[1, 2, 3]

The client has cached the parts attribute.
Solution (1) is not robust, is the parts attribute in the public API ?
>>> cont1.clear()
>>> parts
[1, 2, 3]
>>> cont1.parts, parts
([], [1, 2, 3])

We now have two different objects.

>>> cont2 = Container2()
>>> cont2.add(21)
True

>>> cont2.add(22)
True
>>> cont2.add(23)
True
>>> parts2 = cont2.parts
>>> parts2
[21, 22, 23]
>>> cont2.clear()
>>> parts2
[]
>>> cont1.parts, parts
([], [1, 2, 3])

>>> cont3 = Container3()
>>> cont3.add(31)
True
>>> cont3.add(32)
True
>>> parts3 = cont3.parts
>>> cont3.add(33)
True
>>> parts3
[31, 32, 33]
>>> cont3.clear()
>>> parts3
[]

Test de replace_parts

>>> cont3 = Container3()
>>> len(cont3)
0
>>> parts3 = cont3.parts
>>> cont3.add(30)
True
>>> parts3
[30]
>>> cont3.replace_parts( (31, 32, 33) )
True
>>> parts3
[31, 32, 33]
>>>


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