(second try with an enhanced version)
Executive summary : What idiom do you use for resetting a list ?
lst = |] # (1)
lst[:] = [] # (2)
del lst[:] # (3)
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 = {}
Solution (2) does not suffer the same problem, and (3) has the advantage of
not building an empty list.
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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