这道习题中包含了 Python 中最常见、最难找出来的错误。编写一个叫 Kangaroo 的类,包含以下方法:
- 一个__init__ 方法,初始化一个叫 pounch_contents 的属性为空列表。
- 一个叫put_in_pounch 的方法,将一个任意类型的对象加入pounch_contents。
- 一个__str__ 方法,返回 Kangaroo 对象的字符串表示和 pounch 中的内容。
创建两个 Kangaroo 对象, 将它们命名为 kanga 和 roo , 然后将 roo 加入 kanga 的 pounch 列表, 以此测试你写的代码。
这是我在不完全理解题目时写的代码（请不要参考）：
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| class Kangaroo:
def __init__(self):
self.pounch_contents = []
def put_in_pounch(self, ayobj):
self.pounch_contents.append(ayobj)
def __str__(self):
return str(self.pounch_contents)
kanga = Kangaroo()
roo = Kangaroo()
kanga.put_in_pounch(roo)
print(kanga)
|
其实这是一个找bug的练习。作者想让我们使用pylint这个工具找出他的代码里的bug。
作者的代码（含bug）：
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| """This module contains a code example related to
Think Python, 2nd Edition
by Allen Downey
http://thinkpython2.com
Copyright 2015 Allen Downey
License: http://creativecommons.org/licenses/by/4.0/
"""
from __future__ import print_function, division
"""
WARNING: this program contains a NASTY bug. I put
it there on purpose as a debugging exercise, but
you DO NOT want to emulate this example!
"""
class Kangaroo:
"""A Kangaroo is a marsupial."""
def __init__(self, name, contents=[]):
"""Initialize the pouch contents.
name: string
contents: initial pouch contents.
"""
self.name = name
self.pouch_contents = contents
def __str__(self):
"""Return a string representaion of this Kangaroo.
"""
t = [ self.name + ' has pouch contents:' ]
for obj in self.pouch_contents:
s = ' ' + object.__str__(obj)
t.append(s)
return '\n'.join(t)
def put_in_pouch(self, item):
"""Adds a new item to the pouch contents.
item: object to be added
"""
self.pouch_contents.append(item)
kanga = Kangaroo('Kanga')
roo = Kangaroo('Roo')
kanga.put_in_pouch('wallet')
kanga.put_in_pouch('car keys')
kanga.put_in_pouch(roo)
print(kanga)
# If you run this program as is, it seems to work.
# To see the problem, trying printing roo.
# Hint: to find the problem try running pylint.
|
用pylint分析一番：
% pylint /Users/aoyu/Downloads/ThinkPython2-master/code/BadKangaroo.py
************* Module BadKangaroo
Downloads/ThinkPython2-master/code/BadKangaroo.py:24:0: C0303: Trailing whitespace (trailing-whitespace)
Downloads/ThinkPython2-master/code/BadKangaroo.py:1:0: C0103: Module name "BadKangaroo" doesn't conform to snake_case naming style (invalid-name)
Downloads/ThinkPython2-master/code/BadKangaroo.py:14:0: W0105: String statement has no effect (pointless-string-statement)
Downloads/ThinkPython2-master/code/BadKangaroo.py:25:4: W0102: Dangerous default value [] as argument (dangerous-default-value)
Downloads/ThinkPython2-master/code/BadKangaroo.py:37:8: C0103: Variable name "t" doesn't conform to snake_case naming style (invalid-name)
Downloads/ThinkPython2-master/code/BadKangaroo.py:39:12: C0103: Variable name "s" doesn't conform to snake_case naming style (invalid-name)
------------------------------------------------------------------
Your code has been rated at 7.00/10 (previous run: 7.00/10, +0.00)
其他的都是小毛病，关键在于：
Downloads/ThinkPython2-master/code/BadKangaroo.py:25:4: W0102: Dangerous default value [] as argument (dangerous-default-value)
说把空列表作为参数的默认值，很危险。怎么就危险了呢？看一个简单的例子就明白了：
>>> a = []
>>> b = a
>>> c = a
>>> b.append('a')
>>> a
['a']
>>> b
['a']
>>> c
['a']
a, b, c三个变量都是空列表的引用，其中一个改变，其他的变量也跟着改变。
作者的代码的bug也是同理。每一个 Kangaroo 对象的 pouch_contents 属性的默认值都是一个空列表的引用，一个对象的 pouch_contents 改变，其他对象的 pouch_contents 也跟着改变。
作者的解决办法：
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| """This module contains a code example related to
Think Python, 2nd Edition
by Allen Downey
http://thinkpython2.com
Copyright 2015 Allen Downey
License: http://creativecommons.org/licenses/by/4.0/
"""
from __future__ import print_function, division
"""
WARNING: this program contains a NASTY bug. I put
it there on purpose as a debugging exercise, but
you DO NOT want to emulate this example!
"""
class Kangaroo:
"""A Kangaroo is a marsupial."""
def __init__(self, name, contents=[]):
"""Initialize the pouch contents.
name: string
contents: initial pouch contents.
"""
# The problem is the default value for contents.
# Default values get evaluated ONCE, when the function
# is defined; they don't get evaluated again when the
# function is called.
# In this case that means that when __init__ is defined,
# [] gets evaluated and contents gets a reference to
# an empty list.
# After that, every Kangaroo that gets the default
# value gets a reference to THE SAME list. If any
# Kangaroo modifies this shared list, they all see
# the change.
# The next version of __init__ shows an idiomatic way
# to avoid this problem.
self.name = name
self.pouch_contents = contents
def __init__(self, name, contents=None):
"""Initialize the pouch contents.
name: string
contents: initial pouch contents.
"""
# In this version, the default value is None. When
# __init__ runs, it checks the value of contents and,
# if necessary, creates a new empty list. That way,
# every Kangaroo that gets the default value gets a
# reference to a different list.
# As a general rule, you should avoid using a mutable
# object as a default value, unless you really know
# what you are doing.
self.name = name
if contents == None:
contents = []
self.pouch_contents = contents
def __str__(self):
"""Return a string representaion of this Kangaroo.
"""
t = [ self.name + ' has pouch contents:' ]
for obj in self.pouch_contents:
s = ' ' + object.__str__(obj)
t.append(s)
return '\n'.join(t)
def put_in_pouch(self, item):
"""Adds a new item to the pouch contents.
item: object to be added
"""
self.pouch_contents.append(item)
kanga = Kangaroo('Kanga')
roo = Kangaroo('Roo')
kanga.put_in_pouch('wallet')
kanga.put_in_pouch('car keys')
kanga.put_in_pouch(roo)
print(kanga)
print(roo)
# If you run this program as is, it seems to work.
# To see the problem, trying printing roo.
|
代码里面的object.__str__(obj)怎样理解？
返回它的参数obj所指的对象的字符串表现形式。
举例：
>>> a = 'abc'
>>> object.__str__(a)
"'abc'"
>>> print(a)
abc
>>> b = 123
>>> object.__str__(b)
'123'
>>> print(b)
123
>>> class Point:
... """test"""
...
>>> object.__str__(a)
'<__main__.Point object at 0x7f7db642b700>'