没有__iter__的for循环文档？ [英] docs on for-loop with no __iter__?
问题描述
当你在X中使用for x：时，有人可以向我指出有关应该发生什么的文件
X没有__iter__
方法时的语法？我知道代码：
Can someone point me to the documentation on what''s supposed to happen
when you use the "for x in X:" syntax when X does not have an __iter__
method? I know that the code:
class S：
.... def __len __（self） ：返回42
.... def __getitem __（self，i）：返回i
.... for S in S（）：
class S: .... def __len__(self): return 42
.... def __getitem__(self, i): return i
.... for x in S():
.... print x
尝试打印所有非负整数，从0开始。（注意
__len__方法在42处没有停止它。）显然，正确的
方式是用__iter__，但可能这种行为是
记录在某个地方......
史蒂夫
-
如果你只是动词，你可以用任何语法表达。
- Bucky Katt，Get Fuzzy
.... print x
tries to print all the non-negative integers, starting with 0. (Note
that the __len__ method doesn''t stop it at 42.) Obviously, the right
way to do this is with __iter__, but presumably this behavior is
documented somewhere...
Steve
--
You can wordify anything if you just verb it.
- Bucky Katt, Get Fuzzy
推荐答案
Steven Bethard写道：
Steven Bethard wrote:
有人能指出我吗当你在X中使用for x：时，关于应该发生什么的文档。 X没有__iter__
方法时的语法？
你需要提出一个IndexError
.... def __len __（self）：return 42
.... def __getitem __（self ，i）：返回i
说出来
def __getitem __（self，i）：
if i> = 42：提高IndexError，我
返回i
显然，正确的方法是使用__iter__，但可能是这种行为在某处记录了......
Can someone point me to the documentation on what''s supposed to happen
when you use the "for x in X:" syntax when X does not have an __iter__
method?
You need to raise an IndexError
.... def __len__(self): return 42
.... def __getitem__(self, i): return i
Make that say
def __getitem__(self, i):
if i >= 42: raise IndexError, i
return i
Obviously, the right
way to do this is with __iter__, but presumably this behavior is
documented somewhere...
]注意：for循环期望一个IndexError将为非法索引筹集
]，以便在序列结束时正确检测
]。
''我可以在文档中找到所有内容（搜索docs.python.org
for __getitem__和IndexError）
查看CVS的语言参考，我发现
http://www.python.org/dev/doc/devel/ref/for.html
它声明
]套件然后为每件物品执行一次
]顺序，按升序索引的顺序。
这意味着序列被编入索引，是吗？但是如果
序列实现__iter__那么就没有
可能没有''索引'的基本概念。
这应该修复吗？
Andrew
da *** @ dalkescientific.com
http://docs.python.org/ref/sequence-types.html
] Note: for loops expect that an IndexError will be
] raised for illegal indexes to allow proper detection
] of the end of the sequence.
That''s all I can find in the docs (searched docs.python.org
for __getitem__ and IndexError )
Looking at the language reference from CVS, I found
http://www.python.org/dev/doc/devel/ref/for.html
It states
] The suite is then executed once for each item in
] the sequence, in the order of ascending indices.
That implies the sequence is indexed, yes? But if
the sequence implements __iter__ then there''s no
possibly no underlying idea of ''index''.
Should this be fixed?
Andrew
da***@dalkescientific.com
Andrew Dalke< adalke< at> mindspring.com>写道：
Andrew Dalke <adalke <at> mindspring.com> writes:
http：// docs。 python.org/ref/sequence-types.html
]注意：for循环期望为非法索引引发IndexError以允许正确检测
http://docs.python.org/ref/sequence-types.html
] Note: for loops expect that an IndexError will be
] raised for illegal indexes to allow proper detection
] of the end of the sequence.
谢谢，这就是我要找的东西。我知道它必须在那附近
。 =）大概是有理由不使用len（）来确定序列结束时的
吗？
Steve
Thanks, that''s what I was looking for. I knew it had to be around there
somewhere. =) Presumably there was a reason not to use len() to determine
the end of the sequence?
Steve
" Andrew Dalke" <广告**** @ mindspring.com>在消息中写道
新闻：bM ***************** @ newsread1.news.pas.earthl ink.net ...
< snip>
"Andrew Dalke" <ad****@mindspring.com> wrote in message
news:bM*****************@newsread1.news.pas.earthl ink.net...
<snip>
查看CVS的语言参考，我发现
http://www.python.org/dev/doc/devel/ref/for.html
它声明
然后按照升序索引的顺序对该序列中的每个项目执行一次。
这意味着序列被编入索引，是吗？但是如果序列实现了__iter__那么就没有可能没有''索引'的基本概念。
这应该修复吗？
Andrew
da***@dalkescientific.com
第7.3节（来自上面给出的链接）给出了for的语法。 as：
for_stmt :: =" for target_list" in" expression_list"："套房
[" else" "："套件]
然后开始将组件语法元素描述为，表达式
列表被评估一次;它应该产生一个序列。这似乎有点过时了，因为expression_list也可以是生成器或迭代器。
另外，for如果没有提供__iter__方法，则使用自适应方法尝试模拟迭代器
，通过连续调用__getitem__直到
引发IndexError（此错误在此内部默默吸收
伪迭代器）。
这是一个简单的测试类:(我也实现了__len__认为它将使用
限制对__getitem__的调用，但我们可以从
输出中看到它从未被调用 - 而__getitem__被调用一次
太多，告诉伪迭代器没有更多的条目）。
A级（对象）：
def __init __（self，lst）：
self.list_ = lst [：]
def __len __（自我）：
print self .__ class __.__ name __ +" .__ len __"
返回len（self.list_）
def __getitem __（self，i）：
print self .__ class __.__ name __ +" .__ getitem __ ;
返回self.list_ [i]
class A_with _iter（A）：
def __iter __（self）：
print self .__ class __.__ name __ +" .__ iter __"
return iter（ self.list_）
for（A，A_with_iter）中的cls：
a = cls（[1,2,3]）
print" iterate over％s" ％cls .__ name__
for i in a：
print i
print
输出：
迭代A
A .__ getitem__
1
A .__ getitem__
2
A .__ getitem__
3
A .__ getitem__
迭代A_with_iter
A_with_iter .__ iter__
1
2
3
请注意这个是最近报告的email.Message中的错误的基础
和cgi.FieldStorage。 email.Message没有实现__iter__，它的
__getitem__方法假定项目将像keyed
字典查找一样被检索，而不是使用整数序列访问。所以当__getitem__
在输入字符串上调用.lower（）时，Python会抛出一个错误 - 你不能在一个int上做
lower（）。
Section 7.3 (from the link given above) gives the syntax for "for" as:
for_stmt ::= "for" target_list "in" expression_list ":" suite
["else" ":" suite]
and then begins describing the component syntax elements as, "The expression
list is evaluated once; it should yield a sequence." This seems to be a bit
dated, since expression_list could also be a generator or iterator.
Additionally, "for" uses an adaptive method to try to simulate an iterator
if no __iter__ method is provided, by successively calling __getitem__ until
IndexError is raised (this error gets silently absorbed within this
pseudo-iterator).
Here is a simple test class: (I also implemented __len__ thinking that it
would be used to limit the calls to __getitem__, but we can see from the
output that it is never called - instead __getitem__ gets called one time
too many, telling the pseudo-iterator that there are no more entries).
class A(object):
def __init__(self,lst):
self.list_ = lst[:]
def __len__(self):
print self.__class__.__name__+".__len__"
return len(self.list_)
def __getitem__(self,i):
print self.__class__.__name__+".__getitem__"
return self.list_[i]
class A_with_iter(A):
def __iter__(self):
print self.__class__.__name__+".__iter__"
return iter(self.list_)
for cls in (A, A_with_iter):
a = cls([1,2,3])
print "iterate over %s" % cls.__name__
for i in a:
print i
print
output:
iterate over A
A.__getitem__
1
A.__getitem__
2
A.__getitem__
3
A.__getitem__
iterate over A_with_iter
A_with_iter.__iter__
1
2
3
Note that this is the basis for the recently reported bugs in email.Message
and cgi.FieldStorage. email.Message does not implement __iter__, and its
__getitem__ method assumes that items will be retrieved like keyed
dictionary lookups, not using integer sequence access. So when __getitem__
calls .lower() on the input string, Python throws an error - you can''t do
lower() on an int.
em = email.message_from_file（open（''MAILMESSAGE''））
for i in em：
em = email.message_from_file(open(''MAILMESSAGE''))
for i in em:
... print i
...
Traceback（最近一次调用最后一次）：
文件"< stdin>"，line 1，在文件中/usr/lib/python2.3/email/Message.py" ;,
第304行，在__getitem__
文件" / usr / lib / python2中。 3 / email / Message.py"，
第370行，获取
AttributeError：''int''对象没有属性''lower''
... print i
...
Traceback (most recent call last):
File "<stdin>", line 1, in ?
File "/usr/lib/python2.3/email/Message.py",
line 304, in __getitem__
File "/usr/lib/python2.3/email/Message.py",
line 370, in get
AttributeError: ''int'' object has no attribute ''lower''
- Paul
-- Paul
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