PEP 3106 -- Revamping(改进) dict.keys(), .values() and .items()
1. Abstract(摘要)
This PEP proposes(建议) to change the .keys(), .values() and .items() methods of the built-in dict type to return a set-like or unordered container object whose contents are derived from the underlying(潜在的) dictionary rather than a list which is a copy of the keys, etc.; and to remove the .iterkeys(), .itervalues() and .iteritems() methods.
The approach is inspired(灵感) by that taken in the Java Collections Framework [1].
2.Introduction
It has long been the plan to change the .keys(), .values() and .items() methods of the built-in dict type to return a more lightweight object than a list, and to get rid of .iterkeys(), .itervalues() and .iteritems(). The idea is that code that currently (in 2.x) reads:
for k, v in d.iteritems(): ...
should be rewritten as:
for k, v in d.items(): ...
(and similar for .itervalues() and .iterkeys(), except the latter is redundant since we can write that loop as for k in d.)
Code that currently reads:
a = d.keys() # assume we really want a list here
(etc.) should be rewritten as
a = list(d.keys())
There are (at least) two ways to accomplish(实现) this. The original plan was to simply let .keys(), .values() and .items() return an iterator, i.e. exactly what iterkeys(), itervalues() and iteritems() return in Python 2.x. However, the Java Collections Framework [1] suggests that a better solution is possible: the methods return objects with set behavior (for .keys() and .items()) or multiset (== bag) behavior (for .values()) that do not contain copies of the keys, values or items, but rather reference the underlying dict and pull their values out of the dict as needed.
The advantage(优势) of this approach is that one can still write code like this:
a = d.items()
for k, v in a: ...
# And later, again:
for k, v in a: ...
Effectively, iter(d.keys()) (etc.) in Python 3.0 will do what d.iterkeys() (etc.) does in Python 2.x; but in most contexts we don't have to write the iter() call because it is implied by a for-loop.
在python3.0中,iter(d.keys())等同于python2.0的d.iterkeys() ,但是,在大多数场景下,我们不需要嗲用iter(),因为,可以使用简单的for循环实现。
list可迭代化,但不是迭代器。
https://stackoverflow.com/questions/45458631/how-python-built-in-function-iter-convert-a-python-list-to-an-iterator
The objects returned by the .keys() and .items() methods behave like sets. The object returned by the values() method behaves like a much simpler unordered collection -- it cannot be a set because duplicate values are possible.
Because of the set behavior, it will be possible to check whether two dicts have the same keys by simply testing:
if a.keys() == b.keys(): ...
and similarly for .items().
These operations are thread-safe only to the extent that using them in a thread-unsafe way may cause an exception but will not cause corruption of the internal representation.
As in Python 2.x, mutating a dict while iterating over it using an iterator has an undefined effect and will in most cases raise a RuntimeError exception. (This is similar to the guarantees made by the Java Collections Framework.)
The objects returned by .keys() and .items() are fully interoperable with instances of the built-in set and frozenset types; for example:
set(d.keys()) == d.keys()
is guaranteed to be True (except when d is being modified simultaneously by another thread).
Specification
I'm using pseudo-code to specify the semantics:
class dict:
# Omitting all other dict methods for brevity.
# The .iterkeys(), .itervalues() and .iteritems() methods
# will be removed.
def keys(self):
return d_keys(self)
def items(self):
return d_items(self)
def values(self):
return d_values(self)
class d_keys:
def __init__(self, d):
self.__d = d
def __len__(self):
return len(self.__d)
def __contains__(self, key):
return key in self.__d
def __iter__(self):
for key in self.__d:
yield key
# The following operations should be implemented to be
# compatible with sets; this can be done by exploiting
# the above primitive operations:
#
# <, <=, ==, !=, >=, > (returning a bool)
# &, |, ^, - (returning a new, real set object)
#
# as well as their method counterparts (.union(), etc.).
#
# To specify the semantics, we can specify x == y as:
#
# set(x) == set(y) if both x and y are d_keys instances
# set(x) == y if x is a d_keys instance
# x == set(y) if y is a d_keys instance
#
# and so on for all other operations.
class d_items:
def __init__(self, d):
self.__d = d
def __len__(self):
return len(self.__d)
def __contains__(self, (key, value)):
return key in self.__d and self.__d[key] == value
def __iter__(self):
for key in self.__d:
yield key, self.__d[key]
# As well as the set operations mentioned for d_keys above.
# However the specifications suggested there will not work if
# the values aren't hashable. Fortunately, the operations can
# still be implemented efficiently. For example, this is how
# intersection can be specified:
def __and__(self, other):
if isinstance(other, (set, frozenset, d_keys)):
result = set()
for item in other:
if item in self:
result.add(item)
return result
if not isinstance(other, d_items):
return NotImplemented
d = {}
if len(other) < len(self):
self, other = other, self
for item in self:
if item in other:
key, value = item
d[key] = value
return d.items()
# And here is equality:
def __eq__(self, other):
if isinstance(other, (set, frozenset, d_keys)):
if len(self) != len(other):
return False
for item in other:
if item not in self:
return False
return True
if not isinstance(other, d_items):
return NotImplemented
# XXX We could also just compare the underlying dicts...
if len(self) != len(other):
return False
for item in self:
if item not in other:
return False
return True
def __ne__(self, other):
# XXX Perhaps object.__ne__() should be defined this way.
result = self.__eq__(other)
if result is not NotImplemented:
result = not result
return result
class d_values:
def __init__(self, d):
self.__d = d
def __len__(self):
return len(self.__d)
def __contains__(self, value):
# This is slow, and it's what "x in y" uses as a fallback
# if __contains__ is not defined; but I'd rather make it
# explicit that it is supported.
for v in self:
if v == value:
return True
return False
def __iter__(self):
for key in self.__d:
yield self.__d[key]
def __eq__(self, other):
if not isinstance(other, d_values):
return NotImplemented
if len(self) != len(other):
return False
# XXX Sometimes this could be optimized, but these are the
# semantics: we can't depend on the values to be hashable
# or comparable.
olist = list(other)
for x in self:
try:
olist.remove(x)
except ValueError:
return False
assert olist == []
return True
def __ne__(self, other):
result = self.__eq__(other)
if result is not NotImplemented:
result = not result
return result
PEP 3106 -- Revamping(改进) dict.keys(), .values() and .items()的更多相关文章
- Python3基础 dict keys+values 循环打印字典中的所有键和值
Python : 3.7.0 OS : Ubuntu 18.04.1 LTS IDE : PyCharm 2018.2.4 Conda ...
- 查询set、dict、dict.keys()的速度对比
查找效率:set>dict>list 单次查询中: list set dict O(n) set做了去重,本质应该一颗红黑树 (猜测,STL就是红黑树),复杂度 O(logn): dict ...
- ES2017 keys,values,entries使用
let {keys, values, entries} = Object; let obj = { a: 1, b: 2, c: 3 }; for (let key of keys(obj)) { c ...
- StackExchange.Redis 官方文档(五) Keys, Values and Channels
原文:StackExchange.Redis 官方文档(五) Keys, Values and Channels Keys, Values and Channels 在使用redis的过程中,要注意到 ...
- 37-python基础-python3-字典的常用方法-keys()-values()-items()
有 3 个字典方法,它们将返回类似列表的值,分别对应于字典的键.值和键-值对:keys().values()和 items(). 这些方法返回的值不是真正的列表,它们不能被修改,没有append()方 ...
- ES6扩展——数组的新方法(Array.from、Array.of、Array.fill、Array.includes、keys values entries 、find)
1.Array.from(objec,回调函数)将一个ArrayLike对象或者Iterable对象(类数组对象)转换成一个数组 1)该类数组对象必须具有length属性,用于指定数组的长度.如果没有 ...
- fromkeys() keys() values() items()
fromkeys() >>> dict1={} >>> dict1.fromkeys((1,2,3))#会自动为没有赋值的值建立none {1: None, 2: ...
- keys(),values()和items()
a={'a':11,'b':'bb','c':321}for x in a.items(): print(x) # 每条都输出来print("------------")for ...
- python中的keys、values、items
keys()获取字典中所有的键并放入列表 values()获取字典中所有的值并放入列表 items()获取字典中所有的键值对并放入列表 举个例子: 1 a = { 2 "name" ...
随机推荐
- sql 条件插入
原普通插入语句: insert into seriestable_upload values(New.SeriesID,0); 加条件后:(当不存在该条数据插入) insert into custom ...
- lintcode-63-搜索旋转排序数组 II
63-搜索旋转排序数组 II 跟进"搜索旋转排序数组",假如有重复元素又将如何? 是否会影响运行时间复杂度? 如何影响? 为何会影响? 写出一个函数判断给定的目标值是否出现在数组中 ...
- jquery在页面加载完成后再append的元素事件无效问题
最近遇到一个问题,jquery在页面加载完成后再append的元素,append元素上有onclick事件,但是在append的元素上怎么点击都不会触发onclick事件.就如: <ul cla ...
- 6for Java
class Check{ public boolean validate(String name, String password){ if(name.equals("xuzhaoni ...
- Java的同步容器和并发容器
前言: 之前在介绍Java集合的时候说到,java提供的实现类很少是线程安全的.只有几个比较古老的类,比如Vector.Hashtable等是线程安全的,尤其是Hashtable,古老到连命名规范都没 ...
- vue中表单的动态绑定
有一个表单: <el-form :model="configForm"> </el-form> 如果configForm初始化为{},此对象是动态变化的,如 ...
- Maven中如何将源码之外的文件打包及添加本地jar
<build> <resources> <resource> <directory>src/main/resources</directory&g ...
- 【电影影评】梦之安魂曲-败给了BGM和豆瓣影评
首先,这部电影豆瓣8.7分,一般来说,豆瓣的打分是比较准确的.能反映一个片子的质量,而较少受到环境的影响.但是这种关系当然也不全对,比如某些片子可能特别让某一种人喜欢(如退役军人和军旅题材),而在某些 ...
- [Leetcode] Copy list with random pointer 对带有任意指针的链表深度拷贝
A linked list is given such that each node contains an additional random pointer which could point t ...
- [Leetcode] set matrix zeroes 矩阵置零
Given a m x n matrix, if an element is 0, set its entire row and column to 0. Do it in place. click ...