Python中遍历字典过程中更改元素导致异常的解决方法
先来回顾一下Python中遍历字典的一些基本方法：
脚本:
#!/usr/bin/python
dict={"a":"apple","b":"banana","o":"orange"}
print "##########dict######################"
for i in dict:
print "dict[%s]=" % i,dict[i]
print "###########items#####################"
for (k,v) in dict.items():
print "dict[%s]=" % k,v
print "###########iteritems#################"
for k,v in dict.iteritems():
print "dict[%s]=" % k,v
print "###########iterkeys,itervalues#######"
for k,v in zip(dict.iterkeys(),dict.itervalues()):
print "dict[%s]=" % k,v
执行结果:
##########dict###################### dict[a]= apple dict[b]= banana dict[o]= orange ###########items##################### dict[a]= apple dict[b]= banana dict[o]= orange ###########iteritems################# dict[a]= apple dict[b]= banana dict[o]= orange ###########iterkeys,itervalues####### dict[a]= apple dict[b]= banana dict[o]= orange
嗯，然后我们进入“正题”--
一段关于Python字典遍历的“争论”....
先摘抄下：
#这里初始化一个dict
>>> d = {'a':1, 'b':0, 'c':1, 'd':0}
#本意是遍历dict，发现元素的值是0的话，就删掉
>>> for k in d:
... if d[k] == 0:
... del(d[k])
...
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
RuntimeError: dictionary changed size during iteration
#结果抛出异常了，两个0的元素，也只删掉一个。
>>> d
{'a': 1, 'c': 1, 'd': 0}
>>> d = {'a':1, 'b':0, 'c':1, 'd':0}
#d.keys() 是一个下标的数组
>>> d.keys()
['a', 'c', 'b', 'd']
#这样遍历，就没问题了，因为其实其实这里遍历的是d.keys()这个list常量。
>>> for k in d.keys():
... if d[k] == 0:
... del(d[k])
...
>>> d
{'a': 1, 'c': 1}
#结果也是对的
>>>
#这里初始化一个dict
>>> d = {'a':1, 'b':0, 'c':1, 'd':0}
#本意是遍历dict，发现元素的值是0的话，就删掉
>>> for k in d:
... if d[k] == 0:
... del(d[k])
...
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
RuntimeError: dictionary changed size during iteration
#结果抛出异常了，两个0的元素，也只删掉一个。
>>> d
{'a': 1, 'c': 1, 'd': 0}
>>> d = {'a':1, 'b':0, 'c':1, 'd':0}
#d.keys() 是一个下标的数组
>>> d.keys()
['a', 'c', 'b', 'd']
#这样遍历，就没问题了，因为其实其实这里遍历的是d.keys()这个list常量。
>>> for k in d.keys():
... if d[k] == 0:
... del(d[k])
...
>>> d
{'a': 1, 'c': 1}
#结果也是对的
>>>
其实这个问题本来很简单，就是说如果遍历一个字典，但是在遍历中改变了他，比如增删某个元素，就会导致遍历退出，并且抛出一个dictionary changed size during iteration的异常.
解决方法是遍历字典键值，以字典键值为依据遍历，这样改变了value以后不会影响遍历继续。
但是下面又有一位大神抛出高论：
首先，python 是推荐使用迭代器的，也就是 for k in adict 形式。其次，在遍历中删除容器中的元素，在 C++ STL 和 Python 等库中，都是不推荐的，因为这种情况往往说明了你的设计方案有问题，所有都有特殊要求，对应到 python 中，就是要使用 adict.key() 做一个拷贝。最后，所有的 Python 容器都不承诺线程安全，你要多线程做这件事，本身就必须得加锁，这也说明了业务代码设计有问题的.
但由“遍历中删除特定元素”这种特例，得出“遍历dict的时候，养成使用 for k in d.keys() 的习惯”，我觉得有必要纠正一下。在普通的遍历中，应该使用 for k in adict。
另外，对于“遍历中删除元素”这种需求，pythonic 的做法是 adict = {k, v for adict.iteritems() if v != 0} 或 alist = [i for i in alist if i != 0]
这个写法让我眼前一亮：怎么还有这个语法？
再仔细一看，他可能是这个意思：
#!/usr/bin/env python
# -*- coding=utf-8 -*-
a = {'a':1, 'b':0, 'c':1, 'd':0}
b={}
for k,v in a.items():
if v != 0:
b.update({k:v})
adict = b
del b
print a
#!/usr/bin/env python
# -*- coding=utf-8 -*-
a = {'a':1, 'b':0, 'c':1, 'd':0}
b={}
for k,v in a.items():
if v != 0:
b.update({k:v})
adict = b
del b
print a
不知道对不对。
因为这个写法一开始让我猛然想到三元操作符，仔细一看才发现不是，以前Goolge到有个解决方案
val = float(raw_input("Age: "))
status = ("working","retired")[val>65]
print "You should be",status
val = float(raw_input("Age: "))
status = ("working","retired")[val>65]
print "You should be",status
val>65是个逻辑表达式，返回0或者1，刚好作为前面那个元组的ID来取值，实在是太妙了。。。
不过在Google的资料里面还有一个版本
#V1 if X else V2 s = None a = "not null" if s == None else s print a #'not null'
后来发帖在华蟒用户组(中文Python技术邮件列表)中提到后众多大神解答如下：
>>> alist = [1,2,0,3,0,4,5]
>>> alist = [i for i in alist if i != 0]
>>> alist
[1, 2, 3, 4, 5]
>>> d = {'a':1, 'b':0, 'c':1, 'd':0}
>>> d = dict([(k,v) for k,v in d.iteritems() if v!=0])
>>> d
{'a':1,'c':1'}
如果大于Python>=2.7
还可以用这个写法：
>>> d = {k:v for k,v in d.iteritems() if v !=0 }
Hot AI Tools
Undresser.AI Undress
AI-powered app for creating realistic nude photos
AI Clothes Remover
Online AI tool for removing clothes from photos.
Undress AI Tool
Undress images for free
Clothoff.io
AI clothes remover
AI Hentai Generator
Generate AI Hentai for free.
Hot Article
Hot Tools
Notepad++7.3.1
Easy-to-use and free code editor
SublimeText3 Chinese version
Chinese version, very easy to use
Zend Studio 13.0.1
Powerful PHP integrated development environment
Dreamweaver CS6
Visual web development tools
SublimeText3 Mac version
God-level code editing software (SublimeText3)
Hot Topics
How to convert XML files to PDF on your phone?
Apr 02, 2025 pm 10:12 PM
It is impossible to complete XML to PDF conversion directly on your phone with a single application. It is necessary to use cloud services, which can be achieved through two steps: 1. Convert XML to PDF in the cloud, 2. Access or download the converted PDF file on the mobile phone.
Is the conversion speed fast when converting XML to PDF on mobile phone?
Apr 02, 2025 pm 10:09 PM
The speed of mobile XML to PDF depends on the following factors: the complexity of XML structure. Mobile hardware configuration conversion method (library, algorithm) code quality optimization methods (select efficient libraries, optimize algorithms, cache data, and utilize multi-threading). Overall, there is no absolute answer and it needs to be optimized according to the specific situation.
What is the function of C language sum?
Apr 03, 2025 pm 02:21 PM
There is no built-in sum function in C language, so it needs to be written by yourself. Sum can be achieved by traversing the array and accumulating elements: Loop version: Sum is calculated using for loop and array length. Pointer version: Use pointers to point to array elements, and efficient summing is achieved through self-increment pointers. Dynamically allocate array version: Dynamically allocate arrays and manage memory yourself, ensuring that allocated memory is freed to prevent memory leaks.
Is there any mobile app that can convert XML into PDF?
Apr 02, 2025 pm 08:54 PM
An application that converts XML directly to PDF cannot be found because they are two fundamentally different formats. XML is used to store data, while PDF is used to display documents. To complete the transformation, you can use programming languages and libraries such as Python and ReportLab to parse XML data and generate PDF documents.
How to convert xml into pictures
Apr 03, 2025 am 07:39 AM
XML can be converted to images by using an XSLT converter or image library. XSLT Converter: Use an XSLT processor and stylesheet to convert XML to images. Image Library: Use libraries such as PIL or ImageMagick to create images from XML data, such as drawing shapes and text.
What is the process of converting XML into images?
Apr 02, 2025 pm 08:24 PM
To convert XML images, you need to determine the XML data structure first, then select a suitable graphical library (such as Python's matplotlib) and method, select a visualization strategy based on the data structure, consider the data volume and image format, perform batch processing or use efficient libraries, and finally save it as PNG, JPEG, or SVG according to the needs.
Is there a mobile app that can convert XML into PDF?
Apr 02, 2025 pm 09:45 PM
There is no APP that can convert all XML files into PDFs because the XML structure is flexible and diverse. The core of XML to PDF is to convert the data structure into a page layout, which requires parsing XML and generating PDF. Common methods include parsing XML using Python libraries such as ElementTree and generating PDFs using ReportLab library. For complex XML, it may be necessary to use XSLT transformation structures. When optimizing performance, consider using multithreaded or multiprocesses and select the appropriate library.
How to control the size of XML converted to images?
Apr 02, 2025 pm 07:24 PM
To generate images through XML, you need to use graph libraries (such as Pillow and JFreeChart) as bridges to generate images based on metadata (size, color) in XML. The key to controlling the size of the image is to adjust the values of the <width> and <height> tags in XML. However, in practical applications, the complexity of XML structure, the fineness of graph drawing, the speed of image generation and memory consumption, and the selection of image formats all have an impact on the generated image size. Therefore, it is necessary to have a deep understanding of XML structure, proficient in the graphics library, and consider factors such as optimization algorithms and image format selection.
