search
HomeBackend DevelopmentPython TutorialPython中的引用和拷贝浅析

If an object's value can be modified, the object is said to be mutable. If the value cannot be modified,the object is said to be immutable.

mutable 可变类型,例如 list,set,自定义类型(等价于C#中的引用类型);

immutable 不可变类型,例如string,numbers等(等价于C#中的值类型);

一、引用和拷贝(references and copies)

当程序中使用=赋值操作符时,例如a=b,

对于不可变的对象,a作为b的一个拷贝被创建,a和b将指向不同的内存地址,a和b相互独立。

代码如下:


def TestCopy():
    a = 10
    b = a
    a =20
    print (b) #b still is 10


但是对于可变的对象,a作为b的一个引用被创建,a和b的元素公用相同的内存地址,a和b的元素共享。

代码如下:


def TestRef():
    a=[1,2,3,4]
    b=a   #b is a reference to a
    print (b is a) # True
    b[2] = -100 #change an element in b
    print (a) # a also changed to [1,2,-100,4]

二、深拷贝和浅拷贝(shallow copy and deep copy)

为了避免可变对象指向同一个对象,必须创建一个新的拷贝,而不是引用。
在python中可以对容器对象(例如lists和dictionaries)使用两种拷贝:浅拷贝和深拷贝。
 
浅拷贝创建一个新的对象,但是使用原来对象的元素的引用(如果是不变类型,相当于是拷贝)来填充新对象。可以使用copy.copy()来实现浅拷贝。

代码如下:


def TestShallowCopy():
    a = [ 1, 2, [3,4] ]
    b = list(a) # create a shallow copy of a
    print (b is a) #False
    b.append(100) #append element to b
    print (b)
    print (a) # a is unchanged
    b[2][0] = -100 # modify an element inside b
    print (b)
    print (a)  # a is changed

在这个例子中,a和b共享相同的可变元素。所以修改其中一个list对象中的元素,另一个list对象也会被修改。

深拷贝创建一个新的对象,同时递归地拷贝对象所包含的所有的元素。可以使用copy.deepcopy()来实现深拷贝。

代码如下:


def TestDeepCopy():
  import copy
  a = [1, 2, [3, 4]]
  b = copy.deepcopy(a)
  b[2][0] = -100
  print (b)  # b is changed
  print (a)  # a is unchanged

在这个例子中,a和b是对立的list对象,且他们的元素也相互独立。

三、引用计数和垃圾回收

python中的所有的对象都是引用计数的,一个对象赋值或加入容器时,它的引用计数就会自增,当使用del时或变量赋值为其他值时,引用计数就会自减,当引用计数为0时,python的垃圾回收器就会回收该变量。

代码如下:


def TestGarbageCollection():
  import sys
  print(sys.getrefcount(37))
  a = 37 # Creates an object with value 37
  print(sys.getrefcount(37))
  b = a # Increases reference count on 37
  print(sys.getrefcount(37))
  c = []
  c.append(b) # Increases reference count on 37
  print(sys.getrefcount(37))
  del a # Decrease reference count of 37
  print(sys.getrefcount(37))
  b = 42 # Decrease reference count of 37
  print(sys.getrefcount(37))
  c[0] = 2.0 # Decrease reference count of 37
  print(sys.getrefcount(37))
 
TestGarbageCollection()

运行结果为:

代码如下:


11
12
13
14
13
12
11


为啥一上来就有11个引用了呢?谁知道?
Statement
The content of this article is voluntarily contributed by netizens, and the copyright belongs to the original author. This site does not assume corresponding legal responsibility. If you find any content suspected of plagiarism or infringement, please contact admin@php.cn
How do you append elements to a Python array?How do you append elements to a Python array?Apr 30, 2025 am 12:19 AM

InPython,youappendelementstoalistusingtheappend()method.1)Useappend()forsingleelements:my_list.append(4).2)Useextend()or =formultipleelements:my_list.extend(another_list)ormy_list =[4,5,6].3)Useinsert()forspecificpositions:my_list.insert(1,5).Beaware

How do you debug shebang-related issues?How do you debug shebang-related issues?Apr 30, 2025 am 12:17 AM

The methods to debug the shebang problem include: 1. Check the shebang line to make sure it is the first line of the script and there are no prefixed spaces; 2. Verify whether the interpreter path is correct; 3. Call the interpreter directly to run the script to isolate the shebang problem; 4. Use strace or trusts to track the system calls; 5. Check the impact of environment variables on shebang.

How do you remove elements from a Python array?How do you remove elements from a Python array?Apr 30, 2025 am 12:16 AM

Pythonlistscanbemanipulatedusingseveralmethodstoremoveelements:1)Theremove()methodremovesthefirstoccurrenceofaspecifiedvalue.2)Thepop()methodremovesandreturnsanelementatagivenindex.3)Thedelstatementcanremoveanitemorslicebyindex.4)Listcomprehensionscr

What data types can be stored in a Python list?What data types can be stored in a Python list?Apr 30, 2025 am 12:07 AM

Pythonlistscanstoreanydatatype,includingintegers,strings,floats,booleans,otherlists,anddictionaries.Thisversatilityallowsformixed-typelists,whichcanbemanagedeffectivelyusingtypechecks,typehints,andspecializedlibrarieslikenumpyforperformance.Documenti

What are some common operations that can be performed on Python lists?What are some common operations that can be performed on Python lists?Apr 30, 2025 am 12:01 AM

Pythonlistssupportnumerousoperations:1)Addingelementswithappend(),extend(),andinsert().2)Removingitemsusingremove(),pop(),andclear().3)Accessingandmodifyingwithindexingandslicing.4)Searchingandsortingwithindex(),sort(),andreverse().5)Advancedoperatio

How do you create multi-dimensional arrays using NumPy?How do you create multi-dimensional arrays using NumPy?Apr 29, 2025 am 12:27 AM

Create multi-dimensional arrays with NumPy can be achieved through the following steps: 1) Use the numpy.array() function to create an array, such as np.array([[1,2,3],[4,5,6]]) to create a 2D array; 2) Use np.zeros(), np.ones(), np.random.random() and other functions to create an array filled with specific values; 3) Understand the shape and size properties of the array to ensure that the length of the sub-array is consistent and avoid errors; 4) Use the np.reshape() function to change the shape of the array; 5) Pay attention to memory usage to ensure that the code is clear and efficient.

Explain the concept of 'broadcasting' in NumPy arrays.Explain the concept of 'broadcasting' in NumPy arrays.Apr 29, 2025 am 12:23 AM

BroadcastinginNumPyisamethodtoperformoperationsonarraysofdifferentshapesbyautomaticallyaligningthem.Itsimplifiescode,enhancesreadability,andboostsperformance.Here'showitworks:1)Smallerarraysarepaddedwithonestomatchdimensions.2)Compatibledimensionsare

Explain how to choose between lists, array.array, and NumPy arrays for data storage.Explain how to choose between lists, array.array, and NumPy arrays for data storage.Apr 29, 2025 am 12:20 AM

ForPythondatastorage,chooselistsforflexibilitywithmixeddatatypes,array.arrayformemory-efficienthomogeneousnumericaldata,andNumPyarraysforadvancednumericalcomputing.Listsareversatilebutlessefficientforlargenumericaldatasets;array.arrayoffersamiddlegro

See all articles

Hot AI Tools

Undresser.AI Undress

Undresser.AI Undress

AI-powered app for creating realistic nude photos

AI Clothes Remover

AI Clothes Remover

Online AI tool for removing clothes from photos.

Undress AI Tool

Undress AI Tool

Undress images for free

Clothoff.io

Clothoff.io

AI clothes remover

Video Face Swap

Video Face Swap

Swap faces in any video effortlessly with our completely free AI face swap tool!

Hot Tools

Atom editor mac version download

Atom editor mac version download

The most popular open source editor

EditPlus Chinese cracked version

EditPlus Chinese cracked version

Small size, syntax highlighting, does not support code prompt function

SublimeText3 Mac version

SublimeText3 Mac version

God-level code editing software (SublimeText3)

Safe Exam Browser

Safe Exam Browser

Safe Exam Browser is a secure browser environment for taking online exams securely. This software turns any computer into a secure workstation. It controls access to any utility and prevents students from using unauthorized resources.

SAP NetWeaver Server Adapter for Eclipse

SAP NetWeaver Server Adapter for Eclipse

Integrate Eclipse with SAP NetWeaver application server.