Python interview must ask questions

Python 2020-11-11 15:52:00
python interview ask questions


In past interviews , One of the most frequently asked questions is assignment 、 The principle of deep copy and shallow copy is , Today I'm going to take you to solve this problem .

First , What is the knowledge point of this question design ? We have to figure out the problem first . This problem involves the bottom layer of our computer ---- Memory . So how can we solve this problem now ? This question asks us about assignment 、 What kind of embodiment are deep copy and shallow copy in our memory .

Everybody knows , Everything is object , An object is a memory allocation area , So the popular expression for this question is , Does the assignment partition a region in memory , What's the difference between deep copy and shallow copy zoning ? Let's talk about it one by one

assignment

Assignment is to assign a variable to a value ( name ), So that we can use this value .

There are several types of memory id Fixed data :int type 、 Letter 、 Spaces, etc.

about id Indefinite data , If you assign two variables at the same time , So two variables are the same object , Will point to the memory address where the value is located .

a = 123
b = 123
print(id(a), id(b))
Execution results :
(1590328144, 1590328144)

So other data types ( Like strings , list , Dictionary, etc ), If you assign two variables at the same time , So they are two objects , Points to the memory address of the respective data .

a = [1, 2, 3]
b = [1, 2, 3]
print(id(a), id(b))
Execution results :
1881566679560 1881566679240

If not b assignment , Direct order b = a, So they are a reference relationship ,b quote a The object of , Will point to a The memory address .( We were asked about assignment during the interview , Usually the interviewer also refers to this phenomenon , Answer this )

a = [1, 2, 3]
b = a
print(id(a), id(b))
Execution results :
2166563410440 2166563410440

Shallow copy

Here we're going to use a library copy(python Self contained ). Shallow copy means that when a piece of data has nested data types ( for example [1,2,[3,4,5]]), Only the outer layer of data can be copied out to become a completely independent individual , That is to say, generate a new object , And the second tier data can't be copied out , It's just a quote , The memory address remains unchanged , This will create a phenomenon : Modify the outer data of one of the objects , The data of the other object does not change , And modify the inner data of one of the objects , The data of another object will also change .

import copy
a = [1, 2, 3, [4, 5, 6]]
#  Shallow copy
b = copy.copy(a)
#  Modify the first level data
a[0] = 10
print(a)
print(b)
print(id(a), id(b))
Execution results :
[10, 2, 3, [4, 5, 6]]
[1, 2, 3, [4, 5, 6]]
1724296570184 1724296571464
import copy
a = [1, 2, 3, [4, 5, 6]]
#  Shallow copy
b = copy.copy(a)
#  Modify the second level data
a[3][0] = 10
print(a)
print(b)
print(id(a), id(b))
Execution results :
[1, 2, 3, [10, 5, 6]]
[1, 2, 3, [10, 5, 6]]
1813604181320 1813604182600

Deep copy

The difference between deep copy and shallow copy is , No matter how many data types are nested in the data type , It will completely copy all the data into a new object , Modifying any of these data will not affect the data of the other object .

import copy
a = [1, 2, 3, [4, 5, 6]]
#  Deep copy
b = copy.deepcopy(a)
#  Modify the second level data
a[3][0] = 10
print(a)
print(b)
print(id(a), id(b))
Execution results :
[1, 2, 3, [10, 5, 6]]
[1, 2, 3, [4, 5, 6]]
2274488001864 2274488003144

Conclusion

So see here , You assign a value to us 、 Shallow copy and deep copy get it ? Assignment is to take the data of the object directly and use it , It doesn't change the memory address . Shallow copy will make the first layer of object data copied out to become a new object , So the address changed , But the data from the second and deeper levels will only be quoted , It doesn't generate a new address , So if you change the data in it, the data of both objects will change . Deep copy will copy all the data into a new object , The address has completely changed , So change the data of one of the objects , The other object is not affected .

Later, we met this problem , Answer in this way , Absolutely full .

image

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