In our programming , Variable is the most basic concept , It's as important as a brick we use to build a building , Is indispensable . therefore , Understanding how variables work is crucial .
Platform on the ninth floor , It starts with soil ; Cuddle wood , Start at the slightest ; A journey , Begins with a single step ！
Today, let's talk about Python Those things about variables .
1. Variables are not boxes
Let's look at the following code
1. a = "hello,world" 2. b = a 3. c = [1,2,3]
For us beginners , The assignment of variables is the easiest place to go wrong . What is the most common misunderstanding ？
Define a variable , Just create a variable box in memory , Then put the value of the variable in this box
Let's look at the picture below , This idea is quite wrong . Because of this misunderstanding , So our code may encounter many problems .
What's right ？ What do you do when assigning variables ？
2. Strange variables
1. a = "hello,world 2. b = a 3. c = [1,2,3]
So what exactly does the above code do ？ before this , Let me tell you a story first .
Last century 90 In time , Our famous Python buffet restaurant has opened . seeing the name of a thing one thinks of its function , This restaurant mainly serves buffet , But what? ？ Each customer can only eat one kind of delicious food , And will distribute the delicious food in different rooms , The type of food is not fixed , Provide customers with whatever they need .
On the day , Our little a The students came to the restaurant , Tell the front desk , I want to eat “hello,world”, therefore , A new room was opened at the hotel front desk , Room No ：00010, And put our... In it “hello,world”, And he gave the a A pass , This pass can only lead to 00010 Room number . And record hello,world： Number of people to eat ：1, When a When you want to eat , Just take your pass 00010 Go to room number to get
Before long , Small b Also came to our hotel , Tell the hotel front desk , I want to talk to you a Eat the same thing . So the hotel front desk , And to b A pass ,b According to the pass, you can also go to 00010 Go to room number to get hello,world. The hotel front desk records again ： hello,world： Number of people to eat ：2
Then , We are small c The students also came , He followed b Dissimilarity , He has what he wants to eat . He told the hotel front desk ： I want to eat [1,2,3]. Customers have new needs , The hotel front desk opened another room , Room No ：00020, And also put in it [1,2,3]. Also to c A pass . Then record ： [1,2,3]： Number of people to eat ：1
We can look down with this story
According to this picture , In our story above ：
customer a、b、c： Variable a、b、c\
The hotel ： Memory space
front desk ：Python Interpreter
room ： Memory space divided for objects
Your room number, ： The memory address of the object
food ： All kinds of objects （ character string 、 list 、 Dictionaries 、 Numbers ...）
The number of people recorded at the front desk ： Reference count
Communication Certificate number ： The memory address referenced by the variable
In fact, when we assign a value to a variable , Our variable does not store this value . It's bound to a memory address id, When we want to use the value of this variable , Just look for the stored value of this address in memory
And then the story above , Our little a classmate , Tired of eating hello,world, Now I want to eat 123456, So he ran to the hotel front desk and said , I want to eat now 123456 了 , The hotel front desk didn't say a word , A new room has been opened , Room No 00030, There's... In it 123456, And updated a Classmate's pass , At this time, this communication card can only go to 00030 Room number, eat 123456. The front desk continues to record hello,world： Number of people to eat ：1、123456： Number of people to eat ：1
In the code , We have changed a The value of the variable , What will happen ？
Let's see , change a What happens to the variable ？
a = 123456
Is that going to happen ？
We change a When , It won't change directly a The value stored at the memory address pointed to , Instead, open up a new space to store new values 123456, hold a Change the point to the address of the new space 00030, As shown in the figure below . The right thing to do is ：
our b Students like to imitate , She doesn't want to eat now hello,world 了 . So he ran downstairs to the hotel front desk and said ： I want to eat [1,2,3]. Watch out! , This time, b What the classmate said is , I want to eat [1,2,3], Not that I want to eat and c Same . Although their food is the same . But our front desk doesn't give it directly to b 00020 The pass for room . It's a new room , Your room number, 00040, Inside [1,2,3], And give b A pass points to 00040 Room number . Colleague records （00040）[1,2,3]： Number of people to eat ：1、（00010）hello,world： Number of people to eat ：0
b = [1,2,3]
Why? ？ In fact, it's easy to understand here , Because we b When assigning a value, an object is created . Just create a new object , Will reopen space .
however , like this
b = c
So there is no new object , It's going to be c Is passed to b, They all point to an object . Here, guys, pay attention , Don't be led astray by my example .
This is the time , Our hotel front desk found 00010 In room No hello,world No one has eaten . At this time, the hotel front desk will take this room back , And put the inside hello,world Throw away the food . This is python Garbage collection mechanism .
here , Again, I thought my classmates d,d Tell the hotel front desk , I want to talk to you c Eat the same , The hotel front desk will give it to d Issued a pass ,d According to the pass, you can go to 00020 Go to room number to get [1,2,3]. The hotel front desk records again ： [1,2,3]： Number of people to eat ：2
d = c
But our d Students are very picky , He is not satisfied with the existing [1,2,3], He wants to add something , Just tell the front desk to add food ： Now 00020 The things in the room become [1,2,3,4]
d.append(4)
here , We found a problem ,c The students did nothing , But the food he can eat from [1,2,3] Turned into [1,2,3,4]
What is the problem ？
Why did we a from “hello,world” become 123456 When , Is to open up a new space . But now d from [1,2,3] become [1,2,3,4], But directly modify it in the original memory space ？
This is it. python Classic interview questions ： The variability of objects ？ What are mutable objects , What is immutable object ？
3. Mutable objects versus immutable objects
stay python in , Everything is the object , But these objects are also divided into two categories ：
The variable object （3 individual ）：List（ list ）、Dictionary（ Dictionaries ）、Set（ aggregate ）
Immutable object （3 individual ）：Number（ Numbers ）、String（ character string ）、Tuple（ Tuples ）
Python Variable and immutable data types , It mainly depends on whether the value at the memory address pointed to by the variable will change .
3.2 Immutable object
>>> a = 10000 >>> id(a) 139964684838128 >>> a = 30000 # Immutable object , Change the value of a variable , It's actually instantiating a new object 、 Open up new memory space >>> id(a) # A new memory address is generated , It means that it is no longer the original object 139964684837872 >>>
3.3 The variable object
>>> a = [1,2,3] >>> b = a >>> id(a) 139711046464264 >>> id(b) 139711046464264 >>> b.append(4) # The variable object , Allows you to change the value of an object in place >>> id(b) 139711046464264 # The memory address has not changed , The description is to change the value in the original memory space >>> id(a) 139711046464264 >>> b [1, 2, 3, 4] >>> a [1, 2, 3, 4]
summary ：
The variable object ： The value at the memory address pointed to by the variable can be changed
Immutable object ： The value at the memory address pointed to by the variable cannot be changed .
