Declare
We first put this 123.py
Delete all
Then we type x = 5
x = 5 archive
If you save this file when you save
Just press the fork or cancel
This is just because the Sublime we use is
Free version so it sometimes
Will encourage you to donate or buy
We just press the fork
How to execute our program
back to cmd
Then type python space file name file name is 123.py
Then press Enter to run
You have to remember
If your path is correct
If you forget how to put the path
If you cd to the right place
You have to go back to see that
Run the first python file
Remember the python space when executing
Just press Enter for your file name
Then I press Enter
Nothing is printed after repeated execution
Then we are back now Sublime
Print it out i print x
Is the meaning of printing out my x, right?
print is to print things
Then I will print x now
Then I remember you saw this circle
On behalf of me
After my ctrl s archived it changed to fork
Then i come back cmd
I press on the implementation
Becomes 5
It printed x
it is good
Ok, we now know that print x can print 5
Because x is 5
Key announcement
What is an announcement
宣告 in English is declare declare
Declare therecreationthe meaning of
This word is quite important
creation
When we type x = 5
This program has created x
You imagine this world
You imagine that this world was originally nothing
You imagine that Martian sky is unique
It ’s all sand and no one
No machine is empty
When we enter x = 5
Our world produces x
Then I will draw x into a box
I put something in our world
Draw into boxes
Then the name of this box is x
It contains 5
The name of this box is x
Then it contains 5
I didn't have this box
After we enter x = 5
Our world has this box
So this is why it feels creative
Let me describe another example for you
Suppose I told you today
Trump is the President of the United States Trump is the President of the United States
Then you think about seeing a baby
When a baby is born, it knows nothing
He has nothing to do with this world
Don't know anything
Then you tell this baby
Trump is the President of the United States
Does the baby know
There is Trump in the world
Then he is the President of the United States
He didn't know that Trump was there
He heard you saying that Trump is equal to the President of the United States
He said Oh, there are Trumps in the world
Just like this feeling
When we enter x = 5
We create x
Then we know it is 5
Its value is 5
We do n’t have to be too persistent about part of this term
You just need to know
Just declare that you have a creative meaning
Then you have to realize this feeling
If we now put these
These words are not code, so we delete them first
Then you think about it, if I enter another line
x=10
How many boxes do you think in this world
We created a box
Still created two boxes
What do you think
the answer is
One
Still one
Come and explain to you now
Now there is a box
Then x and then this box contains 5
When we meet the first
Now is the third line when we meet the third line
When x = 10
We just wiped out this 5
Changed to 10 only
We are just going to change its content
Right, so you x = 5 and x = 10
What we are talking about x
Is talking about the same thing, this box
So what you have to understand here is that
When our code first encountered
When x = 5
Only in the first encounter will there be creation
After that, you will only change
Only the first encounter will create
After that, if you encounter it again, it will only change.
So if I write another x =
x=10000 1000 1000
Then we just say change 5 to 10
Then it was changed to 1000
Not so hard
What if I want to create a second box
I have to use a different name
For example
y = 2
Then our program runs
You first create a box which is x
Then it was originally 5
Change it to 10 and change it to 1000
And then on the fifth line
It produces another box which is y
The name of this box is y
Then it contains 2
It contains 2
Just like this, there are two boxes in this world
x and y
Okay, let me explain in terms
But what
I do n’t think people need to be too attached to these terms
You do n’t have to bring a laptop or something
Hard to recite those words
You only need to listen a few more times to understand
The terms x and y are called variables
English is called variable
Can also be abbreviated as var var
Why is it called a variable because it can change
For example, look at
At first x was 5
Later it becomes 10
Later it became 1000 and it was changing
So it is called a variable
Its origin is such that it can change
What about the right side
2 1000 10 these are the values
English is called value value
The x and y on the left are called variables
The 10 1000 2 on the right is called value
If you read the whole sentence
This x = 5 read the whole sentence
Should be read like this
We declare a variable
Its name is x
Then its value is 5
We create a box
That is, we declare a box
Its name is x and its value is 5
So the value is in it
You can imagine the inner content of a person
Just its value
A person's inner x's inner is 5
What about the third line
Again x
Is it x again?
We do n’t create because we already have x
This world already has the name x
How to read this x = 10
This is called we set the value of x to 10
The third line is set to 10
Because we haven't created anymore
So our third line is no longer declared
We will read that we set the value of x to 10
So we changed this 5 to 10
It actually has a term called assignment
assignment is Chinese called setting instruction
Each of these lines is a set instruction
x = 5 x = 10 y = 1000
Not x = 1000 y = 2
Each of these lines is an assignment
How to judge it is an assignment
Because we have an equal sign
That means me
We store 5 in x
You understand it as an equal sign
Save right to left
Save 5 to x
Read as we set the value of x to 5
10 save to x
That is, we set the value of x to 10
1000 saved to x read
We set the value of x to 1000
For example, if I type now
y = 1000
I can tell you
We now set the value of y to 1000
So those assignment variable value
Don't be too obsessed with memorizing its definition
Let's review this program now
We come line by line
Determine what it is doing
The first line x = 5
Because this is the first time we see x
So we have announced the creation of action
Create a box
Its name is x and its value is 5
We printed the second line
In the third line we set the value of x to 10
So I changed it to 10
In the fourth line, we set the value of x to 1000
I changed it again to 1000
The fifth line is the first time we have seen y in our life
So we have announced actions
We create y
A box called y, the value in it is 2
The sixth line y = 1000
We set the value of y to 1000
So we read these six lines like this
Then think about it
If we print now
and
What will be printed
Print out the value of y
The value of y is not just 1000
Let's try
Print out x there is printed out x there is printed out y
Then we perform a look
python 123.py
Then you see it prints 5 and 1000
Why it prints 5
Because x is 5 right
x later became 10 became 1000
That's what happened later
When it was printed, x was still 5
Because it reads from top to bottom
Directly after the execution of the first line
Execute the second line
Later it became 10
Later it became 1000
Then the last printed yy is 1000
So 5 and 1000 are not wrong
Then if you think about it,
with
What will happen
Try it on Enter
An error occurred
NameError
z is not defined
Is that z has not been defined
Simply put, we are still
There is no z in this world
Look at the above, we obviously only have x and y
How can we call it print z
We haven't created z yet
So when we want to ask it to print a
When we haven't created something
There will be this NameError
Then it will tell you
The name you gave has not been defined yet
That's it
