01 Variable
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1. Understanding Variables - Examples from Daily Life
1.1 What is a Variable
- Vari: Change
- Able: Capacity
1.2 An Example
Let's say you're the class monitor and you need to compile the monthly test scores of each student in the class. Each month's score will be recorded on a paper, listing each student's score, for instance:
- Li Lei: 98 points
- Ma Dongmei: 89 points
- Liu Yitong: 96 points
- ...
One day, the teacher wants to see Liu Yitong's scores for January, February, and March. As the class monitor, what do you do? Clearly, directly handing over each month's test score isn't appropriate.
We should copy Liu Yitong's January, February, and March scores onto a separate sheet of paper and then give it to the teacher.
So, why didn't we assign an envelope for each student from the beginning? (It could also be a file bag.) Initially, the envelope is flat. When we put things (data) in it, does it swell? Did it change? Did it gain size? Clearly, yes.
So, the envelope, did it allocate space in the current space we're in to store data and say, "This is an envelope."
Similarities: The refrigerator also allocates space in the current space we're in.
---- Variables allocate space in the computer's memory ---- Variables can be overwritten
2. Creating Variables - Assignment Statements
- Using a variable name to represent or reference a value
- Nuwa made mud figures, the figures had no life. Nuwa waved a willow branch, giving life to the mud figures. At this moment, the mud figure can represent Nuwa's descendants. "Variable: Mud figure, Value: Nuwa"
- The National People's Congress, the delegates of the NPC, are elected by the vast masses of the people. Their rights are not acquired by themselves, so they can say: "I represent the will of the vast masses of the people." "Variable: NPC delegate, Value: Masses"
- Initializing assignment statement: Variable Name = Expression
- Variable Name: What we call this space
- Expression: Similar to a mathematical expression
Logic of the program: From top to bottom, from right to left, finally assigning a value.
name1 = 'WRYH'
name2 = name1
print(name2)
# Variable passing
name1 = "look"
name1 = "WRYH"
print(name1)
# Variable overwrite
x = 1 # 1 assigned to x
x = x + 10 # x represents 1, so x + 10 == 1 + 10, after getting 11, it's assigned to variable x
print(x) # Print output
# The hashtag is for comments, annotations, for easy understanding of the code and future maintenance.
3. Exploring print
3.1 Outputting Multiple Data Simultaneously
a = 1
b = 1
c = 1
print(a, b, c)
# Output: 1 1 1
From the output, it's evident that print outputs multiple variables simultaneously, with each value defaulting to a space between them. So, can we modify this default space? Obviously, yes. Using sep.
3.2 Modifying the Separator for Multiple Simultaneous Outputs
a = 1
b = 1
c = 1
print(a, b, c, sep=".hhhhhh.")
3.3 Modifying the Print Output Ending
a = 1
b = 1
c = 1
print(a)
print(b)
print(c)
# Output:
# 1
# 1
# 1
a = 1
b = 1
c = 1
print(a, end="\n\n\n")
print(b)
print(c)
# Output:
# 1
#
# 1
# 1
a = 1
b = 1
c = 1
print(a, end="hello hello")
print(b)
print(c)
3.4 Using end and sep Simultaneously
a = 1
b = 1
c = 1
print(a, b, c, sep = "~", end = " Hello")
# Output: 1~1~1 Hello
4. Advanced Assignment Methods
4.1 Assigning the Same Value to Multiple Variables Simultaneously
a = b = c = 1
print (a, b, c,) # Assigning the same value to multiple variables simultaneously
4.2 Assigning Different Values to Multiple Variables Simultaneously
a, b, c = 1, 2, 3
print(a, b, c)
Practice
If you have two kids, an older brother named Austin and a younger brother named Jaden.
Austin wants juice, Jaden wants cola. Both brothers have their exclusive cups and don't like using each other's cups.
At home, there's only one cup of juice and one cup of cola. You accidentally pour cola into the older brother's cup and juice into the younger brother's cup.
Question: How do you exchange the juice between the cups of the older and younger brothers?
Austin = "Coke" # This assignment can be understood as pouring juice
Jaden = "juice" # This assignment can be understood as pouring juice
print("Austin", Austin)
print("Jaden", Jaden)
Austin1 = Jaden
Jaden1 = Austin
Austin = Austin1
Jaden = Jaden1
print("Austin", Austin)
print("Jaden", Jaden)
Austin = "Coke" # This assignment can be understood as pouring juice
Jaden = "juice" # This assignment can be understood as pouring juice
print("Austin", Austin)
print("Jaden", Jaden)
Austin1 = Austin
Austin = Jaden
Jaden = Austin1
print("Austin", Austin)
print("Jaden", Jaden)
Austin = "Coke" # This assignment can be understood as pouring juice
Jaden = "juice" # This assignment can be understood as pouring juice
print("Austin", Austin)
print("Jaden", Jaden)
Austin, Jaden = Jaden, Austin
print("Austin", Austin)
print("Jaden", Jaden)
5. Naming Rules for Variables
Combination of uppercase, lowercase, English, numbers, and __, and cannot start with a number;
a1b2c3 = 1 # Cannot start with a numberSystem keywords cannot be used as variable names;
as = 1 #
Cannot be printed As = 1 aS = 1 a_s = 1 ```
Get the keyword list: `help('keywords')`
``` python
Here is a list of the Python keywords. Enter any keyword to get more help.
False class from or
None continue global pass
True def if raise
and del import return
as elif in try
assert else is while
async except lambda with
await finally nonlocal yield
break for not
```
Variables are case-sensitive in Python
n = 1 N = 2 print (n) # 1Variable names cannot contain spaces, but can use underscores
user_name = "hello" username = "hi"Avoid using Python's built-in function names as variable names
print = "123" print(print) # Cannot be printed
6. Exercise
1. In Python, variable names can start with a number.
- [ ] True
- [x] False
2. Which of the following variable names is valid in Python?
- [ ] 2myVar
- [x] myVar2
- [ ] my-var
- [ ] my var
3. Variable names in Python are case-sensitive.
- [x] True
- [ ] False
4. In Python, `my_var` and `myVar` refer to the same variable.
- [ ] True
- [x] False
5. Variable names can be Python keywords.
- [ ] True
- [x] False
6. `None` is a special type in Python, representing no value or an empty value.
- [x] True
- [ ] False
7. Which of the following is a valid variable assignment in Python?
- [x] 123abc = "hello"
- [ ] for = "world"
- [ ] _hidden = "secret"
- [ ] import = 123
8. A variable in Python can be assigned multiple values simultaneously.
- [ ] True
- [x] False
9. Which of the following is a multi-variable assignment in Python?
- [x] a, b, c = 1, 2, 3
- [ ] a = 1, b = 2, c = 3
- [ ] a; b; c = 1; 2; 3
- [ ] a = 1; b = 2; c = 3;
10. Variables must be assigned a value before using them.
- [x] True
- [ ] False
11. The type of variables in Python is static; once assigned, it cannot be changed.
- [ ] True
- [x] False
12. In Python, which represents a global variable?
- [x] var = "hello"
- [ ] global var
- [ ] var(global)
- [ ] def var():
13. Which represents deleting a variable?
- [ ] delete x
- [x] del x
- [ ] remove x
- [ ] destroy x
```python
my_variable = "Hello, World!"
print(my_variable)
# Delete the variable
del my_variable
# Attempting to access a deleted variable will raise a NameError
# print(my_variable)
```
14. `x = 5`, then `y = x`, if you change the value of `y`, the value of `x` will also change.
- [ ] True
- [x] False
15. `x = [1, 2, 3]`, then `y = x`, if you change the list content of `y`, the content of `x` will also change.❌
- [x] True
- [ ] False
16. In Python, variables can only store basic data types, such as integers or strings.
- [ ] True
- [x] False
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