Python Quick Guide - Step 1 Basic Syntax and Data Types (4) - Variables and Assignment
- This course is designed to help you learn the basics of Python programming as quickly as possible through hands-on practice.
- The “Style Guide” sections primarily cover guidelines from PEP8 for writing clean Python code.
- You can run and see the results of each code example.
Feel free to experiment with the code - reloading the page will reset the content.
This is a continuation of “Step 1: Basic Syntax and Data Types”.
1.4. Variables and Assignment
1.4.1. What are Variables
Variables are fundamental elements in programming.
While specifications may vary slightly depending on the programming language,
in Python, think of variables as “labels” that point to data.
(Not the box containing the value itself, but more like a name tag attached to that box)
graph LR
subgraph "Objects (Values)"
obj1[10]
obj2["Python"]
obj3[True]
end
subgraph "Variables (Labels)"
age --> obj1
name --> obj2
is_active --> obj3
end
style obj1 fill:#a5d8ff,stroke:#333,stroke-width:2px
style obj2 fill:#ffc078,stroke:#333,stroke-width:2px
style obj3 fill:#8ce99a,stroke:#333,stroke-width:2px
- Multiple variables can act as labels for the same value (this will be explained in detail later)
- Associating a value with a variable is called assignment
- For assignment, write the variable on the left side of the equals sign (
=) and the value on the right
- Leave spaces before and after the assignment operator (
=).
Create a program that converts Celsius (℃) to Fahrenheit (°F).
- Assign 25 to a variable called
celsius - Convert it to Fahrenheit using the formula
(celsius * 9/5) + 32 - Assign the result to a variable called
fahrenheit - Display both temperatures like “25℃ is 77.0°F”
-
First try displaying it using the methods we’ve already covered
-
Then try displaying it using the following format (which we’ll cover in section 1.4.4):
print(f"{celsius}℃ is {fahrenheit}°F")
-
1.4.2. Variable Naming Rules
When naming variables, follow these rules:
- Use letters (
a-z,A-Z), numbers, and underscores (_) - Do not start with a number
- Do not use reserved words (
if,for,while, etc.)
- Variable names should use lowercase letters with words separated by underscores (
_).
- Choose variable names that clearly indicate their purpose
- Avoid overly short names (
i,j, etc.) - Particularly avoid lowercase L (
l), uppercase O (O), and uppercase I (I) as they have poor visibility
- Avoid overly short names (
- Non-English variable names are possible but not recommended
- Variables whose values don’t change during a program are called constants.
For constants, it’s recommended to use all uppercase letters with words separated by underscores (_).- In reality, Python doesn’t have a feature to declare “unchangeable constants.”
Uppercase variable names are a convention among developers meaning “this value shouldn’t be changed,”
but technically, they can be modified just like regular variables.
- In reality, Python doesn’t have a feature to declare “unchangeable constants.”
1.4.3. Variable Reassignment
- Variable values can be reassigned.
- This is essentially an operation of relabeling to point to a different value (object).
graph LR
subgraph "Step 2: After Reassignment"
greeting2[greeting] --> value2["Good morning!"]
value1_old["Hello!"]
end
subgraph "Step 1: Initial Assignment"
greeting --> value1["Hello!"]
end
style value1 fill:#ffc078,stroke:#333,stroke-width:2px
style value2 fill:#ffc078,stroke:#333,stroke-width:2px
style value1_old fill:#ffc078,stroke:#333,stroke-width:2px,opacity:0.3
- Variables can also be reassigned to values of different types.
- You can reassign a variable using its current value.
- This may be confusing if you think of the equals sign (
=) as in mathematics,
but in programming, the equals sign represents the “assignment” operation.
(As we’ll see later, “equality” is represented by double equals (==))
- This may be confusing if you think of the equals sign (
- You can use these shorthand notations:
Calculate the final price of an item that costs 5000 yen before tax, with a 20% discount and then a 10% tax added.
- Assign 5000 to a variable called
total - Apply a 20% discount
- Add 10% tax
- Display the final price as “The final payment amount is ○○ yen”
1.4.4. Using Variables: String Formatting (f-strings)
As you’ve seen in the exercises above, there are times when you want to embed variable values within strings.
Python’s f-strings make this easy.
- Create an f-string by adding
fbefore the string - Use variables inside the string by writing
{variable_name}
- f-strings can include not just variables but also the results of calculations
- When displaying
floatvalues, you can specify the number of decimal places
- You can also pad numbers with zeros
- You can align text right, left, or center with specified width
- f-strings have many other features. For more details, please refer to:
f-strings are a relatively new feature introduced in Python 3.6.
Previously, the following methods were used:
f-strings are more readable and concise than these methods, so we recommend using them whenever possible.
Create a self-introduction using f-strings.
- Assign your name to a variable called
name(e.g., “John Smith”) - Assign your age to a variable called
age(e.g., 25) - Assign your hobby to a variable called
hobby(e.g., “programming”) - Assign your height in cm to a variable called
height(e.g., 175.5) - Using f-strings, display all this information in one line:
- “My name is ○○. I am ○○ years old, and my hobby is ○○. My height is ○○ cm.”
- Display the height with 1 decimal place