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Naming variables

Learn valid variable naming rules.

Variable naming rules

A variable name must start with a letter or underscore; it cannot start with a digit. It can contain letters, digits and underscores.

Valid names: user_age, _count, total2. Invalid names: 2nd (starts with a digit), user-age (has a hyphen), class (reserved word).

Python is case sensitive: age and Age are different variables. Reserved words (if, for, class, def) cannot be used.

The snake_case convention

The Python community convention is to use snake_case: lowercase words separated by underscores.

  • user_age not userAge
  • total_price not totalprice
  • is_valid not isvalid

For boolean variables the is_ or has_ prefix is common. This convention is defined in PEP 8.

Practice Declare two variables

Assign "Alex" to user_name, assign 25 to user_age. Then print print(user_name).

Starter code

main.py
# Declare the variables

Sample solution

main.py
user_name = "Alex"
user_age = 25
print(user_name)

Expected output

Alex

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 Which is a valid variable name?

  • 2nd_place
  • total_count Correct answer
  • user-name
  • class

Explanation: total_count starts with a letter and uses an underscore.

2 Declare the age variable

Question 
user_____ = 25
Answer 
user_age = 25

Explanation: user_age = 25 follows snake_case naming.

3 Match the naming principles

Term Meaning
Starts with a letter Valid name
Starts with a digit Invalid name
Reserved word Cannot be used
snake_case Python convention

All lessons in Beginner

1 Python basics

  1. What is Python?
  2. REPL and print
  3. Comments
  4. Indentation blocks
  5. python -c and scripts
  6. Summary - Python basics

2 Variables

  1. Variable assignment
  2. Multiple assignment
  3. Naming variables
  4. id and type
  5. Swapping variables
  6. Summary - Variables

3 Data types

  1. int and float
  2. bool
  3. str basics
  4. None
  5. Type conversion
  6. Summary - Data types

4 Project: Calculator

  1. Project design
  2. Add/subtract
  3. Multiply/divide
  4. Error handling
  5. Putting it together

5 Operators

  1. Arithmetic
  2. Comparison
  3. Logical
  4. Bitwise
  5. Membership and identity
  6. Summary - Operators

6 String basics

  1. f-string
  2. Concatenation
  3. str.format
  4. Raw strings
  5. Triple-quote
  6. Summary - String basics

7 Conditionals

  1. The if statement
  2. elif and else
  3. Nested if
  4. Truthiness
  5. Conditional expression
  6. Summary - Conditionals

8 Loops

  1. The for loop
  2. range
  3. The while loop
  4. break and continue
  5. Loop else
  6. Summary - Loops

9 Project: Shopping cart

  1. Cart design
  2. Adding items
  3. Calculating totals
  4. Updating quantity
  5. Putting it together

10 Lists

  1. Declaring lists
  2. Index and slice
  3. List methods
  4. in and count
  5. Sorting lists
  6. Summary - Lists

11 List comprehension

  1. Comprehension basics
  2. Filtered comprehension
  3. Dict and set comprehension
  4. Nested comprehension
  5. Generator expression
  6. Summary - List comprehension

12 Tuple, set and dict

  1. Tuple
  2. Set
  3. Dict basics
  4. Dict methods
  5. Dict comprehension
  6. Summary - Tuple, set and dict

13 Functions

  1. def
  2. return
  3. Docstring
  4. Default parameters
  5. *args and **kwargs
  6. Summary - Functions

14 Function parameters

  1. Positional parameters
  2. Keyword parameters
  3. Keyword-only
  4. Positional-only
  5. Type hints basics
  6. Summary - Function parameters

15 Project: To-do CLI

  1. Task model
  2. Adding tasks
  3. Listing tasks
  4. Completing tasks
  5. Putting it together

16 Advanced string methods

  1. split and join
  2. strip, lstrip, rstrip
  3. replace
  4. find and index
  5. startswith, endswith
  6. Summary - Advanced strings

17 Input and output

  1. input()
  2. int/float conversion
  3. print parameters
  4. f-string formatting
  5. sys.argv
  6. Summary - Input and output

18 Error handling

  1. try/except
  2. else and finally
  3. raise
  4. Exception hierarchy
  5. Custom exceptions
  6. Summary - Error handling

19 Modules

  1. import
  2. from import
  3. as alias
  4. __name__ == '__main__'
  5. Standard library
  6. Summary - Modules

20 Beginner final

  1. Project design
  2. Data model
  3. Business logic
  4. Error handling
  5. Putting it together