Python Variables and Data Types: Beginner Lesson Slides

Slide-by-slide content

1. 1. Python Variables and Data Types

   Beginner lesson · Names, values and Python's built-in types

   Speaker notes

   Ask: who has written any code before, even a single line? Reassure beginners that Python reads close to plain English. Tell them every code example in this deck has been run on Python 3, so they can type it in and check.

2. 2. By the end of this lesson you can

   • Create variables with valid, readable names
   • Recognise int, float, str, bool and None
   • Use list, tuple, dict and set
   • Check types with type() and convert them
   • Explain dynamic typing
   Speaker notes

   Read the objectives aloud. If computers are available, ask learners to open a Python shell now and try each example as you go.

3. 3. What is a variable?

   • A name that refers to a value
   • Created the first time you assign to it
   • No type declaration needed: age = 15
   • Reassign any time: age = 16
   Speaker notes

   Compare a variable to a label stuck on a value rather than a box. Ask: what does = mean in maths, and what does it mean here? In Python it means assign, not equals; == tests equality.

4. 4. Naming rules

   • Letters, digits and underscores only
   • Cannot start with a digit: 2nd_place is invalid
   • Case-sensitive: score and Score are different
   • Keywords such as class, if and for are not allowed
   • Style (PEP 8): snake_case, e.g. total_marks
   Speaker notes

   Type 2nd_place = 1 in the shell to show the SyntaxError. Ask: why might score and Score in the same program cause a bug? Python treats them as two separate variables.

5. 5. Good and bad names

   Clear names

   • student_name
   • total_marks
   • is_passed
   • max_speed

   Avoid

   • 2nd_place: starts with a digit
   • total-score: hyphen means minus
   • class: a keyword
   • x, tm: unclear meaning
   Speaker notes

   The first three names in the right column cause errors; the last one runs but is hard to read. Ask learners to rename tm to something meaningful. Code is read far more often than it is written.

6. 6. Numbers: int and float

   • int: whole numbers such as 42, -7, 0
   • float: decimals such as 3.14 and 98.6
   • 7 / 2 gives 3.5; 7 // 2 gives 3
   • Mixing them gives a float: 2 + 1.5 is 3.5
   • 0.1 + 0.2 shows 0.30000000000000004
   Speaker notes

   Run 0.1 + 0.2 live: it surprises everyone. Explain that floats are stored in binary, so some decimals are approximations. Ask: which operator would you use to share 7 sweets equally between 2 people?

7. 7. Text: str

   • Text in single or double quotes: 'Delhi', "Namaste"
   • '10' is text, not a number
   • '10' + '5' gives '105'
   • len('Python') gives 6
   Speaker notes

   Show that + joins strings instead of adding them. Ask: what would '10' * 3 give? It repeats the text: '101010'. A common error is trying to add a str and an int, which raises a TypeError.

8. 8. True, False and None

   • bool has two values: True and False
   • Comparisons give a bool: 5 > 3 is True
   • Capitals matter: true gives a NameError
   • None means no value yet; its type is NoneType
   Speaker notes

   Type true in the shell to show NameError: name 'true' is not defined. Ask: where might you use None? For example, a phone number that has not been entered yet.

9. 9. Collections: list and tuple

   • list: ordered and changeable: marks = [78, 91, 85]
   • marks.append(66) adds an item
   • Positions start at 0: marks[0] is 78
   • tuple: ordered but fixed: point = (3, 4)
   Speaker notes

   After append, marks is [78, 91, 85, 66]. Ask: why would you choose a tuple for map coordinates? Because they should not change by accident.

10. 10. Collections: dict and set

    • dict: key–value pairs: student = {'name': 'Asha', 'age': 14}
    • Look up by key: student['name'] gives 'Asha'
    • set: unique items, no fixed order
    • {2, 3, 3} becomes {2, 3}
    Speaker notes

    Relate a dict to a school ID card: each label (key) has a value. Ask: how could a set help count how many different states students come from? It removes duplicates automatically.

11. 11. Can it change after it is made?

    Mutable

    • list
    • dict
    • set

    Immutable

    • int, float and bool
    • str
    • tuple
    • None
    Speaker notes

    Show that t[0] = 9 on a tuple raises TypeError: 'tuple' object does not support item assignment. The same happens for a str. Reassigning a name is allowed; changing an immutable value in place is not.

12. 12. Checking types with type()

    • type(42) → <class 'int'>
    • type(3.14) → <class 'float'>
    • type('42') → <class 'str'>
    • type([1, 2]) → <class 'list'>
    Speaker notes

    Ask learners to predict type(3.0) before running it; the answer is float, because of the decimal point. type() is the quickest way to debug a value that behaves strangely.

13. 13. Dynamic typing

    • A name can refer to a value of any type
    • x = 5, then x = 'five' is allowed
    • The value has the type, not the name
    • Check with type(x) when unsure
    Speaker notes

    Run x = 5, type(x), then x = 'five', type(x): the result changes from int to str. Ask: what problem could this cause in a long program? A name may hold a different type than you expect.

14. 14. Converting between types

    • int('25') gives 25
    • float('3.5') gives 3.5
    • str(45) gives '45'
    • int('abc') raises a ValueError
    • input() always returns a str
    Speaker notes

    This is the most common beginner bug: adding a number typed by the user without converting it. Ask: what does int('3.5') do? It raises a ValueError; use float() first.

15. 15. A canteen bill in code

    • price = 45
    • qty = int('3')
    • total = price * qty
    • print(f'Total: Rs {total}')
    • Output: Total: Rs 135
    Speaker notes

    In a real program qty would come from input(), which returns the text '3', so int() is needed. Ask: what would print if we forgot int()? '3' * 45 repeats the text, a classic surprise.

16. 16. Quick check

    Which of these is a valid variable name in Python?

    • 2nd_place
    • total-score
    • student_name
    • class
    Speaker notes

    Answer: C, student_name. 2nd_place starts with a digit, total-score contains a minus sign, and class is a reserved keyword.

17. 17. Key takeaways

    • A variable is a name that refers to a value
    • Names: letters, digits, underscores; no leading digit
    • Core types: int, float, str, bool, None
    • Collections: list, tuple, dict, set
    • Use type() to check and int(), float(), str() to convert
    Speaker notes

    Recap with one live example per bullet. Then take the five-question quiz on the page and set a short task: store your name, age and three favourite foods in suitable variables and print their types.

Key terms

Variable

A name that refers to a value, created when you first assign to it.

Data type

The kind of a value, such as int, float or str, which decides what you can do with it.

Dynamic typing

The type belongs to the value, so the same name can refer to values of different types over time.

Keyword

A reserved word such as class, if or for that cannot be used as a variable name.

Mutable

Can be changed after it is created, like a list, dict or set.

Immutable

Cannot be changed in place after creation, like an int, str or tuple.

Type conversion

Turning a value into another type with functions such as int(), float() and str().

Quick quiz

Frequently asked questions