Python Basics – Part 6: Type Conversion and Lists

In this part of my 5-week Python training series, we’ll cover two key concepts that appear everywhere in Python:
type conversion (casting) and lists, one of the most important data structures.

Type Conversion

Changing a value from one type to another is called type conversion or casting.

int()

Converts a string or float to an integer.

input_str = "12345"
input_int = int(input_str)
print(input_int, type(input_int))

Output:

12345 <class 'int'>

Now you can perform arithmetic operations:

result = input_int + 2000
print(result)

Output:

14345

💡 Strings cannot be used directly in math operations:

# result = input_str + 2000  ❌ TypeError
print(int(input_str) + 2000)  # ✅

str()

Converts numbers to strings.

number = 123
output = "The number is " + str(number)
print(output)

Output:

The number is 123

You can combine this with random numbers:

from random import randint
lotto = randint(1, 49)
print("Your lottery number is " + str(lotto))

float()

Converts integers or numeric strings into floating-point numbers.

print(float(123))
print(float("123.45"))

Output:

123.0
123.45

bool()

Converts a value into a Boolean (True or False).

print(bool("Hello"))  # True
print(bool(""))       # False

For numbers:

print(bool(0))     # False
print(bool(-123))  # True
print(bool(0.0))   # False
print(bool(777.7)) # True

Rule:
Only empty strings (""), zero (0, 0.0), and empty collections (like [], {}, set()) evaluate to False.
Everything else is True.

Lists

Lists are ordered, mutable sequences — perfect for storing multiple values in one variable.

Creating Lists

list1 = [1, 3, 5, 7]
print(list1)
print(type(list1))

Output:

[1, 3, 5, 7]
<class 'list'>

Indexing

List indices start at 0.

print(list1[0])  # 1
print(list1[1])  # 3
print(list1[2])  # 5
print(list1[3])  # 7

Modifying Elements

list1[0] = 11
print(list1)

Output:

[11, 3, 5, 7]

Mixed Data Types

Lists can store values of different types.

list1 = [1, 2.2, "Hello", True, [1, 2]]
print(list1)

Output:

[1, 2.2, 'Hello', True, [1, 2]]

Why Lists Are Useful

Instead of storing separate variables:

name1 = "Peter"
name2 = "Paul"
name3 = "Mary"
name4 = "Jane"

You can store them all in one list:

names = ["Peter", "Paul", "Mary", "Jane"]
print(names)

Common List Methods

.append()

Adds an element to the end of a list.

list2 = [1, 3, 5, 7, 9]
list2.append(23)
print(list2)

Output:

[1, 3, 5, 7, 9, 23]

.pop()

Removes and returns the last item of the list.

list3 = [1, 3, 5, 7, 9, 11]
list3.pop()
print(list3)

Output:

[1, 3, 5, 7, 9]

You can also capture the removed value:

deleted = list3.pop()
print(deleted, list3)

Output:

9 [1, 3, 5, 7]

Combining Lists and Dictionaries

You can combine two lists into pairs using zip():

t = [1, 2, 3]
v = ['a', 'b', 'c']

for x, y in zip(t, v):
    print(x, y)

Output:

1 a
2 b
3 c

Creating Dictionaries from Lists

t = [1, 2, 3]
v = ['a', 'b', 'c']

d = dict(zip(v, t))
print(d)

Output:

{'a': 1, 'b': 2, 'c': 3}

Accessing dictionary values:

print(d['a'])  # 1

Built-in list() Function

Converts any iterable into a list.

print(list(range(5)))
print(list(range(1, 30, 2)))
print(list("Hello"))

Output:

[0, 1, 2, 3, 4]
[1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27, 29]
['H', 'e', 'l', 'l', 'o']

Sorting Lists

Use .sort() to sort a list in place.

list4 = [11, 33, 54, 47, 19, 111]
list4.sort()
print(list4)

Output:

[11, 19, 33, 47, 54, 111]

Descending order:

list4.sort(reverse=True)
print(list4)

Output:

[111, 54, 47, 33, 19, 11]

Sorting strings:

list5 = ['D', 'A', 'B', 'F', 'P']
list5.sort()
print(list5)

Output:

['A', 'B', 'D', 'F', 'P']

Comparing Strings and Numbers

print('A' < 'B')  # True
print(65 < 62)    # False

Strings are compared using their ASCII values.

Summary

In this lesson, we learned:

• How to convert between data types (int(), str(), float(), bool())
• Basic list operations: creation, indexing, modification
• Useful list methods: .append(), .pop(), .sort()
• Using zip() to combine lists and dict() to create dictionaries
• Converting iterables to lists with list()

Next up: String methods, slicing, and indexing in depth!