beginner⏱55 minLección 3 de 10

Variables & Data Types

Master Python variables, primitive data types (int, float, str, bool), type conversion, and naming conventions.

Variables & Data Types

Variables are the foundation of every program. They store data that your program manipulates. In Python, working with variables is intuitive and flexible.

What is a Variable?

A variable is a named location in memory that stores a value. Think of it as a labeled box where you can put different items.

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Creating Variables in Python

python

# Variable assignment
age = 25
name = "Alice"
height = 1.75
is_student = True

# Multiple assignment
x, y, z = 10, 20, 30
a = b = c = 0

# Displaying variables
print(f"Name: {name}")
print(f"Age: {age}")
print(f"Height: {height}m")
print(f"Student: {is_student}")

Output:

Name: Alice
Age: 25
Height: 1.75m
Student: True

ℹ️Note

Unlike languages like C or Java, Python doesn't require you to declare the type of a variable. Python infers the type from the value you assign. This is called "dynamic typing."

Python's Primitive Data Types

Python has several built-in data types. Let's explore the four most fundamental ones.

Overview of Data Types

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Type	Python Name	Example	Description
Integer	`int`	`42`, `-7`, `0`	Whole numbers (positive, negative, zero)
Float	`float`	`3.14`, `-0.5`, `2.0`	Decimal numbers
String	`str`	`"hello"`, `'world'`	Text/characters
Boolean	`bool`	`True`, `False`	Truth values

Integers (int)

Integers represent whole numbers without a decimal point.

Integer Operations

python

# Creating integers
positive = 100
negative = -50
zero = 0
large_number = 1_000_000  # Underscores for readability

# Arithmetic with integers
a = 15
b = 4

print(f"Addition: {a} + {b} = {a + b}")       # 19
print(f"Subtraction: {a} - {b} = {a - b}")    # 11
print(f"Multiplication: {a} * {b} = {a * b}") # 60
print(f"Division: {a} / {b} = {a / b}")       # 3.75 (returns float!)
print(f"Floor Division: {a} // {b} = {a // b}") # 3
print(f"Modulus: {a} % {b} = {a % b}")        # 3
print(f"Exponent: {a} ** {b} = {a ** b}")     # 50625

Output:

Addition: 15 + 4 = 19
Subtraction: 15 - 4 = 11
Multiplication: 15 * 4 = 60
Division: 15 / 4 = 3.75
Floor Division: 15 // 4 = 3
Modulus: 15 % 4 = 3
Exponent: 15 ** 4 = 50625

💡Tip

Python integers have unlimited precision! You can work with numbers as large as your memory allows:

python

>>> 2 ** 1000
107150860718626732094842504906000181056140481170553360744375038837...

Floats (float)

Floats represent numbers with decimal points.

Float Operations and Precision

python

# Creating floats
pi = 3.14159
temperature = -5.5
scientific = 6.022e23  # Scientific notation: 6.022 × 10²³

# Float operations
x = 10.5
y = 3.2

print(f"Addition: {x + y}")       # 13.7
print(f"Multiplication: {x * y}") # 33.6

# Float precision caveat
print(f"\n0.1 + 0.2 = {0.1 + 0.2}")  # 0.30000000000000004 (!)
print(f"0.1 + 0.2 == 0.3: {0.1 + 0.2 == 0.3}")  # False!

Output:

Addition: 13.7
Multiplication: 33.6

0.1 + 0.2 = 0.30000000000000004
0.1 + 0.2 == 0.3: False

⚠️Warning

Floating-point arithmetic can have precision issues due to how computers represent decimals in binary. For financial calculations, use the decimal module instead of floats.

python

# Using decimal for precise calculations
from decimal import Decimal

a = Decimal('0.1')
b = Decimal('0.2')
print(f"Decimal: {a + b}")  # 0.3 (exact!)

Strings (str)

Strings represent text and are enclosed in quotes.

Creating and Using Strings

python

# Different ways to create strings
single_quotes = 'Hello'
double_quotes = "World"
triple_quotes = """This is a
multi-line string"""

# String operations
first_name = "Alice"
last_name = "Smith"

# Concatenation
full_name = first_name + " " + last_name
print(f"Full name: {full_name}")

# Repetition
separator = "-" * 30
print(separator)

# String length
message = "Python Programming"
print(f"Length: {len(message)}")  # 18

# Accessing characters (indexing)
word = "Python"
print(f"First character: {word[0]}")    # P
print(f"Last character: {word[-1]}")    # n
print(f"Third character: {word[2]}")    # t

Output:

Full name: Alice Smith
------------------------------
Length: 18
First character: P
Last character: n
Third character: t

String Methods

python

text = "  Hello, World!  "

print(f"Original: '{text}'")
print(f"Upper: '{text.upper()}'")
print(f"Lower: '{text.lower()}'")
print(f"Title: '{text.title()}'")
print(f"Strip: '{text.strip()}'")
print(f"Replace: '{text.replace('World', 'Python')}'")
print(f"Split: {text.strip().split(',')}")

Output:

Original: '  Hello, World!  '
Upper: '  HELLO, WORLD!  '
Lower: '  hello, world!  '
Title: '  Hello, World!  '
Strip: 'Hello, World!'
Replace: '  Hello, Python!  '
Split: ['Hello', 'World!']

F-Strings (Formatted String Literals)

python

name = "Alice"
age = 30
score = 95.678

# Basic f-string
print(f"My name is {name} and I am {age} years old.")

# Expressions in f-strings
print(f"Next year I will be {age + 1}.")

# Formatting numbers
print(f"Score: {score:.2f}")      # 2 decimal places: 95.68
print(f"Score as percent: {score:.1%}")  # Not applicable here, but useful
print(f"Right aligned: {name:>10}")     # Right aligned in 10 chars
print(f"Center aligned: {name:^10}")    # Center aligned

Output:

My name is Alice and I am 30 years old.
Next year I will be 31.
Score: 95.68
Right aligned:      Alice
Center aligned:  Alice

Booleans (bool)

Booleans represent truth values: True or False.

Boolean Basics

python

# Boolean values
is_active = True
is_admin = False

print(f"Active: {is_active}")       # True
print(f"Admin: {is_admin}")         # False
print(f"Type: {type(is_active)}")   # <class 'bool'>

# Boolean from comparisons
x = 10
y = 20

print(f"\n{x} > {y}: {x > y}")       # False
print(f"{x} < {y}: {x < y}")         # True
print(f"{x} == {y}: {x == y}")       # False
print(f"{x} != {y}: {x != y}")       # True
print(f"{x} >= 10: {x >= 10}")       # True

Truthiness in Python

python

# Python evaluates many values as True or False in boolean context
print(f"bool(1): {bool(1)}")         # True
print(f"bool(0): {bool(0)}")         # False
print(f"bool('hello'): {bool('hello')}")  # True
print(f"bool(''): {bool('')}")       # False
print(f"bool([]): {bool([])}")       # False (empty list)
print(f"bool([1, 2]): {bool([1, 2])}")  # True (non-empty list)
print(f"bool(None): {bool(None)}")   # False

ℹ️Note

In Python, the following are considered "falsy": False, None, 0, 0.0, "", [], {}, set(). Everything else is "truthy".

Type Conversion (Casting)

You often need to convert between types. Python provides built-in functions for this.

Converting Between Types

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python

# String to int
age_str = "25"
age_int = int(age_str)
print(f"int('25') = {age_int}")        # 25
print(f"Type: {type(age_int)}")        # <class 'int'>

# String to float
price_str = "19.99"
price_float = float(price_str)
print(f"float('19.99') = {price_float}")  # 19.99

# Int/float to string
number = 42
number_str = str(number)
print(f"str(42) = '{number_str}'")     # '42'

# Float to int (truncates, doesn't round!)
pi = 3.14159
pi_int = int(pi)
print(f"int(3.14159) = {pi_int}")     # 3

# Boolean to int/float
print(f"int(True) = {int(True)}")      # 1
print(f"int(False) = {int(False)}")    # 0
print(f"float(True) = {float(True)}")  # 1.0

Practical Example: Input Conversion

python

# calculator_input.py
# Getting numeric input from user

print("=== Rectangle Area Calculator ===")

# input() always returns a string, so we must convert
width_str = input("Enter width: ")
height_str = input("Enter height: ")

# Convert strings to floats
width = float(width_str)
height = float(height_str)

# Calculate area
area = width * height

print(f"\nRectangle Dimensions:")
print(f"  Width: {width}")
print(f"  Height: {height}")
print(f"  Area: {area}")

Sample output:

=== Rectangle Area Calculator ===
Enter width: 5.5
Enter height: 3.2

Rectangle Dimensions:
  Width: 5.5
  Height: 3.2
  Area: 17.6

Variable Naming Conventions

Python has specific rules and conventions for naming variables.

Rules (Must Follow)

Rule	Valid	Invalid
Start with letter or underscore	`name`, `_age`	`1name`, `2nd`
Only letters, numbers, underscores	`user_name`, `var2`	`user-name`, `var@2`
Case-sensitive	`age`, `Age`, `AGE` are different	-
Not a reserved keyword	-	`if`, `for`, `class`, `def`

Conventions (Should Follow)

python

# Snake case for variables (recommended)
user_name = "Alice"
total_amount = 100.50
is_active = True

# Constants (all uppercase)
PI = 3.14159
MAX_RETRIES = 3
API_KEY = "secret123"

# Private variables (leading underscore - convention only)
_internal_value = 42
_temp_data = []

# Bad names - avoid these!
x = 10          # Too vague
temp123 = "hi"  # Not descriptive
myVariableName = "Alice"  # camelCase not standard in Python

💡Tip

Choose descriptive names! student_age is much better than sa or x. Your future self (and other developers) will thank you.

Checking Types with type() and isinstance()

python

# Using type()
value1 = 42
value2 = 3.14
value3 = "hello"
value4 = True

print(f"type(42) = {type(value1)}")         # <class 'int'>
print(f"type(3.14) = {type(value2)}")       # <class 'float'>
print(f"type('hello') = {type(value3)}")    # <class 'str'>
print(f"type(True) = {type(value4)}")       # <class 'bool'>

# Using isinstance() for type checking
print(f"\nisinstance(42, int): {isinstance(42, int)}")          # True
print(f"isinstance(3.14, int): {isinstance(3.14, int)}")        # False
print(f"isinstance(3.14, float): {isinstance(3.14, float)}")    # True
print(f"isinstance(True, int): {isinstance(True, int)}")        # True (bool is subclass of int!)

Real-World Example: Student Record System

python

# student_record.py
# A simple student record using different data types

# Student information
student_id = 1001                    # int
name = "Maria Santos"                # str
age = 22                             # int
gpa = 3.85                           # float
is_enrolled = True                   # bool
courses = ["Math", "Physics", "CS"]  # list

# Display student record
print("=" * 40)
print("       STUDENT RECORD")
print("=" * 40)
print(f"Student ID: {student_id}")
print(f"Name: {name}")
print(f"Age: {age}")
print(f"GPA: {gpa:.2f}")
print(f"Enrolled: {'Yes' if is_enrolled else 'No'}")
print(f"Courses: {', '.join(courses)}")
print("=" * 40)

# Type summary
print("\nData Types Used:")
print(f"  student_id: {type(student_id).__name__}")
print(f"  name: {type(name).__name__}")
print(f"  age: {type(age).__name__}")
print(f"  gpa: {type(gpa).__name__}")
print(f"  is_enrolled: {type(is_enrolled).__name__}")

Output:

========================================
       STUDENT RECORD
========================================
Student ID: 1001
Name: Maria Santos
Age: 22
GPA: 3.85
Enrolled: Yes
Courses: Math, Physics, CS
========================================

Data Types Used:
  student_id: int
  name: str
  age: int
  gpa: float
  is_enrolled: bool

Practice Exercises

Exercise 1: Type Identification

Identify the type of each value:

42
"42"
42.0
True
"True"
0

Exercise 2: Variable Creation

Create variables for a product: name (str), price (float), quantity (int), and in_stock (bool). Print a summary.

Exercise 3: Type Conversion

Convert the string "3.14159" to a float, then to an int. What happens to the decimal part?

Exercise 4: String Operations

Given text = "Python Programming", write code to:

Get the length
Convert to uppercase
Replace "Python" with "Java"
Get the first 6 characters

Exercise 5: F-String Formatting

Create an f-string that displays a price of $19.99 with 2 decimal places, right-aligned in 10 characters.

Exercise 6: Naming Convention Fix

Fix these variable names to follow Python conventions:

myVariableName
2ndPlace
user-email
TOTALTAX

Exercise 7: Temperature Conversion Program

Write a program that converts Celsius to Fahrenheit and Kelvin:

F = C × 9/5 + 32
K = C + 273.15

Exercise 8: Type Checker Function

Write a function that takes any value and prints its type and whether it's truthy or falsy.

Summary

In this lesson, you learned:

How to create and use variables in Python
The four primitive data types: int, float, str, bool
Arithmetic operations and their behavior with different types
String operations, methods, and f-string formatting
Boolean values and truthiness in Python
Type conversion between different data types
Variable naming rules and conventions
How to check types using type() and isinstance()

Variables and data types are the building blocks of every Python program. Master these fundamentals before moving on to operators and expressions.

Progreso30%

Python Setup & First Program

Operators & Expressions

Contenido del Curso

1. How Computers Work 2. Python Setup & First Program 3. Variables & Data Types 4. Operators & Expressions 5. Control Flow: Conditionals 6. Control Flow: Loops 7. Functions 8. Data Structures: Lists & Tuples 9. Data Structures: Dictionaries & Sets 10. Imperative Algorithm Projects