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Module 004: Type Conversion and Casting

Phase: 1. Fundamentals | Estimated time: 1.5 hours | Milestone Project: No

Prerequisites

Learning Objectives

By the end of this module, you will be able to:

  • Convert between int, float, str, and bool using built-in functions
  • Distinguish implicit conversion (automatic) from explicit conversion (casting)
  • Anticipate and handle common conversion errors like ValueError
  • Explain why "42" + 3 fails but int("42") + 3 works

Why This Matters

Real-world data rarely arrives in the type you need. User input comes as strings, files contain text, and APIs return JSON. Converting between types is a daily task in every Python developer's life.

Concept Explanation

Implicit Conversion (Automatic)

Python automatically converts between some types when it makes sense. This always happens in a safe direction — widening to avoid data loss:

# int + float → float (no data loss)
result = 3 + 2.0
print(result)      # 5.0
print(type(result))  # <class 'float'>

# int + bool → int (True=1, False=0)
print(10 + True)   # 11
print(10 + False)  # 10

Python does not implicitly convert strings to numbers:

print("42" + 3)  # TypeError: can only concatenate str (not "int") to str

This gives an error because Python refuses to guess your intent.

Explicit Conversion (Casting)

You control conversion using the type functions as constructors:

Function Converts To Example Result
int(x) integer int("42") 42
float(x) float float("3.14") 3.14
str(x) string str(42) "42"
bool(x) boolean bool(1) True

int() — Convert to Integer

print(int(3.99))     # 3 (truncates toward zero)
print(int("42"))     # 42
print(int(True))     # 1
print(int(False))    # 0

float() — Convert to Float

print(float(3))      # 3.0
print(float("3.14")) # 3.14
print(float("1e3"))  # 1000.0
print(float(True))   # 1.0

str() — Convert to String

print(str(42))       # "42"
print(str(3.14))     # "3.14"
print(str(True))     # "True"

bool() — Convert to Boolean

The bool conversion follows truthiness rules:

  • Numbers: 0 is False, everything else is True
  • Strings: empty string "" is False, everything else is True
  • None is always False
print(bool(1))       # True
print(bool(0))       # False
print(bool(-5))      # True
print(bool(""))      # False
print(bool("Text"))  # True
print(bool(None))    # False

Common Conversion Errors

int("hello")   # ValueError: invalid literal for int() with base 10: 'hello'
int("3.14")    # ValueError: invalid literal for int() with base 10: '3.14'
float("3,14")  # ValueError (comma decimal not allowed in Python)

Always ensure the string has the right format before converting. Leading/trailing whitespace is fine:

print(int("  42  "))  # 42

Common Pitfalls

  1. Trying to convert "3.14" directly to int: int("3.14") raises ValueError. Convert to float first: int(float("3.14")) → 3.
  2. Forgetting that bool("False") is True: Any non-empty string is True, including "False" and "0".
  3. Assuming int() rounds: int(3.9) yields 3, not 4. It truncates, not rounds.
  4. Concatenating string numbers without conversion: "Total: " + 42 fails. Use "Total: " + str(42).
  5. Relying on implicit conversion: Python rarely converts automatically. Always cast explicitly for clarity.

Hands-On Walkthrough

Let's build a small program that demonstrates type conversion using a real scenario — a user's age input (from a file, simulated here):

# Simulating data we read from somewhere
age_string = "25"
height_string = "1.75"
has_license = "True"

# Convert to proper types
age = int(age_string)
height = float(height_string)
license_bool = bool(has_license)

print("Age:", age, type(age))
print("Height:", height, type(height))
print("Has license:", license_bool, type(license_bool))

# Now we can do math
print("Next year you will be:", age + 1)

# String concatenation with str()
print("You are " + str(age) + " years old.")

Expected output:

Age: 25 <class 'int'>
Height: 1.75 <class 'float'>
Has license: True <class 'bool'>
Next year you will be: 26
You are 25 years old.

Now let's experiment with truncation vs rounding:

print("int(3.99) =", int(3.99))  # 3, truncation
print("int(3.14) =", int(3.14))  # 3, truncation
print("int(-2.7) =", int(-2.7))  # -2, truncates toward zero
print("round(3.99) =", round(3.99))  # 4.0, rounding

Key Takeaways

  • Implicit conversion happens automatically in safe directions (int → float, int → bool).
  • Explicit conversion uses int(), float(), str(), bool().
  • int() truncates toward zero — it does not round.
  • bool("") and bool(0) are False; all other values are generally True.
  • int("3.14") raises ValueError — convert to float() first.
  • Strings with numbers can be converted if they contain valid numeric characters.
  • Always convert input data explicitly before doing operations.
  • Use str() to combine non-string values with strings using +.

Further Reading

Next Module

Learn how to get input from users in Module 005: Basic Input and Output.