Python Basics — Type Conversion

Type Conversion

Type conversion (also called type casting) is the process of converting a value from one data type to another. Python supports two kinds: implicit (automatic) and explicit (manual).

Implicit Type Conversion (Coercion)

Python automatically converts one type to another when it makes sense — without you asking:

# int + float → float (Python "promotes" the int to float)
result = 5 + 3.0
print(result)        # 8.0
print(type(result))  # <class 'float'>

# int + bool → int (True = 1, False = 0)
result = 10 + True
print(result)        # 11
print(type(result))  # <class 'int'>

Implicit Conversion Hierarchy

When mixing types in an operation, Python promotes to the “wider” type:

bool → int → float → complex

# bool + int → int
print(True + 5)      # 6

# int + float → float
print(3 + 2.5)       # 5.5

# float + complex → complex
print(1.5 + 2j)      # (1.5+2j)

Python does NOT implicitly convert between strings and numbers:

# ❌ This causes a TypeError
# print("Age: " + 25)  # TypeError: can only concatenate str to str

Explicit Type Conversion (Casting)

Use built-in functions to manually convert between types:

int() — Convert to Integer

print(int(3.7))       # 3 (truncates decimal, does NOT round)
print(int(3.2))       # 3
print(int(-3.7))      # -3 (truncates toward zero)
print(int("42"))      # 42 (string to int)
print(int(True))      # 1
print(int(False))     # 0
print(int("0b1010", 2))  # 10 (binary string to int)
print(int("0xFF", 16))   # 255 (hex string to int)

Common error:

# ❌ Cannot convert a non-numeric string
# int("hello")  # ValueError: invalid literal for int()

# ❌ Cannot directly convert a float string
# int("3.14")   # ValueError — convert to float first
print(int(float("3.14")))  # 3 ✅

float() — Convert to Float

print(float(42))       # 42.0
print(float("3.14"))   # 3.14
print(float("42"))     # 42.0
print(float(True))     # 1.0
print(float("inf"))    # inf (positive infinity)
print(float("-inf"))   # -inf (negative infinity)

str() — Convert to String

print(str(42))         # "42"
print(str(3.14))       # "3.14"
print(str(True))       # "True"
print(str(None))       # "None"
print(str([1, 2, 3]))  # "[1, 2, 3]"

bool() — Convert to Boolean

# Numbers
print(bool(0))         # False
print(bool(1))         # True
print(bool(-5))        # True (any non-zero number)
print(bool(0.0))       # False

# Strings
print(bool(""))        # False (empty string)
print(bool("hello"))   # True (non-empty string)
print(bool("False"))   # True! (non-empty string — the content doesn't matter)

# Collections
print(bool([]))        # False (empty list)
print(bool([0]))       # True (non-empty list, even if element is falsy)

# None
print(bool(None))      # False

Conversion Functions Summary

Function	Converts To	Examples
int(x)	Integer	int(3.7) → 3, int("42") → 42
float(x)	Float	float(42) → 42.0, float("3.14") → 3.14
str(x)	String	str(42) → "42", str(True) → "True"
bool(x)	Boolean	bool(0) → False, bool("hi") → True
list(x)	List	list("abc") → ["a","b","c"]
tuple(x)	Tuple	tuple([1,2]) → (1, 2)
set(x)	Set	set([1,1,2]) → {1, 2}

Common Conversion Errors

Conversion	Input	Error	Fix
int("hello")	Non-numeric string	ValueError	Validate input first
int("3.14")	Float string	ValueError	Use int(float("3.14"))
int([1, 2])	List	TypeError	Use int(my_list[0])
float("abc")	Non-numeric string	ValueError	Validate input first

Handling Conversion Errors

user_input = "not a number"

try:
    number = int(user_input)
    print(f"You entered: {number}")
except ValueError:
    print("That's not a valid number!")

Output:

That's not a valid number!

Type Checking with isinstance()

Use isinstance() to check if a value is of a certain type:

x = 42
print(isinstance(x, int))       # True
print(isinstance(x, float))     # False
print(isinstance(x, (int, float)))  # True (check multiple types)

name = "Alice"
print(isinstance(name, str))    # True

isinstance() vs type():

# type() checks the exact type
print(type(True) == int)           # False (type is bool, not int)

# isinstance() respects inheritance (bool is a subclass of int)
print(isinstance(True, int))      # True

Best practice: Use isinstance() over type() comparisons because it handles inheritance correctly.

Practical Examples

Example 1: User Input Calculation

# input() always returns a string — you must convert it
price = float(input("Enter the price: "))
quantity = int(input("Enter the quantity: "))
total = price * quantity
print(f"Total: ${total:.2f}")

Example 2: Building a Report String

name = "Alice"
score = 95
passed = True

# Convert everything to strings for concatenation
report = name + " scored " + str(score) + " — Passed: " + str(passed)
print(report)  # Alice scored 95 — Passed: True

# Better approach: use f-strings (no manual conversion needed)
report = f"{name} scored {score} — Passed: {passed}"
print(report)  # Alice scored 95 — Passed: True

Common Mistakes

Mistake	Example	Fix
Assuming int() rounds	int(3.7) → 3 (truncates)	Use round(3.7) → 4
bool("False") is True	Non-empty string = truthy	Check value, not just bool()
Forgetting to cast input	input() returns string	Use int(input(...))
Chaining impossible casts	int("3.14") fails	int(float("3.14"))

Summary

• Implicit conversion happens automatically (int → float, bool → int)
• Explicit conversion uses functions: int(), float(), str(), bool()
• int() truncates (doesn’t round) — use round() if you want rounding
• input() always returns a string — convert with int() or float()
• Use isinstance() to check types (better than type())
• Handle conversion errors with try/except for user-facing code
• f-strings avoid the need for manual str() conversion in most cases

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