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Python Basics

Python Type Conversions

1. What is Type Conversion?

  • Converting a value from one data type to another
  • Also called type casting
  • Not all conversions are possible — invalid ones raise an error
  • Common need: input() always returns a string, but you may need a number

2. The 4 Conversion Functions

Function Converts to Example
int() Integer int("4") → 4
float() Float float("4.5") → 4.5
bool() Boolean bool("hello") → True
str() String str(45) → "45"

These functions return the converted value — they do not change the original variable

3. Converting to int

x = "4"
y = int(x)       # y = 4 (int), x is still "4" (str)
print(y + 4)     # 8  ✅

What works and what doesn't:

int("4")       # ✅  4
int("4 ")      # ✅  4  (strips whitespace)
int(4.7)       # ✅  4  (truncates decimal — no rounding)
int("4hello")  # ❌  ValueError — not a valid int
int("4.5")     # ❌  ValueError — use float() instead

int() on a float truncates (chops off decimals), it does NOT round

4. Converting to float

float("4.5")   # ✅  4.5
float("4")     # ✅  4.0
float(4)       # ✅  4.0
float("4.7x")  # ❌  ValueError

5. Converting to bool

Strings → bool

Value Result
Any non-empty string ("hello", "0", " ") True
Empty string "" False 
bool("hello")   # True
bool("0")       # True  ← still True! It's a non-empty string
bool(" ")       # True  ← even a space is True
bool("")        # False ← only empty string is False

Numbers → bool

Value Result
Any non-zero number (1, -9, 0.2) True
0 or 0.0 False 
bool(2)         # True
bool(-9)        # True
bool(0.2)       # True
bool(0)         # False
bool(0.0)       # False

6. Converting to str

str(0)       # "0"
str(23)      # "23"
str(True)    # "True"
str(4.5)     # "4.5"

Verify it's a string via concatenation:

str(23) + "0"    # "230"  ← proves it's a string, not int (23+0 would be 23)

7. Practical Example — Fixing input() Type

# ❌ Crashes — input returns a string, can't add int to string
number = input("Enter a number: ")
result = number + 5    # TypeError

# ✅ Option 1 — convert at input
number = int(input("Enter a number: "))
result = number + 5

# ✅ Option 2 — convert when using
number = input("Enter a number: ")
result = int(number) + 5

Use float() instead of int() if the user might enter a decimal:

number = float(input("Enter a number: "))
result = number + 5    # works for both 4 and 4.3

If the user types a non-number (e.g. "hello"), the program will crash — error handling is covered in a later lesson

8. Conversion Compatibility Cheat Sheet

From → To int() float() bool() str()
str (numeric) ✅ ✅ ✅ ✅
str (non-numeric) ❌ ❌ ✅ ✅
int ✅ ✅ ✅ ✅
float ✅ (truncates) ✅ ✅ ✅
bool ✅ ✅ ✅ ✅

9. Key Takeaways & Recap

  1. Use int(), float(), bool(), str() to convert between types
  2. Conversion functions return the new value — original variable is unchanged
  3. int("4.5") fails — convert to float() first, or use float() directly
  4. int(4.7) → 4 — truncates, never rounds
  5. bool() is False only for 0, 0.0, or "" — everything else is True
  6. Always convert input() result before using it in math operations
  7. Invalid conversions raise a ValueError — no workaround except error handling