Python Type Conversion – int(), float(), str(), bool() and More

Implicit Type Conversion

Python automatically converts types when it's safe to do so. This only happens when there's no risk of losing data.

# Python automatically converts int to float when mixed
result = 5 + 2.0      # int + float = float
print(result)          # 7.0
print(type(result))    # <class 'float'>

# Another example
x = True + 5  # bool + int = int (True == 1)
print(x)      # 6

Explicit Type Conversion – The Big 5

Use these built-in functions to explicitly convert types: int(), float(), str(), bool(), list().

# int() - convert to integer
print(int(3.9))       # 3 (truncates, does NOT round)
print(int("42"))      # 42
print(int(True))      # 1
print(int(False))     # 0

# float() - convert to float
print(float(5))       # 5.0
print(float("3.14"))  # 3.14

# str() - convert to string
print(str(42))        # "42"
print(str(3.14))      # "3.14"
print(str(True))      # "True"

# bool() - convert to boolean
print(bool(0))        # False
print(bool(1))        # True
print(bool(""))       # False
print(bool("hello"))  # True

Converting Collections

Python can convert between list, tuple, set, and dict (with some restrictions).

my_list = [1, 2, 3, 2, 1]

print(tuple(my_list))   # (1, 2, 3, 2, 1) - ordered, immutable
print(set(my_list))     # {1, 2, 3} - unique values only
print(list(set(my_list)))  # back to list, but unordered

# String to list of characters
print(list("hello"))  # ['h', 'e', 'l', 'l', 'o']

Conversion Errors – When Conversion Fails

Not all conversions are valid. Always handle potential ValueError when converting user input.

# This will raise ValueError
try:
    x = int("hello")   # Cannot convert "hello" to int
except ValueError as e:
    print(f"Error: {e}")

# Safe conversion pattern
def safe_int(value, default=0):
    try:
        return int(value)
    except (ValueError, TypeError):
        return default

print(safe_int("123"))   # 123
print(safe_int("abc"))   # 0 (default)

Type Conversion: Explicit Casts and the input() Trap

Python doesn't silently convert types the way some languages do — you cast explicitly with int(), str(), float(). The #1 beginner bug lives here: input() always returns a string, even for numbers.

age = input("Age? ")     # ALWAYS a str, e.g. "30"
age + 1                   # ❌ TypeError: can't add int to str
age = int(input("Age? ")) # ✅ convert first

int("42")                 # 42
int("3.9")                # ❌ ValueError — use int(float("3.9"))
str(42)                   # "42"
float("1e3")              # 1000.0

The input() rule: wrap numeric input in int() or float() immediately. Guard against bad input with try/except ValueError, since a user typing "abc" into int() crashes. Note int() truncates floats toward zero (int(3.9) == 3) — use round() if you want nearest. Understanding truthiness helps too: bool() of empty strings, empty lists, 0, and None is all False.

Interview Questions on Type Conversion

• What is the difference between implicit and explicit type conversion?
• What are the main type conversion functions in Python?
• What happens when you convert a float to an int?
• What error is raised when converting an invalid string to a number?
• How do you convert a list to a set, and what side effect does that have?
• Why can’t you add a string and an integer directly?
• How do you safely convert user input to a number?

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