Type Conversion

5 min · The Basics

Type Conversion

Type conversion (also called type casting) is the process of converting a value from one data type to another. Python provides built-in functions that allow you to explicitly convert between different types.

This is especially important when working with user input (which is always a string) or when you need to perform operations that require specific types.

Basic Type Conversions

Python provides several conversion functions. Here are the most common ones:

conversion.py
# String to Integer
age_str = "25"
age = int(age_str)  # 25 (integer)
print(age, type(age))  # 25 <class 'int'>

# String to Float
price_str = "19.99"
price = float(price_str)  # 19.99 (float)
print(price, type(price))  # 19.99 <class 'float'>

# Number to String
num = 42
text = str(num)  # "42" (string)
print(text, type(text))  # 42 <class 'str'>

# Float to Integer (truncates, doesn't round)
x = 3.7
y = int(x)  # 3 (drops decimal part)
z = 3.9
w = int(z)  # 3 (still 3, not 4!)

Common Conversion Functions

Here are the most frequently used conversion functions with examples:

int(x) — Convert to integer (removes decimal part)
float(x) — Convert to floating-point number
str(x) — Convert to string representation
bool(x) — Convert to boolean (True/False)
list(x) — Convert iterable to list
tuple(x) — Convert iterable to tuple

String to Number Conversions

Converting strings to numbers is common when dealing with user input or reading from files:

string_to_number.py
# String to integer
num_str = "42"
num = int(num_str)
print(num + 8)  # 50 (can now do math)

# String to float
price_str = "19.99"
price = float(price_str)
print(price * 2)  # 39.98

# Converting numbers with different bases
binary = "1010"
decimal = int(binary, 2)  # Convert binary to decimal
print(decimal)  # 10

hexadecimal = "FF"
decimal2 = int(hexadecimal, 16)  # Convert hex to decimal
print(decimal2)  # 255

Number to String Conversions

Converting numbers to strings is useful for displaying values or formatting output:

number_to_string.py
# Integer to string
age = 25
age_str = str(age)
print("I am " + age_str + " years old")

# Float to string
pi = 3.14159
pi_str = str(pi)
print("Pi is " + pi_str)

# Using f-strings (better way - automatic conversion)
age = 25
print(f"I am {age} years old")  # Automatic conversion!

Boolean Conversions

Python has specific rules for what evaluates to True and False:

boolean_conversion.py
# Numbers to boolean
print(bool(0))      # False
print(bool(1))      # True
print(bool(42))     # True (any non-zero number)
print(bool(-5))    # True

# Strings to boolean
print(bool(""))      # False (empty string)
print(bool("Hello")) # True (non-empty string)

# None to boolean
print(bool(None))    # False

# Common pattern: check if value exists
name = ""
if bool(name):
    print("Name provided")
else:
    print("No name")

Error Handling with Conversions

Converting invalid values will raise errors. It's important to handle these cases:

conversion_errors.py
# These will cause errors:
# int("hello")      # ValueError: invalid literal
# int("12.5")       # ValueError: invalid literal (use float first)
# float("abc")      # ValueError: could not convert

# Safe conversion pattern
user_input = "25"
try:
    age = int(user_input)
    print(f"Age: {age}")
except ValueError:
    print("Invalid number!")

# Convert float string to int (two steps)
price_str = "19.99"
price_float = float(price_str)  # First to float
price_int = int(price_float)    # Then to int
print(price_int)  # 19

Implicit vs Explicit Conversion

Python sometimes converts types automatically (implicit), but explicit conversion is clearer:

implicit_explicit.py
# Implicit conversion (automatic)
result = 10 + 5.5  # int + float = float
print(result)  # 15.5 (float)

# Explicit conversion (recommended for clarity)
num1 = int(10)
num2 = float(5)
result = num1 + num2  # More explicit

# String concatenation requires explicit conversion
age = 25
# message = "I am " + age  # ERROR! Can't concatenate str + int
message = "I am " + str(age)  # Correct: explicit conversion
print(message)

⚠️ Common Conversion Errors

• int("12.5") will fail - must use float("12.5") first, then int()

• int("hello") will fail - string must contain valid number

• Converting None to int/float will fail

• Always validate or use try/except when converting user input!

Practical Example: Safe Input Conversion

Here's a pattern for safely converting user input:

safe_conversion.py
# Safe conversion with validation
user_age = "25"

if user_age.isdigit():  # Check if all characters are digits
    age = int(user_age)
    print(f"You are {age} years old")
else:
    print("Please enter a valid number")

# For floats, check differently
price_input = "19.99"
try:
    price = float(price_input)
    print(f"Price: ${price}")
except ValueError:
    print("Invalid price format")

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