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19. Type Conversion and Casting

1. Introduction​

Sometimes you need to change a value from one type to another.
For example, converting a string "123" into a number 123.
This process is called type conversion or casting.

Python supports two types of conversions:

  • Implicit Conversion (Type Casting by Python)
  • Explicit Conversion (Manual Casting by the Programmer)

2. Implicit Type Conversion​

Python automatically converts smaller data types to larger ones when needed.

Example:

x = 10        # int
y = 2.5       # float
z = x + y     # int + float
print(z)      # 12.5
print(type(z))  # <class 'float'>

Here, Python automatically converted x into a float.

3. Explicit Type Conversion​

You can manually convert a value to another type using built-in functions.

  • Convert to integer:
x = int("42")
print(x)        # 42
print(type(x))  # <class 'int'>
  • Convert to float:
y = float("3.14")
print(y)        # 3.14
  • Convert to string:
z = str(100)
print(z)        # "100"
print(type(z))  # <class 'str'>

4. Converting Between Data Types​

  • List to Tuple:
numbers = [1, 2, 3]
t = tuple(numbers)
print(t)  # (1, 2, 3)
  • Tuple to List:
t = (4, 5, 6)
lst = list(t)
print(lst)  # [4, 5, 6]
  • List to Set:
numbers = [1, 2, 2, 3]
s = set(numbers)
print(s)  # {1, 2, 3}
  • Dictionary Keys to List:
person = {"name": "Alice", "age": 25}
keys = list(person.keys())
print(keys)  # ['name', 'age']

5. Converting Booleans​

Booleans (True or False) can also be converted.

print(int(True))   # 1
print(int(False))  # 0
print(bool(0))     # False
print(bool(42))    # True
print(bool(""))    # False
print(bool("hi"))  # True

6. Common Pitfalls​

  • Converting non-numeric strings to numbers will fail:
x = int("hello")  # ValueError
  • Floating-point to integer conversion truncates (does not round):
print(int(3.99))  # 3

7. When to Use Conversion​

  • When reading user input (always a string by default).
  • When working with files or APIs that return text.
  • When preparing data for mathematical operations.

8. Next Steps​

✅ You now understand how to convert between Python data types.
In the next chapter, we’ll dive into operators and how they work with different types.