There may be times when you want to specify a type onto a variable. This can be done with casting. Python is an object-oriented language, and as such it uses classes to define data types, including its primitive types.

Casting in Python is therefore done using constructor functions.

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1. Implicit Casting

Implicit casting (or implicit type conversion) is automatically performed by the Python interpreter without any user involvement. Python automatically converts a smaller data type to a larger data type to prevent data loss.

For example, when you add an int and a float, Python will implicitly convert the int to a float.

integer_number = 123
float_number = 1.23

new_number = integer_number + float_number

print(type(integer_number)) # <class 'int'>
print(type(float_number))   # <class 'float'>

print("Value:", new_number) # Value: 124.23
print("Type:", type(new_number)) # <class 'float'>

As seen above, Python implicitly cast the integer 123 to 123.0 before adding it to 1.23.

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2. Explicit Casting (int(), float(), str())

Explicit casting is when you force a variable to be a specific data type using built-in functions.

Casting to Integers (int())

Constructs an integer number from an integer literal, a float literal (by rounding down to the previous whole number), or a string literal (providing the string represents a whole number).

x = int(1)     # x will be 1
y = int(2.8)   # y will be 2
z = int("3")   # z will be 3

Casting to Floats (float())

Constructs a float number from an integer literal, a float literal, or a string literal (providing the string represents a float or an integer).

x = float(1)     # x will be 1.0
y = float(2.8)   # y will be 2.8
z = float("3")   # z will be 3.0
w = float("4.2") # w will be 4.2

Casting to Strings (str())

Constructs a string from a wide variety of data types, including strings, integer literals, and float literals.

x = str("s1")  # x will be 's1'
y = str(2)     # y will be '2'
z = str(3.0)   # z will be '3.0'

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3. When to use casting

Casting is primarily used when:

1. Handling User Input: The input() function always returns a string. If you need the user to input a number to perform math, you must cast it.

   age = input("Enter your age: ")
   # age is a string. If we want to add to it, we must cast:
   age_next_year = int(age) + 1

2. String Concatenation: Python will not implicitly cast integers to strings during concatenation. You must do it explicitly.

   score = 100
   # print("Your score is " + score) # ERROR!
   print("Your score is " + str(score)) # Correct

3. Data Processing: When parsing files (like CSV or text files) where numbers are read as text strings and need to be converted to actual numeric types for data analysis.