Python Casting

Why cast?

Sometimes a value has the wrong type for what you want to do with it. Text typed by a user arrives as a string even when it looks like a number, and you cannot do arithmetic on it until you convert it. Casting solves this by building a new value of the target type from the value you already have.

Converting to numbers

The int() function turns a suitable string or a float into an integer, and float() turns a suitable string or an int into a float. When int() receives a float it truncates toward zero, discarding the fractional part rather than rounding it.

int("10")      # 10
int(9.8)        # 9, the .8 is dropped
float("3.5")   # 3.5
float(7)        # 7.0

print(int("10") + float(7))   # 17.0

Converting to text

The str() function turns any value into its string form, which is essential when you want to join a number to a message. Trying to add a number directly to a string raises a TypeError, so cast the number first.

score = 95
message = "Your score is " + str(score)
print(message)     # Your score is 95

Converting to boolean

• bool() returns False for empty or zero-like values: 0, 0.0, "", [], {}, and None.
• bool() returns True for every other value, including any non-empty string or list.
• Casting can fail, so guard user input with try/except or validate it first.
• Casting always builds a new value; the original variable keeps its own type.

print(bool(0))        # False
print(bool(""))      # False
print(bool("no"))    # True, any non-empty string is truthy
print(bool([1, 2]))   # True