Casting

Temporarily convert values from one data type to another.

Casting changes the type of a value. It is especially important because keyboard input arrives as text even when the user types digits.

Predict

Read the code first. Do not run it yet.

casting-predict.py

number = "12"
print(number + "3")
print(int(number) + 3)

Predict before running

Predict both outputs.
Why does the first line join text instead of adding numerically?
What does int() change?

Investigate

Use this code to explain exactly how the program works.

casting-investigate.py

age = int(input("Age: "))
next_age = age + 1
print(next_age)

Investigate the code

Which operation performs the cast?
What type does input() return before casting?
Why is the cast needed before addition?

Common casts

Cast	Purpose	Example
`int()`	convert to an integer	`int("14")` → 14
`float()`	convert to a real number	`float("2.5")` → 2.5
`str()`	convert to text	`str(25)` → "25"

price.py

price = float(input("Price: "))
quantity = int(input("Quantity: "))
total = price * quantity
print("Total:", total)

The text must be suitableint("ten") cannot produce an integer. Validation is covered later.

Self-study guide

Casting temporarily converts a value from one data type to another. It is useful when the current type is unsuitable for the next operation—for example, converting keyboard input from text into a number before arithmetic.

Casting does not magically make every value convertible. int("15") works, but int("fifteen") does not because that text is not a valid integer representation.

OCR focusUnderstand that data types can be temporarily changed through casting and explain where that is useful. OCR ERL lists str(), int(), float(), real() and bool(); Python uses str(), int(), float() and bool() but has no real() function.

OCR Exam Reference Language ↔ Python

OCR Exam Reference Language / term	Python	What to remember
`int("3")`	`int("3")`	Converts suitable text or a compatible value to an integer.
`float("4.52")`	`float("4.52")`	Converts to a Python floating-point value.
`real("4.52")`	`float("4.52")`	OCR ERL can use `real()`; Python normally uses `float()`.
`str(345)`	`str(345)`	Converts a value to text.
`bool(...)`	`bool(...)`	Boolean conversion exists in both, but be careful: `bool("False")` is `True` in Python because a non-empty string is truthy.

Read both formsIn an examination you may see OCR Exam Reference Language even when you have learned the idea in Python. Translate the structure, not just the symbols.

Worked example

A very common pattern is input → cast → calculate → output.

lesson-04-worked.py

age_text = input("Age: ")
age = int(age_text)
next_age = age + 1

print("Next age:", next_age)

Output

Next age: 16

Work through it line by line

input() returns the text "15".
int(age_text) creates the integer 15.
Only after casting can the program safely perform integer arithmetic.
The original variable age_text remains a string; the converted result was stored separately in age.

Another example

Casting can also be useful when joining a number into text.

lesson-04-example-2.py

score = 42
message = "Score: " + str(score)
print(message)

Output

Score: 42

Why this example matters

String concatenation requires strings on both sides of +.
str(score) provides a text version of the numeric value.

Common mistakes

Avoid these errors

Assuming input() returns an integer automatically.
Using int() for decimal text such as "4.5"; use float() instead.
Using bool("False") expecting False; non-empty strings are truthy in Python.

Exam-safe warningChoose the cast that matches the data you actually need. Casting is purposeful conversion, not something to apply to every value.

Exam-ready checklist

You should now be able to

Explain why casting is needed in a given program.
Use int(), float() and str() appropriately in Python.
Recognise OCR real() as corresponding to Python float() in this course.
Predict whether a cast is valid for a particular value.

What you have learned

Casting temporarily changes a value from one type to another.
Use int(), float() and str() when appropriate.
Numeric keyboard input normally needs casting before arithmetic.