Introduction to Python

1 Explain why the following code causes an error: age = input("Age? ") then print(age + 1) 3

One mark for each of the following, to a maximum of 3.

• The input function returns a string
• Python cannot add a number to a string
• The fix is to convert the input using int()

Model answer. The input function always returns a string, so age holds the text "14" rather than the number 14 even when the user types digits. Python cannot add the integer 1 to a string, so it raises a TypeError. The fix is to convert the input to a whole number as it is read, by writing age = int(input("Age? ")), after which the addition works correctly.

2 Write a Python program that asks the user for their name and their age, then prints a message saying whether they are old enough to watch a 12 rated film. 5

One mark for each of the following, to a maximum of 5.

• Asks for the name using input
• Asks for the age and converts it to an integer
• Uses an if statement with a correct condition
• Prints a suitable message when old enough
• Uses else to print a message when not old enough

Model answer. name = input("What is your name? ") age = int(input("How old are you? ")) if age >= 12: print(name + ", you can watch a 12 rated film.") else: print(name + ", you are too young for a 12 rated film.") The name is read as a string, which is correct because it is text. The age is converted to an integer so it can be compared with a number. The condition uses greater than or equal to, so someone who is exactly 12 is correctly allowed.

3 State the purpose of the int() function and the str() function in Python. 2

One mark for each of the following, to a maximum of 2.

• int() converts a value into a whole number
• str() converts a value into text or a string

Model answer. The int() function converts a value into a whole number, which is needed when input has been read from the user as text but arithmetic or a numerical comparison has to be performed on it. The str() function does the opposite, converting a value such as a number into a string, which is needed when you want to join it to other text using the plus operator.

4 A program should print 'Pass' for a mark of 50 or more and 'Fail' otherwise, but a mark of exactly 50 prints 'Fail'. Explain the likely cause and the correction. 3

One mark for each of the following, to a maximum of 3.

• The condition uses greater than rather than greater than or equal to
• A mark of exactly 50 is not greater than 50, so it fails the test
• Change the condition to mark >= 50

Model answer. The likely cause is that the condition has been written as if mark > 50 rather than if mark >= 50. A mark of exactly 50 is not greater than 50, so the condition is false and the else branch runs, printing Fail. The correction is to change the operator to greater than or equal to, writing if mark >= 50, so that a mark of exactly 50 is correctly counted as a pass.

5 Explain why indentation matters in Python. 2

One mark for each of the following, to a maximum of 2.

• Indentation shows which lines belong inside a structure such as an if statement or a loop
• Incorrect indentation causes an error or makes the program behave incorrectly

Model answer. In Python, indentation is not just formatting: it is how the language knows which lines belong inside an if statement, a loop or a function. Lines indented under an if only run when the condition is true, while a line that is not indented runs regardless. If the indentation is inconsistent Python raises an IndentationError and the program will not run at all, and if it is consistent but wrong the program will run while doing something the programmer did not intend.