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A1.5 Scope, lifetime and debugging in an IDE

Programming techniques and object-oriented programming · A level · OCR H446 2.2.1, AQA 7517 4.1.1.13, Eduqas A500QS 1.8 · about 20 min

What this lesson is about

Local and global variables, why locals are good practice, and breakpoints, stepping, watches and tracebacks.

• Scope and lifetime
• Shadowing, and changing a global
• Why local variables are good practice
• Debugging tools

Questions 6 marks in all

1. [1 mark]What is the lifetime of a local variable?

   1. AFrom when its subroutine is called until the subroutine returns
   2. BThe whole time the program runs
   3. CUntil the variable is printed
   4. DUntil the next global variable is declared
   Answer: A. A local is created on each call and destroyed when that call ends.

2. [1 mark]What does this program print?

   speed = 80
   
   def slow():
       speed = 30
       return speed
   
   print(slow(), speed)

   Answer:

   30 80

   The assignment inside slow makes a new local speed; the global speed is still 80.

3. [1 mark]Why is it good practice to use local variables rather than global variables in subroutines?

   Tick every answer that is true.

   1. AThe subroutine can be tested and reused on its own
   2. BA value cannot be changed unexpectedly by another part of the program
   3. CThe same name can be used safely in different subroutines
   4. DLocal variables can be used anywhere in the program
   Answer: A, B, C. Locals keep subroutines independent; being usable anywhere describes globals.

4. [1 mark]A program gives the wrong total, but only on the 40th time round a loop. Which IDE feature lets you run at full speed and then pause at the line that updates the total?

   1. AA breakpoint
   2. BSyntax highlighting
   3. CAuto-completion
   4. DCode folding
   Answer: A. A breakpoint pauses the run at a chosen line, so you can inspect variables there.

5. [1 mark]What does this program print?

   total = 0
   
   def add(n):
       global total
       total = total + n
   
   add(5)
   add(7)
   print(total)

   Answer:

   12

   global makes total inside add refer to the global variable, so both calls change it.

6. [1 mark]An exception is not handled and Python prints a traceback. What does the traceback show?

   1. AThe chain of subroutine calls that led to the line where the exception was raised
   2. BEvery variable's value
   3. CThe lines that ran successfully
   4. DA suggested fix
   Answer: A. Read it to see which call passed the bad value to the line that failed.

The task: fix the scope bug

# the two lines every program starts with: the commands, then the robot
from bugbot import *
connect()

steps = 0

def step():
    forward(50, distance=5)
    steps = steps + 1

while distance() > 25:
    step()
print("steps:", steps)

# the two lines every program starts with: the commands, then the robot
from bugbot import *
connect()

def step(count):
    print(f"step {count + 1}: {distance()} cm")
    forward(50, distance=5)
    return count + 1

steps = 0
while distance() > 25:
    steps = step(steps)
print("steps:", steps)