Python Multiple Choice Questions

Mar 12, 2015 • Joseph Long

Question 1: Program Flow and Exceptions

This is based on the Python: Errors and Exceptions lesson.

In Python, suppose you have the following snippet of code to average some data points and write the average to a new file called avg_measurement.txt:

measurements = [1.0, 1.0, 4.0]
uncertainties = [0.1, 0.02, 0.3, 0.11]
average_measurement = sum(measurements) / len(measurements)
print "Average measurement = {}".format(average_measurement)

if len(uncertainties) != len(measurements):
    raise ValueError("You must have one uncertainty value for each measurement")

# write the average measurement to a file
with open('avg_measurement.txt', 'w') as f:
    f.write("Average measurement = {}".format(average_measurement))

After you run this program in the terminal, which of these things have happened?

  1. avg_measurement.txt contains the message “Average measurement = 2.0”, and “Average measurement = 2.0” has been printed out in the terminal
  2. avg_measurement.txt has not been created, and “Average measurement = 2.0” has been printed out in the terminal
  3. avg_measurement.txt contains the message “Average measurement = 2.0”, and “Average measurement = 2.0” has not been printed out in the terminal

Question 2: Variable Names and Loops

This is based on the Python: Making Choices lesson.

Your coworker has a list called data, and is trying to pull all the data points greater than 50.0 into a new list called big_nums.

1  data = [1.0, 100.0, 100.0, 1.0, 100.0]
2
3  for item in data:
4      if item > 50.0:
5          big_nums.append(item)
6
7  print big_nums

When they run the code above, they get a NameError saying big_nums is not defined on line 5. They expected to see [100.0, 100.0, 100.0] printed out. How should that be fixed?

  1. By replacing big_nums.append(item) with big_nums = [item] on line 5
  2. By adding big_nums = [] between line 4 and 5
  3. By adding big_nums = [] on line 2

Those who choose 1. recognize that big_nums is not defined, and that you can define it with item inside. That code will run, but at the end it will only ever contain a single list element.

Those who choose 2. recognize that big_nums needs to be defined, but make the mistake defining it inside the loop, which resets it to empty at every iteration.

Those who choose 3. get it right. The list must be defined before line 5, or else there will be a NameError. However, if the list is defined inside the loop (after line 3), it won’t produce the desired behavior.