HW1: Writing Black Box Tests
Due Monday by 11:59pm Points 15
Motivation
This week we took an in depth look at our first testing technique: black box testing. One
of the key elements of black box testing is that the tester does not have access to the source. In
this assignment, you will be given a specification and list of requirements. This will give you
experience writing unit tests in a black box testing scenario.
Course Learning Outcome(s):
Apply testing techniques, including black-box and white-box techniques, automatic testing
activities, and regression testing (CLO 4)
Module Learning Outcome(s):
Apply black box testing techniques
Description
For this assignment, you will be writing unit tests based on the following
specifications/requirements.
You will write a series of unit tests to test a function called credit_card_validator (written for you)
that is passed a sequence of digits as a string that represents a credit card number. This function
will return True if it is a valid credit card number, otherwise, it will return False .
Depending on the credit card issuer, the length of a credit card number can range between 10 and
19 digits. The first few digits of the number are the issuer prefix. Each credit card issuer has an
assigned range of numbers. For example, only Visa credit card numbers may begin with 4 , while
American Express card numbers must begin with either a 34 or 37 . Sometimes, credit card
providers are assigned multiple ranges. For example, MasterCard card numbers must start with the
numbers between 51 through 55 or 2221 through 2720 (inclusive).
The last digit of the number is referred to as the check digit and acts as a checksum. Most credit
cards calculate this check digit using the Luhn algorithm (see resources below for how this is
calculated).
In order to limit the scope of this assignment, we are going to limit the number of credit card issuers
to 3: Visa, MasterCard, and American Express. Each has its own prefixes and length requirements.
Visa

Length: 16
MasterCard
Prefix(es): 51 through 55 and 2221 through 2720
Length: 16
American Express
Prefix(es): 34 and 37
Length: 15
Your task is to create a series of tests that attempt to reveal bugs in the implementation. As this is
black box testing, you will not have access to the source so you must use what you have learned
this week to generate test cases.
You will be submitting your code to Gradescope which will auto grade your tests. In order to get full
credit on the assignment, you will need to locate all 6 bugs in the code (refer to the rubric for full
details). Some are easier than others. Bug 5 is easy to miss without using Partition Testing and Bug
6 requires using what you know about common errors to design your tests.
You are free to determine how you generate your test cases. You may do it completely manually, or
use an automated tool like the TSLgenerator. No matter how you generate your test cases, in your
file testing file (tests.py), you need to include a comment for each test case describing:
What the test case (i.e. credit card number) is meant to verify
How you determined what to use as the test case
Here is an example:

Verifies if Master Cards with valid lengths and invalid check bits returns False

Picked using Category Partition Testing

def test11(self):
self.assertFalse(credit_card_validator(“….”))
You also need to ensure you have test cases that do a good job covering the input domain. This
means that at the very least, you need to have a test case for each of the prefix ranges listed
above.
Please submit all your tests, even the ones that do not find bugs. Remember, you are practicing
writing a testing suite, which can be used to test the code again if changes are made. There may
be a situation where a previously passing test fails when someone updates credit_card_validator .
Finally, your test suite needs to be free of linting errors using the PEP8 standard; this will be
important later when working on shared repositories. If you are unfamiliar with linting, please see
the resources below. The easiest way to accomplish this is to ensure that there are no “squiggly”
lines under your code in PyCharm (You will need to change PyCharm’s default line length to 79 to
match PEP8). You can also use the PEP8 Online tool below to copy and paste your code to verify it
has no errors.
Restrictions

For this assignment, you are prohibited from using Random Testing. Yes, Random Testing is a type
of Black Box Testing, but you will be working with this approach in a later module (Exploration:
Random Testing). You will not receive points for any bugs triggered by Random Testing. This
means that you cannot use code to generate the test cases, you need to come up with them
yourself. Please restrict yourself to using other Black Box Testing techniques: Error Guessing,
Partition Testing, and Boundary Value Testing.
Hints
You will need to include
if name == ‘__main__’:
unittest.main()
It is best to only have a single assert in any test. Once one fails, the rest of the code in the test
isn’t executed.
You may assume only strings of digits are being sent to credit_card_validator
I used the TSLgenerator to create roughly 35 test cases and then picked some to break
Use what you learned about Error Guessing and Boundary Values to find tricky bugs
You will need to use from credit_card_validator import credit_card_validator in your tests.py
To ensure your tests are correctly importing the function for testing, you may put the following
dummy code into a file called credit_card_validator.py
def credit_card_validator(num):
pass
You may submit as many times as you want to Gradescope to check how well your test suite
performs
What to turn in
Submit to Gradescope your testing suite; it must be named tests.py
This file will include at the top of each test a comment describing your test generation