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## Overview
This blog focuses on the ["Mutation Analysis"](https://www.fuzzingbook.org/html/MutationAnalysis.html) chapter of
-[The Fuzzing Book](https://www.fuzzingbook.org/). As we have discussed in pervious class sessions, there are many ways to test a program, this week we are focussing on a new type of testing: mutation analysis. Today we are discussing where this type of testing fits into the world of software engineering, how it adds to existing testing techniques we have discussed, and the limitations of this type of testing. We will be taking these practices and connecting them to our team’s current efforts with the [execexam](https://github.com/GatorEducator/execexam) tool, as well as how this chapter builds on the concepts we discussed last week from ["Fuzzing: Breaking Things with Random Inputs"](https://www.fuzzingbook.org/html/Fuzzer.html).
+[The Fuzzing Book](https://www.fuzzingbook.org/). As we have discussed in previous class sessions, there are many ways
+to test a program, this week we are focussing on a new type of testing: mutation analysis. Today we are discussing where
+this type of testing fits into the world of software engineering, how it adds to existing testing techniques we have
+discussed, and the limitations of this type of testing. We will be taking these practices and connecting them to our team’s
+current efforts with the [execexam](https://github.com/GatorEducator/execexam) tool, as well as how this chapter builds on
+the concepts we discussed last week from ["Fuzzing: Breaking Things with Random Inputs"](https://www.fuzzingbook.org/html/Fuzzer.html).
## Summary
Mutation is a unique technique used to check the effectiveness of our test suites. We mutate the code by adding a
-bug into our code to see if our test suites are designed to find the bugs. The theory is, as decribed in the ["Mutation Analysis"](https://www.fuzzingbook.org/html/MutationAnalysis.html) chapter of
-[The Fuzzing Book](https://www.fuzzingbook.org/) that **"if [the test suite] fails to detect such mutations, it will also miss real bugs."**
+bug into our code to see if our test suites are designed to find the bugs. The theory is, as decribed in the
+["Mutation Analysis"](https://www.fuzzingbook.org/html/MutationAnalysis.html) chapter of
+[The Fuzzing Book](https://www.fuzzingbook.org/) that
+**"if [the test suite] fails to detect such mutations, it will also miss real bugs."**
### Limitations to Fuzzing
-Mutation differs froms standard structural coverage because structural coverage fails to identify of the program executions from the test suite are giving us the correct output. This is dangerous as a stand alone testing measure, because we may recieve 100% code coverage and assume our code is perfect, but in reality, that we may not be identifying all the bugs. This issue stems from the fact that test suites aren't designed to find specific bugs in code, but rather to test if a function produced output without error. This is not to say that code coverage is useless in our programs, but this chapter shows us that there is more to the picture, on top of code coverage. Overall, mutation is an important step to testing that helps us developers uncover hidden bugs and have more confidence in our code.
+Mutation differs froms standard structural coverage because structural coverage fails to identify of the program
+executions from the test suite are giving us the correct output. This is dangerous as a stand alone testing measure,
+because we may recieve 100% code coverage and assume our code is perfect, but in reality, that we may not be identifying
+all the bugs. This issue stems from the fact that test suites aren't designed to find specific bugs in code, but rather
+to test if a function produced output without error. This is not to say that code coverage is useless in our programs,
+but this chapter shows us that there is more to the picture, on top of code coverage. Overall, mutation is an important
+step to testing that helps us developers uncover hidden bugs and have more confidence in our code.
-Below is a section of code from the["Mutation Analysis"](https://www.fuzzingbook.org/html/MutationAnalysis.html) chapter of [The Fuzzing Book](https://www.fuzzingbook.org/), which highlights the above issue. This test may show that `execute_the_program_as_a_whole` has 100% code coverage, but as we have discovered, that may not be enough in terms of testing.
+Below is a section of code from the["Mutation Analysis"](https://www.fuzzingbook.org/html/MutationAnalysis.html)
+chapter of [The Fuzzing Book](https://www.fuzzingbook.org/), which highlights the above issue.
+This test may show that `execute_the_program_as_a_whole` has 100% code coverage,
+but as we have discovered, that may not be enough in terms of testing.
```python
def ineffective_test_2():
@@ -40,24 +56,147 @@ def ineffective_test_2():
Click to Expand for the Answer
-The above code may show that the code is 100% covered, but does not give us any indication on the tests ability to discover bugs in `execute_the_program_as_a_whole`. This is **not** ideal for giving us an understanding on whether or not we are producing and shipping quality code to our [execexam](https://github.com/GatorEducator/execexam) tool.
+The above code may show that the code is 100% covered, but does not give us any indication on the tests ability to
+discover bugs in `execute_the_program_as_a_whole`. This is **not** ideal for giving us an understanding on
+whether or not we are producing and shipping quality code to our [execexam](https://github.com/GatorEducator/execexam) tool.
### Injecting Artificial Faults
-Fear not! The book provides us with a testing tool to ensure our test suites are also testing for bugs: mutation analysis. Mutation analysis is a technique to evaluate the effectiveness of a test suite by introducing artificial faults, known as mutations, into the program's code. The goal is to see if the test suite can detect these intentional errors. For example, a mutation might involve changing a `+` to a `-` in a function. If the test suite misses this, it indicates the test is ineffective. The effectiveness of a test suite is measured by its ability to detect these artificial faults, known as the mutation score.
+Fear not! The book provides us with a testing tool to ensure our test suites are also testing for bugs: mutation analysis.
+Mutation analysis is a technique to evaluate the effectiveness of a test suite by introducing artificial faults, known as
+mutations, into the program's code. The goal is to see if the test suite can detect these intentional errors. For example,
+a mutation might involve changing a `+` to a `-` in a function. If the test suite misses this, it indicates the test is
+ineffective. The effectiveness of a test suite is measured by its ability to detect these artificial faults, known as
+the mutation score.
-This approach is based on the idea that each part of a program has a similar probability of containing errors. By assessing how well a test suite identifies these mutations, we can gauge its ability to find real bugs. Mutation analysis can be applied to static test suites, fuzzers, and other testing frameworks, helping to ensure that they effectively prevent faults. In essence, mutation analysis acts like fuzzing for test suites — if a mutated program passes through the test suite without being flagged as faulty, it reveals a potential weakness in the testing process.
+This approach is based on the idea that each part of a program has a similar probability of containing errors.
+By assessing how well a test suite identifies these mutations, we can gauge its ability to find real bugs. Mutation
+analysis can be applied to static test suites, fuzzers, and other testing frameworks, helping to ensure that they
+effectively prevent faults. In essence, mutation analysis acts like fuzzing for test suites — if a mutated program
+passes through the test suite without being flagged as faulty, it reveals a potential weakness in the testing process.
-Fault injection involves creating specific errors to test the effectiveness of a test suite, but it has limitations. Generating unbiased, hard-to-detect faults is labor-intensive, prone to developer bias, and may miss important bugs.
+Fault injection involves creating specific errors to test the effectiveness of a test suite, but it has limitations. Generating
+unbiased, hard-to-detect faults is labor-intensive, prone to developer bias, and may miss important bugs.
-Mutation analysis offers a better alternative. It operates on the assumption that most programming errors are small, single-token mistakes, which are often caught by compilers. By generating all possible small variations of a program —called mutants— and testing them against a suite, mutation analysis assesses how well the suite can "kill" these mutants, i.e., detect the errors. The effectiveness of a test suite is measured by the proportion of mutants it successfully identifies, offering a more systematic and automated way to evaluate its quality.
+Mutation analysis offers a better alternative. It operates on the assumption that most programming errors are small,
+single-token mistakes, which are often caught by compilers. By generating all possible small variations of a
+program —called mutants— and testing them against a suite, mutation analysis assesses how well the suite can "kill"
+these mutants, i.e., detect the errors. The effectiveness of a test suite is measured by the proportion of mutants
+it successfully identifies, offering a more systematic and automated way to evaluate its quality.
### A Simple Function Mutator
+Below is code from the [Fuzzing Book](https://www.fuzzingbook.org/html/MutationAnalysis.html), which demonstrates a simple mutator.
+
+```{python}
+import ast
+import inspect
+
+def triangle(a, b, c):
+ if a == b:
+ if b == c:
+ return 'Equilateral'
+ else:
+ return 'Isosceles'
+ elif b == c:
+ return 'Isosceles'
+ elif a == c:
+ return 'Isosceles'
+ else:
+ return 'Scalene'
+
+class MuFunctionAnalyzer:
+ def __init__(self, fn, log=False):
+ self.fn = fn
+ self.name = fn.__name__
+ src = inspect.getsource(fn)
+ self.ast = ast.parse(src)
+ self.src = ast.unparse(self.ast) # normalize
+ self.mutator = self.mutator_object()
+ self.nmutations = self.get_mutation_count()
+ self.un_detected = set()
+ self.mutants = []
+ self.log = log
+
+ def mutator_object(self, locations=None):
+ return StmtDeletionMutator(locations)
+
+ def register(self, m):
+ self.mutants.append(m)
+
+ def finish(self):
+ pass
+
+ def get_mutation_count(self):
+ self.mutator.visit(self.ast)
+ return self.mutator.count
+
+class Mutator(ast.NodeTransformer):
+ def __init__(self, mutate_location=-1):
+ self.count = 0
+ self.mutate_location = mutate_location
+
+ def mutable_visit(self, node):
+ self.count += 1 # statements start at line no 1
+ if self.count == self.mutate_location:
+ return self.mutation_visit(node)
+ return self.generic_visit(node)
+
+class StmtDeletionMutator(Mutator):
+ def visit_Return(self, node): return self.mutable_visit(node)
+ def visit_Delete(self, node): return self.mutable_visit(node)
+
+ def visit_Assign(self, node): return self.mutable_visit(node)
+ def visit_AnnAssign(self, node): return self.mutable_visit(node)
+ def visit_AugAssign(self, node): return self.mutable_visit(node)
+
+ def visit_Raise(self, node): return self.mutable_visit(node)
+ def visit_Assert(self, node): return self.mutable_visit(node)
+
+ def visit_Global(self, node): return self.mutable_visit(node)
+ def visit_Nonlocal(self, node): return self.mutable_visit(node)
+
+ def visit_Expr(self, node): return self.mutable_visit(node)
+
+ def visit_Pass(self, node): return self.mutable_visit(node)
+ def visit_Break(self, node): return self.mutable_visit(node)
+ def visit_Continue(self, node): return self.mutable_visit(node)
+ def mutation_visit(self, node): return ast.Pass()
+
+MuFunctionAnalyzer(triangle).nmutations
+```
+
+**Activity: How do you think the code block above mutates code?**
+
+ Click to Expand for the Answer
+
+The code block uses three classes defined inside it `MuFunctionAnalyzer`, `Mutator`, and `StmtDeletionMutator`
+to carry out different actions associated with mutating code. With the specific example call in the code block,
+the class `MuFunctionAnalyzer` is called with the `triangle` function passed in. The example then specifically
+calls the `nmutations` method found in the `MuFunctionAnalyzer` class. This specific code prints out the number
+of possible mutations that can be made to the code specified, in this case 5. There are 5 possible mutations
+because the `triangle` function only has one type of mutation, the 5 return statements.
+
+
+
### Limitations to Mutators
-One challenge in mutation analysis is that not all generated mutants are faulty. For instance, consider a basic python function, think about how many different ways you could add a mutator to it. Now consider this, while most mutants introduce faults, some may be the same as the original program in functionality, which may not in itself be a bug. These **equivalent mutants** do not represent actual faults, making it hard to accurately judge the effectiveness of a test suite. For example, if a mutation score is 70%, it's unclear if the remaining 30% are undetected faults or equivalent mutants. This ambiguity makes it difficult to determine how much a test suite can be improved.
+One challenge in mutation analysis is that not all generated mutants are faulty. For instance, consider a basic
+python function, think about how many different ways you could add a mutator to it. Now consider this, while
+most mutants introduce faults, some may be the same as the original program in functionality, which may not
+in itself be a bug. These **equivalent mutants** do not represent actual faults, making it hard to accurately
+judge the effectiveness of a test suite. For example, if a mutation score is 70%, it's unclear if the remaining
+30% are undetected faults or equivalent mutants. This ambiguity makes it difficult to determine how much a test
+suite can be improved.
+
+## Reflection
+
+In this article, we discussed the limitations of fuzzing, an example of an ineffective test, injecting artificial faults,
+a code example for a simple mutator, and limitations to mutators. Like fuzzing, mutators have their benefits and uses,
+but they also have their faults.
+
+{{< include /_fuzzingbook-reference.qmd >}}
-## Reflection
\ No newline at end of file
+{{< include /_back-blog.qmd >}}
\ No newline at end of file