quality.html

---
title: "Quality"
quote:
  text: "Opera happens because a large number of things amazingly fail to go wrong."
  author: "Terry Pratchett"
---

# What problems are we trying to solve?

- Correctness
  - "Does the code do what it's supposed to?"
  - "It is getting the right answer?"
- Verifiability
  - "How do I know I can trust this result?"

---

# It's harder than it looks

- Hard to know if you're getting the right answer when you don't know what the right answer is
- Harder when you don't know how close is close enough
- Harder still when the answers depend on deep domain knowledge
- Focus on the process and hope it produces the right results

---

# Coding style

## Before

```
def count(ln):
    s = ['a', 'A', 'the',
             'The', 'and']
    n = 0
    for i in range(len(ln)):
        line = ln[i]
        stuff = line.split()
        for word in stuff:
            # print(word)
            j = s.count(word)
            if (j > 0) == True:
                n = n + 1
    return n

import sys

lines = sys.stdin.readlines()
# print('number of lines', len(lines))
n = count(lines)
print('number', n)
```

---

# Coding style

## After

```
import sys

STOPS = {'a', 'A', 'the', 'The', 'and'}

def count(reader):
    n = 0
    for line in reader:
        for word in line.split():
            if word in STOPS:
                n += 1
    return n

if __name__ == '__main__':
    n = count(sys.stdin)
    print('number', n)
```

---

# Lint

- The original `lint` checked for common problems in C
  - "It's legal, but you shouldn't do it that way"
- Readable code contains fewer bugs
- Use pycodestyle for Python, lintr for R, etc.
- Perform these checks in continuous integration

---

<h1 class="project-lead">As project lead</h1>

- Add linting to continuous integration checks
- Use default settings
  - Will save many hours of argument
  - And reduce unfamiliarity for newcomers
- Show people how to run checks before submitting PRs

---

# Code review

- Bacchelli & Bird 2013: the most cost-effective way to ensure code quality
  - And to spread knowledge through a team
- Cohen et al 2013: most of the benefit comes from the first reviewer in the first hour
- Petre & Wilson 2014: effective code review of scientific software requires domain knowledge

---

<h1 class="project-lead">As project lead</h1>

- Create a short checklist to guide newcomers
  - No more than a dozen points
  - Intended for them to outgrow
- Review first contributions promptly and warmly
  - Sholler et al 2019: increases the chances that they'll become regular contributors

---

# Assert early, assert often

- `assert(condition, message)`
  - "There are no NAs in the address column"
  - "All ages are between 0 and 130"
- Reduces distance between something going wrong and the problem being noticed
- Allows readers to check their mental model
- Forces author to think about what "right" actually means

---

# Logging

- Don't add and remove print statements
- Use `log(level, message)`
  - `level` is `DEBUG`, `INFO`, `WARNING`, `ERROR`
- When running, `setLogging(level, destination)`
  - Only see messages at that level or above
  - `destination` can be standard error, a file, etc.

---

# Unit testing

- Packages should have unit tests
- Use small synthetic datasets
  - Always record the seed for random number generation
- Run–inspect–record–perturb
- Use code coverage to detect untested lines or paths
  - covr for R, coverage.py for Python
- Check that asserts actually assert
  - An alarm that doesn't go off is worse than none at all

---

# This seems like a lot of work

- It actually saves time
  - Improving quality increases efficiency
  - In fact, it's the only thing that does
- And it takes less time than retracting a paper
  - Or reading a pull request that breaks something

---

<h1 class="exercise">Check style</h1>

1. Run a linting program on your existing software. How many problems does it report? How many do you agree with?
1. Add linting to your build system so that others can run it with a single command.

---

<h1 class="exercise">How close is close enough?</h1>

1. Pick a recently reported result.
1. Have team members separately write down how different it would have to be to make them nervous. (They must each provide a specific number.)
1. Compare answers.

---

<h1 class="exercise">What's your coverage?</h1>

1. What fraction of your existing code is there to check that the rest of it works correctly?
1. If you have unit tests, use a coverage tool to see what they don't currently check.
1. How many of your assertions are tested?