A quick taste of pytest

# Testing in Python

It is more fun to write tests on a weekday than it is to look for bugs during the weekend.

-Jacob Kaplan Moss

---

# Overview

## Thinking about programs in different ways
## Pytest
## Where to learn more

---
## Thinking about programs in different ways.

Tests help you think about your code in specific ways.

They 
- invite you to keep your code decoupled (generally a good practice)
- run your code in new ways
- help other programmers understand where to hook into your code

Automated tests help you

- decrease time to run tests
- make sure you don't break something while fixing a bug

???

(That's probably too general an approach, as you'll shortly see.)
---

### Thinking about programs in different ways.

How many ways can python interpret the following program?

```
# hello.py
def greet(msg="Students and professionals, good evening!"):
    "Print a greeting to standard output"
    print(msg)
```

---
Python has at least half dozen modules for interpreting python code, each designed to provide another perspective to the program.

---
layout: true

### Thinking about programs in different ways

```
# hello.py
def greet(msg="Students and professionals, good evening!"):
    "Print a greeting to standard output"
    print(msg)
```
---
### as documentation and as source code

```bash

*$ python -m pydoc hello

Help on module hello:

NAME
    hello

FILE
    /home/buckles/Documents/learn/python/hello.py

FUNCTIONS
    hello()
        Print a greeting to standard output

$
```
---
### as currently running code in a REPL session
```bash
*$ python -i hello.py
Students and professionals, good evening!
>>> greet("This presentation is on testing in python.")
This presentation is on testing in python.
>>>
```
???
Python allows us to drop into the repl and run more commands after executing this program.

---
### as a series of function calls to be optimized
```bash
*$ python -m profile hello.py
Students and professionals, good evening!
       4 function calls in 0.001 seconds

Ordered by: standard name

ncalls  tottime  percall  cumtime  percall filename:lineno(function)
    1    0.001    0.001    0.001    0.001 :0(setprofile)
    1    0.000    0.000    0.000    0.000 hello.py:2(<module>)
    1    0.000    0.000    0.000    0.000 hello.py:2(greet)
    1    0.000    0.000    0.001    0.001 profile:0(<code object <module> at 0x7f7cade89a30, file "hello.py", line 2>)
    0    0.000             0.000          profile:0(profiler)
```
???
If this is a wildly successful hello world program, then perhaps we'll want to optimize.

---
### as code to debug
```bash
*$ python -m pdb hello.py
> /home/buckles/Documents/learn/python/hello.py(2)<module>()
-> def greet():
(Pdb) list
  1     # hello.py
  2  -> def greet():
  3         "Print a greeting to standard output"
  4         print("Students and professionals, good evening!")
  5
  6     def test_hello():
  7         # how do we check IO?
  8         greet()
  9
 10     if "__main__" == __name__:
 11         greet()
(Pdb)
```

---
### as code with tests to run
```bash

$ python -m unittest hello

----------------------------------------------------------------------
Ran 0 tests in 0.000s

OK
$
```

(This would be more interesting if there was a test to run.)

???
Whoops, no tests in that code though.
---
layout: false

# Pytest: a small taste

- discovery
- detailed error output
- parametrization

(Small gotcha: for historical reasons, pytest is run with a dot between py and test: `py.test`)

---
class: center, middle
### Pytest discovery is simple: if it starts with "test_" pytest will pick it up and run it.
---

### test discovery (i.e. let py.test figure out what needs to run)

```
# hello.py
def greet(who="Students and professionals"):
    "Create a greeting"
    return "{}, good evening!".format(who)

def test_hello():
    greeting = greet()
    assert greeting == "Students and professionals, good evening!"

if "__main__" == __name__:
    greet()
```

```
$ py.test hello.py
=============================== test session starts ================================
platform linux2 -- Python 2.7.6 -- py-1.4.27 -- pytest-2.6.3
plugins: capturelog
collected 1 items

hello.py .

============================= 1 passed in 0.02 seconds =============================
$
```
---
layout: true

### detailed failure output

---

```
# hello.py
def greet(who="Students and professionals"):
    "Create a greeting"
    return "{}, good evening!".format(who)

def test_hello():
    greeting = greet("Ladies and Gentlemen")
    assert greeting == "Students and professionals, good evening!"

if "__main__" == __name__:
    greet()

```
---

hello.py F

===================================== FAILURES =====================================
____________________________________ test_hello ____________________________________

def test_hello():
            greeting = greet("Ladies and Gentlemen")
            >       assert greeting == "Students and professionals, good evening!"
            E       assert 'Ladies and G...good evening!' == 'Students and ...good evening!'
            E         - Ladies and Gentlemen, good evening!
            E         + Students and professionals, good evening!

hello.py:9: AssertionError
============================= 1 failed in 0.04 seconds =============================
$
```
---

---

Some tips:

- Put your tests into a different module/file from the code they test. 
- For bigger projects, put them in a whole different package/directory.

```
# hello.py
def greet(who="Students and professionals"):
    "Create a greeting"
    return "{}, good evening!".format(who)

if "__main__" == __name__:
    greet()
```

```
# test_hello.py

import pytest
import hello

@pytest.mark.parametrize('who', 'Boss')
def test_hello(who):
    greeting = hello.greet(who)
    assert greeting == "{}, good evening!".format(who)
```

---
```
*$ py.test
=============================== test session starts ================================
platform linux2 -- Python 2.7.6 -- py-1.4.27 -- pytest-2.6.3
plugins: capturelog
collected 4 items

test_hello.py ....

============================= 4 passed in 0.05 seconds =============================
$
```
Hey, look, now that our tests are in `test_hello.py` py.test can find it without being told what file to look in!

???
Anyone notice something odd here? I see four dots representing four tests.

---

```
$ py.test -v
=============================== test session starts ================================
platform linux2 -- Python 2.7.6 -- py-1.4.27 -- pytest-2.6.3 -- /home/buckles/.virtualenvs/test-mdp/bin/python
plugins: capturelog
collected 4 items

test_hello.py::test_hello[B] PASSED
test_hello.py::test_hello[o] PASSED
test_hello.py::test_hello[s] PASSED
test_hello.py::test_hello[s] PASSED

============================= 4 passed in 0.05 seconds =============================
$

```

Whoops. Forgot that strings act as lists of letters.
---

Let's try that again with a real list this time.
```
# test_hello.py

import pytest
import hello

@pytest.mark.parametrize('who', [
            "Ladies and gentlemen",
            "Students and professionals",
            "Hackers and engineers"])
def test_greet(who):
    greeting = hello.greet(who)
    assert greeting == "{}, good evening!".format(who)
```

---

```
============================= test session starts ==============================
platform linux2 -- Python 2.7.6 -- py-1.4.27 -- pytest-2.6.3 -- /home/buckles/.virtualenvs/test-mdp/bin/python
plugins: capturelog
collecting ... collected 3 items

test_hello.py::test_greet[Ladies and gentlemen] PASSED
test_hello.py::test_greet[Students and professionals] PASSED
test_hello.py::test_greet[Hackers and engineers] PASSED

=========================== 3 passed in 0.05 seconds ===========================
```
That looks more like it.

---
layout: false
class: center, middle

# Resources
---

### Sites

[Python Tools Taxonomy](https://wiki.python.org/moin/PythonTestingToolsTaxonomy)

[PythonTesting.net](http://pythontesting.net) Blog

[pyvideo.org](http://pyvideo.org/search?models=videos.video&q=testing)

### People (Look for their talks on pyvideo)

Holger Krekel - py.test guy, located in Germany

[Michael Foord](https://agileabstractions.com/) Creator of the Python "Mocks" library

### Mailing Lists

[Python testing mailing list](http://lists.idyll.org/listinfo/testing-in-python)

[Mailing List Archives](http://lists.idyll.org/pipermail/testing-in-python/)