Add basic testing intro

exercises/problem/climate_model.py

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+solar_constant = 1370
+sigma = 5.67e-8 # Stefan-Boltzmann constant
+
+def absorbed_solar_radiation(albedo):
+    return solar_constant*(1-albedo)/4
+
+def emitted_radiation(emissivity, temperature):
+    return emissivity * sigma * temperature**4
+
+def energy_balanced_temperature(emissivity, albedo):
+    # dT/dt = 0 => in = out
+    return (solar_constant*(1-albedo)/(4*emissivity * sigma))**0.25
+
+def emissivity_given_temp(albedo, temperature):
+    return solar_constant*(1-albedo)/(4*sigma*temperature**4)

exercises/problem/plot4.py

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+import climate_model
+
+import xarray
+import matplotlib.pyplot as plt
+
+global_ds = xarray.open_dataset("../data/HadCRUT.5.0.0.0_analysis_summary-series_global_annual.nc")
+
+fig, axs = plt.subplots(1, 1, figsize=(10,6), layout='constrained')
+
+# Absolute mean temperature between 1961 and 1990
+# https://crudata.uea.ac.uk/cru/data/temperature/abs_glnhsh.txt
+absolute_value = 13.974 + 273.5
+time = global_ds['time'].data
+temperature = global_ds['tas_mean'].data + absolute_value
+albedo = 0.3
+
+epsilon = climate_model.emissivity_given_temp(albedo, temperature)
+
+axs.plot(time, epsilon)
+
+axs.set_xlabel("Year")
+axs.set_ylabel("Effective emissivity of Earth")
+
+plt.show()
+

exercises/testing_exercises.qmd

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+## Exercises
+
+In the example repository, there is a file called 'climate_model.py'.
+This file contains some functions that are used in 'plot4.py'
+
+Task:
+
+1. Write some basic tests for these functions (think about inputs that give known
+   outputs)
+2. Think about what properties/behaviours these functions should have, and write
+   tests for them
+3. Look at the other python files, and see if any functions can be extracted and
+   tested

src/_pytest.qmd

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+# Testing with pytest
+
+## What is pytest?
+
+:::{.columns}
+::::{.column width=50%}
+- A testing *framework* for python
+- Makes it a lot easier to write tests
+- Is used extensively by python packages
+- Automatically discovers and runs tests
+::::
+::::{.column width=50%}
+![](img/pytest.png){style="width:50%;"}
+::::
+:::
+
+## pytest basics
+
+\
+
+- Tests are .py files and live in a `tests/` directory
+- Each test group is a function with prefix `test_`
+- Use the `assert` keyword to validate an expression is true
+
+\
+
+:::{.columns}
+::::{.column width=30%}
+```{.python filename="src/add.py"}
+def add(a, b):
+  return a+b
+```
+```{.python filename="tests/test.py"}
+def test_add():
+  assert add(0, 0) == 0
+  assert add(1, 0) == 1
+  assert add(0, 1) == 1
+  assert add(1, 1) == 2
+  ...
+```
+::::
+::::{.column width=70%}
+
+```{=html}
+<div class="code-with-filename-file"><pre><strong>terminal</strong></pre></div>
+<div class="term-container">
+<span class="term-fg1">===================================== test session starts =====================================</span>
+platform linux -- Python 3.14.7, pytest-9.1.1, pluggy-1.6.0
+rootdir: &#47;tmp&#47;example
+<span class="term-fg1">collected 1 item                                                                              </span>
+&nbsp;
+tests&#47;test.py <span class="term-fg32">.                                                                         [100%]</span>
+&nbsp;
+<span class="term-fg32">====================================== </span><span class="term-fg32 term-fg1">1 passed</span><span class="term-fg32"> in 0.00s ======================================</span>
+</div>
+```
+::::
+:::
+
+## pytest basics
+
+What if we implemented `add` wrong?
+
+```{.python filename="src/add,py"}
+def add(a, b):
+  return a-b # Incorrect
+```
+
+\
+
+```{=html}
+<div class="term-container">
+<span class="term-fg32 term-fg1">[jack@nixos:~]$</span> pytest tests&#47;test.py
+platform linux -- Python 3.14.7, pytest-9.1.1, pluggy-1.6.0
+rootdir: &#47;tmp&#47;example
+<span class="term-fg1">collected 1 item                                                                              </span>
+&nbsp;
+tests&#47;test.py <span class="term-fg31">F                                                                         [100%]</span>
+&nbsp;
+========================================== FAILURES ===========================================
+<span class="term-fg31 term-fg1">__________________________________________ test_add ___________________________________________</span>
+&nbsp;
+    <span class="term-fgi94">def</span><span class="term-fgi90"> </span><span class="term-fgi92">test_add</span>():
+      <span class="term-fgi94">assert</span> add(<span class="term-fgi94">0</span>, <span class="term-fgi94">0</span>) == <span class="term-fgi94">0</span>
+      <span class="term-fgi94">assert</span> add(<span class="term-fgi94">1</span>, <span class="term-fgi94">0</span>) == <span class="term-fgi94">1</span>
+&gt;     <span class="term-fgi94">assert</span> add(<span class="term-fgi94">0</span>, <span class="term-fgi94">1</span>) == <span class="term-fgi94">1</span>
+<span class="term-fg31 term-fg1">E     assert -1 == 1</span>
+<span class="term-fg31 term-fg1">E      +  where -1 = add(0, 1)</span>
+&nbsp;
+<span class="term-fg31 term-fg1">tests&#47;test.py</span>:7: AssertionError
+<span class="term-fg36 term-fg1">=================================== short test summary info ===================================</span>
+<span class="term-fg31">FAILED</span> tests&#47;test.py::<span class="term-fg1">test_add</span> - assert -1 == 1
+<span class="term-fg31">====================================== </span><span class="term-fg31 term-fg1">1 failed</span><span class="term-fg31"> in 0.02s ======================================</span>
+</div>
+```
+
+## Parametrized tests
+
+- pytest implements a lot of useful tools to make writing tests easier
+- One of these is test parametrization, which allows us to reuse tests for different
+  inputs
+- We only need to give the `pytest.mark.parametrize` decorator:
+  - The names of the input variables
+  - A list of tuples of inputs
+
+```{.python filename="tests/test.py"}
+@pytest.mark.parametrize("x,y,expected", [(0, 0, 0), (1, 0, 1), (0, 1, 1), (1, 1, 2)])
+def test_add(x, y, expected):
+  assert add(x, y) == expected
+```
+
+## What to test
+
+- Beyond using fixed points, we can think about how a function should behave
+- For example, our add function should have the following *properties*
+  - `add(x,0) == x`
+  - `add(x, y) == add(y, x)`
+  - `add(x, add(y, z)) == add(add(x, y), z)`
+
+## Writing code for unit testing
+
+\
+
+- Unit testing forces us to write code that is minimal
+- Decompose functions into their smallest logical unit
+- Smaller functions are easier to test (and review!)
+  - Fewer/simpler inputs
+  - Can help to think of edge cases
+
+### Example
+
+:::{.columns}
+::::{.column width=50%}
+Easier to test this:
+```{.python}
+def complicated_computation(data):
+  z = computation_1(data.x, data.y)
+  return computation_2(data.t, data.w, z)
+
+def computation_1(x, y):
+  result = atan2(x, y)
+  # Maybe do something else here
+  return result
+
+def computation_2(t, w, z):
+  result = exp((w+z)/t)
+  # Maybe do something else here
+  return result
+```
+::::
+::::{.column width=50%}
+Than this:
+```{.python}
+def complicated_computation(data):
+  z = atan2(data.x, data.y)
+  z = ...
+  result = exp((data.w+z)/data.t)
+  result = ...
+  return result
+```
+::::
+:::