Development

docs/collections.md

@@ -92,7 +92,7 @@ class diagram panel
     print(species)         # ["burmese", "carpet", "ball", "blood"]
     ```
 
-- **Acccess a range of multiple items at once:** 
+- **Access a range of multiple items at once:** 
 
     - **Slice with `list[start:end]`** to return a new list containing items from the `start` index up to (but not including) the `end` index.
 
@@ -333,6 +333,20 @@ class diagram panel
 
 ### Going further { data-card-link="skip" }
 
+??? warning "In-place list methods return None"
+    `append()`, `insert()`, `extend()`, `sort()`, `reverse()`, and `remove()` all change the list directly and return `None` — not the changed list. Reassigning the variable to one of their results replaces the list itself with `None`, and the next call on it raises `AttributeError: 'NoneType' object has no attribute '...'`.
+
+    ```python-ref
+    species = species.append("carpet")  # species is now None, not the updated list
+    species.sort()                      # AttributeError: 'NoneType' object has no attribute 'sort'
+    ```
+
+    Call the method on its own line instead — the list was already changed in place, nothing to reassign.
+
+    ```python-ref
+    species.append("carpet")  # correct — no assignment needed
+    ```
+
 ??? run "Practice with lists"
     Each box below is fully editable — write your answer, then click Run.
 
@@ -485,7 +499,7 @@ flowchart LR
     snake["species"]  # "ball"
     ```
 
-- **`get()`** does the same thing, but returns `None` if the key is not in the dict. You can provide an optinal default value to fall back on that will be returned if they key is not in the dict.
+- **`get()`** does the same thing, but returns `None` if the key is not in the dict. You can provide an optional default value to fall back on that will be returned if the key is not in the dict.
 
     ```python-ref
     snake.get("species")        # "ball"

docs/functions.md

@@ -121,6 +121,28 @@ describe(species="ball", venomous=True)    # length_ft still uses its default
         pass    # placeholder — does nothing, but prevents a syntax error
     ```
 
+??? warning "Mutable default argument"
+    A default value is only ever created **once**, when the function is defined — not fresh on every call. For a list or dict default, that means every call sharing that default is silently reading and writing the *same* object, so it keeps growing across calls instead of starting empty each time.
+
+    ```python-ref
+    def add_sighting(species, log=[]):    # log=[] is created once, not per call
+        log.append(species)
+        return log
+
+    add_sighting("ball")       # ["ball"]
+    add_sighting("burmese")    # ["ball", "burmese"] — the same list, not a fresh one
+    ```
+
+    Default to `None` instead, and create the list inside the function body:
+
+    ```python-ref
+    def add_sighting(species, log=None):
+        if log is None:
+            log = []
+        log.append(species)
+        return log
+    ```
+
 ??? run "Run a function example"
     All the examples above, combined into one script:
 

docs/index.md

@@ -28,6 +28,8 @@ hide:
 
     [**`Terminal application`**](workspace.md#using-the-terminal-optional)
 
+    [**`virtual environments`**](workspace.md#virtual-environments-optional)
+
 -   :material-cube-outline:{ .lg .middle } [__Foundations__](foundations.md)
 
     Storing, displaying, and inputting values.
@@ -102,7 +104,7 @@ hide:
 
     Repeat a block of code multiple times.
 
-    [**`for`**](loops.md#for-loops): [`enumerate`](loops.md#loop-with-index-and-value) [`loop a set number of times`](loops.md#loop-a- certain-number-of-times) [`loop through a collection`](loops.md#loop-through-a-collection) [`range`](loops.md#iterable-range) [`reversed`](loops.md#loop-in-reverse) [`zip`](loops.md#loop-with-index-and-value)
+    [**`for`**](loops.md#for-loops): [`enumerate`](loops.md#loop-with-index-and-value) [`loop a set number of times`](loops.md#loop-a-certain-number-of-times) [`loop through a collection`](loops.md#loop-through-a-collection) [`range`](loops.md#iterable-range) [`reversed`](loops.md#loop-in-reverse) [`zip`](loops.md#loop-with-index-and-value)
 
     [**`while`**](loops.md#while-loops): [`and`](loops.md#logical-operators) [`boolean expressions`](loops.md#boolean-expressions) [`counter and flag names`](loops.md#counter-and-flag-names) [`flag`](loops.md#using-a-flag) [`not`](loops.md#logical-operators) [`or`](loops.md#logical-operators) [`sentinel`](loops.md#sentinel)
 
@@ -124,7 +126,7 @@ hide:
 
     [**`def`**](functions.md#defining-a-function): [`default parameter values`](functions.md#default-parameter-values) [`docstrings`](functions.md#docstrings) [`keyword arguments`](functions.md#keyword-arguments) [`return`](functions.md#return-values)
 
-    [**`*args`**](functions.md#args), [**`**kwargs`**](functions.md#kwargs)
+    [**`flexible arguments`**](functions.md#flexible-arguments): [`*args`](functions.md#args) [`**kwargs`](functions.md#kwargs)
 
     [**`scope`**](functions.md#scope): [`local vs global`](functions.md#local-vs-global-variables)
 
@@ -196,13 +198,11 @@ hide:
 
     How to understand, manage, and fix errors.
 
-    [**`try, except`**](errors.md#handling-errors): [`exception types`](errors.md#common-exception-types)
-
-    [**`tracebacks`**](errors.md#reading-errors): [`how to read a traceback`](errors.md#how-to-read-a-traceback)
+    [**`kinds of errors`**](errors.md#kinds-of-errors): [`syntax errors`](errors.md#syntax-errors) [`runtime errors`](errors.md#runtime-errors) [`logic errors`](errors.md#logic-errors)
 
-    [**`debugger`**](errors.md#using-a-debugger): [`run in debug mode`](errors.md#step-1-run-in-debug-mode) [`set breakpoints`](errors.md#step-0-set-breakpoints) [`what you can do at a breakpoint`](errors.md#step-2-what-you-can-do-at-a-breakpoint)
+    [**`fixing errors`**](errors.md#fixing-errors): [`tracebacks`](errors.md#reading-a-traceback) [`reading a syntax error message`](errors.md#reading-a-syntax-error-message) [`try, except`](errors.md#catch-with-tryexcept) [`debugging strategies`](errors.md#debugging-strategies) [`isolate the problem`](errors.md#isolate-the-problem) [`print debugging`](errors.md#print-debugging) [`rubber duck debugging`](errors.md#read-it-out-loud) [`debugger tool`](errors.md#debugger-tool) [`TODO, FIXME`](errors.md#flag-as-todofixme) [`testing`](errors.md#detect-errors-with-testing)
 
-    [**`debugging strategies`**](errors.md#debugging-strategies): [`isolate the problem`](errors.md#isolate-the-problem) [`print debugging`](errors.md#print-debugging) [`rubber duck debugging`](errors.md#read-it-out-loud) [`TODO, FIXME`](errors.md#flag-it-with-todofixme)
+    [**`detect errors with testing`**](errors.md#detect-errors-with-testing)
 
 </div>
 </div>
@@ -252,31 +252,23 @@ hide:
 </div>
 
 <div class="pt-category pt-category--wide pt-lib--1" markdown="block">
-#### Desktop UIs { .pt-homepage-heading }
+#### Testing { .pt-homepage-heading }
 
 <div class="grid cards" markdown="block">
 
--   :material-application-outline:{ .lg .middle } [__Tkinter__](libraries/tkinter.md) [:material-language-python:](libraries/tkinter.md){ .pt-lib-badge .pt-lib-badge--builtin title="Built-in — included with Python" }
-
-    Creating desktop applications: text, buttons, dropdowns, forms, output, etc.
-
-    [**`Tk`**](libraries/tkinter.md#creating-a-window)
-
-    [**`Button`**](libraries/tkinter.md#widgets): [`Button`](libraries/tkinter.md#button) [`Entry`](libraries/tkinter.md#entry) [`Label`](libraries/tkinter.md#label)
-
-    [**`pack`**](libraries/tkinter.md#layout-managers): [`grid`](libraries/tkinter.md#grid) [`pack`](libraries/tkinter.md#pack)
+-   :material-test-tube:{ .lg .middle } [__pytest__](libraries/pytest.md) [:material-download-outline:](libraries/pytest.md){ .pt-lib-badge .pt-lib-badge--third-party title="Third-party — install separately with pip" }
 
-    [**`configure`**](libraries/tkinter.md#configuring-widgets): [`reading and changing options`](libraries/tkinter.md#reading-and-changing-options)
+    Writing and running tests: assertions, fixtures, and parametrizing.
 
-    [**`command`**](libraries/tkinter.md#handling-events): [`binding events`](libraries/tkinter.md#binding-events) [`command callbacks`](libraries/tkinter.md#command-callbacks)
+    [**`writing and running a test`**](libraries/pytest.md#writing-and-running-a-test): [`from the command line`](libraries/pytest.md#from-the-command-line)
 
-    [**`Style`**](libraries/tkinter.md#styling-with-ttk): [`customizing a style`](libraries/tkinter.md#customizing-a-style)
+    [**`reading a failure`**](libraries/pytest.md#reading-a-failure)
 
-    [**`messagebox`**](libraries/tkinter.md#dialogs): [`file dialogs`](libraries/tkinter.md#file-dialogs) [`message boxes`](libraries/tkinter.md#message-boxes)
+    [**`fixtures`**](libraries/pytest.md#fixtures)
 
-    [**`winfo_width`**](libraries/tkinter.md#introspecting-widgets): [`winfo methods`](libraries/tkinter.md#winfo-methods)
+    [**`parametrizing tests`**](libraries/pytest.md#parametrizing-tests)
 
-    [**`putting it together`**](libraries/tkinter.md#putting-it-together): [`a simple form`](libraries/tkinter.md#a-simple-form)
+    [**`testing for exceptions`**](libraries/pytest.md#testing-for-exceptions)
 
 </div>
 </div>
@@ -389,6 +381,36 @@ hide:
 </div>
 </div>
 
+<div class="pt-category pt-category--wide pt-lib--1" markdown="block">
+#### Desktop UIs { .pt-homepage-heading }
+
+<div class="grid cards" markdown="block">
+
+-   :material-application-outline:{ .lg .middle } [__Tkinter__](libraries/tkinter.md) [:material-language-python:](libraries/tkinter.md){ .pt-lib-badge .pt-lib-badge--builtin title="Built-in — included with Python" }
+
+    Creating desktop applications: text, buttons, dropdowns, forms, output, etc.
+
+    [**`Tk`**](libraries/tkinter.md#creating-a-window)
+
+    [**`Button`**](libraries/tkinter.md#widgets): [`Button`](libraries/tkinter.md#button) [`Entry`](libraries/tkinter.md#entry) [`Label`](libraries/tkinter.md#label)
+
+    [**`pack`**](libraries/tkinter.md#layout-managers): [`grid`](libraries/tkinter.md#grid) [`pack`](libraries/tkinter.md#pack)
+
+    [**`configure`**](libraries/tkinter.md#configuring-widgets): [`reading and changing options`](libraries/tkinter.md#reading-and-changing-options)
+
+    [**`command`**](libraries/tkinter.md#handling-events): [`binding events`](libraries/tkinter.md#binding-events) [`command callbacks`](libraries/tkinter.md#command-callbacks)
+
+    [**`Style`**](libraries/tkinter.md#styling-with-ttk): [`customizing a style`](libraries/tkinter.md#customizing-a-style)
+
+    [**`messagebox`**](libraries/tkinter.md#dialogs): [`file dialogs`](libraries/tkinter.md#file-dialogs) [`message boxes`](libraries/tkinter.md#message-boxes)
+
+    [**`winfo_width`**](libraries/tkinter.md#introspecting-widgets): [`winfo methods`](libraries/tkinter.md#winfo-methods)
+
+    [**`putting it together`**](libraries/tkinter.md#putting-it-together): [`a simple form`](libraries/tkinter.md#a-simple-form)
+
+</div>
+</div>
+
 <div class="pt-category pt-category--wide pt-lib--1" markdown="block">
 #### Computer vision { .pt-homepage-heading }
 
@@ -430,7 +452,7 @@ hide:
     |  | Learn to do it yourself | Have AI do it for you |
     |---|---|---|
     | **Writing & struggling with code** | :material-check:{ .pt-icon-success } **Productive struggle** is what builds understanding<ul><li>You can solve the problem again on your own</li><li>Adapt the answer, and catch when it's wrong</li></ul> | :material-close:{ .pt-icon-fail } Being handed the answer skips [the friction that builds understanding](https://bjorklab.psych.ucla.edu/wp-content/uploads/sites/13/2016/04/EBjork_RBjork_2011.pdf)<ul><li>If you skip that struggle, you *won't develop the knowledge* to solve the problem again, adapt the answer, or recognize when it's wrong.</li></ul> |
-    | **Reading & verifying code** | :material-check:{ .pt-icon-success } Understanding programming fundamentals makes AI more useful — you can read code you didn't write, and check it before you trust it<ul><li>Spot mistakes</li><li>Understand *why* a solution works</li><li>Communicate your problem to AI more effectively</li></ul> | :material-close:{ .pt-icon-fail } Inefficient communciation with AI if you don't fully understand what's going on, and AI code can look correct while being **wrong and insecure**<ul><li>You can't tell *why* a solution works</li><li>Researchers are already documenting this skill gap in [students who rely on AI code generation](https://dl.acm.org/doi/10.1145/3617367) before they've [built their own foundation](https://dl.acm.org/doi/10.1145/3624720)</li><li>One [Stanford study](https://dl.acm.org/doi/10.1145/3576915.3623157) found developers using AI wrote *less* secure code — but were *more* confident it was secure</li></ul> |
+    | **Reading & verifying code** | :material-check:{ .pt-icon-success } Understanding programming fundamentals makes AI more useful — you can read code you didn't write, and check it before you trust it<ul><li>Spot mistakes</li><li>Understand *why* a solution works</li><li>Communicate your problem to AI more effectively</li></ul> | :material-close:{ .pt-icon-fail } Inefficient communication with AI if you don't fully understand what's going on, and AI code can look correct while being **wrong and insecure**<ul><li>You can't tell *why* a solution works</li><li>Researchers are already documenting this skill gap in [students who rely on AI code generation](https://dl.acm.org/doi/10.1145/3617367) before they've [built their own foundation](https://dl.acm.org/doi/10.1145/3624720)</li><li>One [Stanford study](https://dl.acm.org/doi/10.1145/3576915.3623157) found developers using AI wrote *less* secure code — but were *more* confident it was secure</li></ul> |
     | **On the job** | :material-check:{ .pt-icon-success } Employers are still hiring for understanding, not prompting<ul><li>Code review, debugging, and interviews all test whether you can reason about code</li><li>And judge whether it's correct</li><li>Learning to program on your own turns AI into a tool you can direct and verify, instead of one you're assuming got it right</li></ul> | :material-close:{ .pt-icon-fail } AI will likely be available at work too — but it isn't what's being tested |
 
     </div>

docs/javascripts/pyodide_runner.js

@@ -111,10 +111,10 @@
         const pyodide = await loadPyodideRuntime();
 
         // Pure-stdlib code runs as-is, but third-party packages (numpy,
-        // pandas, ...) ship as separate Pyodide wheels that must be fetched
-        // before the `import` inside the snippet will succeed.
+        // pandas, pytest, ...) ship as separate Pyodide wheels that must be
+        // fetched before the `import` inside the snippet will succeed.
         const source = codeBlock.textContent;
-        const neededPackages = ["numpy", "pandas"].filter((pkg) =>
+        const neededPackages = ["numpy", "pandas", "pytest"].filter((pkg) =>
           new RegExp(`\\bimport\\s+${pkg}\\b|\\bfrom\\s+${pkg}\\b`).test(source)
         );
         if (neededPackages.length) {

docs/libraries/collections.md

@@ -14,7 +14,7 @@ description: >-
     `from collections import ...`. For the built-in `list`, `dict`, `tuple`, and `set` types
     themselves, see [Collections](../collections.md).
 
-The **`collections`** module adds specialized containers with added functionaility on top of the
+The **`collections`** module adds specialized containers with added functionality on top of the
 built-in [`str`](../types.md#strings) [`list`](../collections.md#lists) [`dict`](../collections.md#dictionaries) [`tuple`](../collections.md#tuples) and [`set`](../collections.md#sets).
 
 <div class="pt-jump-table" markdown="block">

docs/libraries/index.md

@@ -54,31 +54,23 @@ Libraries allow us to apply Python to real tasks. These are a few popular ones,
 </div>
 
 <div class="pt-category pt-category--wide pt-lib--1" markdown="block">
-#### Desktop UIs { .pt-homepage-heading }
+#### Testing { .pt-homepage-heading }
 
 <div class="grid cards" markdown="block">
 
--   :material-application-outline:{ .lg .middle } [__Tkinter__](tkinter.md) [:material-language-python:](tkinter.md){ .pt-lib-badge .pt-lib-badge--builtin title="Built-in — included with Python" }
-
-    Creating desktop applications: text, buttons, dropdowns, forms, output, etc.
-
-    [**`Tk`**](tkinter.md#creating-a-window)
+-   :material-test-tube:{ .lg .middle } [__pytest__](pytest.md) [:material-download-outline:](pytest.md){ .pt-lib-badge .pt-lib-badge--third-party title="Third-party — install separately with pip" }
 
-    [**`Button`**](tkinter.md#widgets): [`Button`](tkinter.md#button) [`Entry`](tkinter.md#entry) [`Label`](tkinter.md#label)
+    Writing and running tests: assertions, fixtures, and parametrizing.
 
-    [**`pack`**](tkinter.md#layout-managers): [`grid`](tkinter.md#grid) [`pack`](tkinter.md#pack)
+    [**`writing and running a test`**](pytest.md#writing-and-running-a-test): [`from the command line`](pytest.md#from-the-command-line)
 
-    [**`configure`**](tkinter.md#configuring-widgets): [`reading and changing options`](tkinter.md#reading-and-changing-options)
+    [**`reading a failure`**](pytest.md#reading-a-failure)
 
-    [**`command`**](tkinter.md#handling-events): [`binding events`](tkinter.md#binding-events) [`command callbacks`](tkinter.md#command-callbacks)
+    [**`fixtures`**](pytest.md#fixtures)
 
-    [**`Style`**](tkinter.md#styling-with-ttk): [`customizing a style`](tkinter.md#customizing-a-style)
+    [**`parametrizing tests`**](pytest.md#parametrizing-tests)
 
-    [**`messagebox`**](tkinter.md#dialogs): [`file dialogs`](tkinter.md#file-dialogs) [`message boxes`](tkinter.md#message-boxes)
-
-    [**`winfo_width`**](tkinter.md#introspecting-widgets): [`winfo methods`](tkinter.md#winfo-methods)
-
-    [**`putting it together`**](tkinter.md#putting-it-together): [`a simple form`](tkinter.md#a-simple-form)
+    [**`testing for exceptions`**](pytest.md#testing-for-exceptions)
 
 </div>
 </div>
@@ -191,6 +183,36 @@ Libraries allow us to apply Python to real tasks. These are a few popular ones,
 </div>
 </div>
 
+<div class="pt-category pt-category--wide pt-lib--1" markdown="block">
+#### Desktop UIs { .pt-homepage-heading }
+
+<div class="grid cards" markdown="block">
+
+-   :material-application-outline:{ .lg .middle } [__Tkinter__](tkinter.md) [:material-language-python:](tkinter.md){ .pt-lib-badge .pt-lib-badge--builtin title="Built-in — included with Python" }
+
+    Creating desktop applications: text, buttons, dropdowns, forms, output, etc.
+
+    [**`Tk`**](tkinter.md#creating-a-window)
+
+    [**`Button`**](tkinter.md#widgets): [`Button`](tkinter.md#button) [`Entry`](tkinter.md#entry) [`Label`](tkinter.md#label)
+
+    [**`pack`**](tkinter.md#layout-managers): [`grid`](tkinter.md#grid) [`pack`](tkinter.md#pack)
+
+    [**`configure`**](tkinter.md#configuring-widgets): [`reading and changing options`](tkinter.md#reading-and-changing-options)
+
+    [**`command`**](tkinter.md#handling-events): [`binding events`](tkinter.md#binding-events) [`command callbacks`](tkinter.md#command-callbacks)
+
+    [**`Style`**](tkinter.md#styling-with-ttk): [`customizing a style`](tkinter.md#customizing-a-style)
+
+    [**`messagebox`**](tkinter.md#dialogs): [`file dialogs`](tkinter.md#file-dialogs) [`message boxes`](tkinter.md#message-boxes)
+
+    [**`winfo_width`**](tkinter.md#introspecting-widgets): [`winfo methods`](tkinter.md#winfo-methods)
+
+    [**`putting it together`**](tkinter.md#putting-it-together): [`a simple form`](tkinter.md#a-simple-form)
+
+</div>
+</div>
+
 <div class="pt-category pt-category--wide pt-lib--1" markdown="block">
 #### Computer vision { .pt-homepage-heading }