Add classnames util function

README.md

@@ -187,6 +187,19 @@ def current_user_panel():
 
 The `render` function takes an `indent` argument, which is a integer used to control how many spaces are used as indentation in the generated HTML. The default is 0, meaning the entire HTML string will be returned on a single line. You may wish to use (say) `indent=2` for development, and `indent=0` for production (essentially minifying your HTML).
 
+## Generating class names
+
+A function is provided that can be used to generate strings of class names based on various arguments. This is closely based on the [classnames](https://github.com/JedWatson/classnames/) JavaScript library.
+
+```python
+from hotmetal.utils.classnames import classnames
+
+
+def header(title):
+    title_is_green = title.lower() == "green"
+    return ("h1", {"class": classnames("title", {"green": title_is_green})}, [title])
+```
+
 ## Testing tools
 
 When writing tests for components, it's often useful to be able to search through a tree to find particular nodes and make assertions about them. To help with this, `hotmetal` provides a `find` function, which takes an iterable of nodes and a predicate callable, and returns a generator that yields nodes that match the predicate (using depth-first pre-order traversal of the nodes, much like the browser's [`querySelectorAll`](https://developer.mozilla.org/en-US/docs/Web/API/Document/querySelectorAll) function).

hotmetal/utils/classnames.py

@@ -0,0 +1,18 @@
+def classnames(*args):
+    classes = []
+    for arg in args:
+        if isinstance(arg, bool):
+            continue
+        if isinstance(arg, dict):
+            for key, value in arg.items():
+                if value:
+                    classes.append(key)
+        elif isinstance(arg, str) and arg.strip():
+            classes.append(arg.strip())
+        elif isinstance(arg, (int, float)) and arg:
+            classes.append(str(arg))
+        elif isinstance(arg, (list, set)):
+            child = classnames(*arg)
+            if child:
+                classes.append(child)
+    return " ".join(classes)

tests/utils/test_classnames.py

@@ -0,0 +1,79 @@
+from hotmetal.utils.classnames import classnames
+from unittest import TestCase
+
+
+class ClassnamesTestCase(TestCase):
+    def test_dict_keys_with_truthy_values(self):
+        self.assertEqual(
+            classnames({"a": True, "b": False, "c": 0, "d": None, "e": 1}),
+            "a e",
+        )
+
+    def test_joins_multiple_classnames_and_ignore_falsy_values(self):
+        self.assertEqual(
+            classnames("a", 0, None, True, 1, "b"),
+            "a 1 b",
+        )
+
+    def test_heterogenous_args(self):
+        self.assertEqual(
+            classnames({"a": True}, "b", 0),
+            "a b",
+        )
+
+    def test_result_trimmed(self):
+        self.assertEqual(
+            classnames("", "b", {}, ""),
+            "b",
+        )
+
+    def test_returns_empty_string_for_empty_args(self):
+        self.assertEqual(
+            classnames({}),
+            "",
+        )
+
+    def test_supports_list_of_classnames(self):
+        self.assertEqual(
+            classnames(["a", "b"]),
+            "a b",
+        )
+
+    def test_supports_sets_of_classnames(self):
+        self.assertTrue(
+            classnames({"a", "b"}) in {"a b", "b a"},
+        )
+
+    def test_joins_lists_args_with_string_args(self):
+        self.assertEqual(
+            classnames(["a", "b"], "c"),
+            "a b c",
+        )
+        self.assertEqual(
+            classnames("c", ["a", "b"]),
+            "c a b",
+        )
+
+    def test_handles_lists_that_include_falsy_and_true_values(self):
+        self.assertEqual(
+            classnames(["a", 0, None, False, True, "b"]),
+            "a b",
+        )
+
+    def test_handles_lists_that_include_lists(self):
+        self.assertEqual(
+            classnames(["a", ["b", "c"]]),
+            "a b c",
+        )
+
+    def test_handles_lists_that_include_dicts(self):
+        self.assertEqual(
+            classnames(["a", {"b": True, "c": False}]),
+            "a b",
+        )
+
+    def test_handles_nested_lists_that_include_empty_nested_lists(self):
+        self.assertEqual(
+            classnames(["a", [[]]]),
+            "a",
+        )