Refine Attribute Resolution (#97)

* Wrap up low level placeholder handling into PlaceholderConfig helper class.

* Draft version of special class handling with merging and distinct sequence vs dict typing.

* Use True instead of None since we only init from literal.

* Add style equivalent to class merging.

* Remove old style processing code.

* Add mixed class types back in but without sequence nesting.

* Allow attributes created via aria dict to clear literal attributes.

* Allow attributes created via data dict to clear literal attributes.

* Move special class and style handling state and logic into helper classes.

* Expand style testing.

* Sp.

* Try to uniform design with prior processor.

* Make things *right.

* Rename to expander to accommodate allocator, pull up into main resolution.

* Re-format.

* Remove excess docstrings.

* Adjust docs, drop classnames helper.

* Ignore None to easily support passing through a kwarg with a None default.

* Allow special attributes to be None to allow pass-through from components or template functions.

* Try to add all forms to class attr smoketest.

* Restrict each class attr to either toggle or add list for now.

* Drop support for booleans in interpolated class attributes.

* Add space separated classes to smoketest.

Author: ianjosephwilson

README.md

@@ -130,23 +130,29 @@ button = html(t'<button class="{classes}">Click me</button>')
 # <button class="btn btn-primary active">Click me</button>
 ```
 
-For flexibility, you can also provide a list of strings, dictionaries, or a mix
-of both:
+The `class` attribute can also be a dictionary to toggle classes on or off:
 
 ```python
-classes = ["btn", "btn-primary", {"active": True}, None, False and "disabled"]
-button = html(t'<button class="{classes}">Click me</button>')
+classes = {"active": True, "btn-primary": True}
+button = html(t'<button class={classes}>Click me</button>')
 # <button class="btn btn-primary active">Click me</button>
 ```
 
-See the
-[`classnames()`](https://github.com/t-strings/tdom/blob/main/tdom/classnames_test.py)
-helper function for more information on how class names are combined.
+The `class` attribute can be specified more than once. The values are merged
+from left to right. A common use case would be to update and/or extend default
+classes:
+
+```python
+classes = {"btn-primary": True, "btn-secondary": False}
+button = html(t'<button class="btn btn-secondary" class={classes}>Click me</button>')
+assert str(button) == '<button class="btn btn-primary">Click me</button>'
+```
 
 #### The `style` Attribute
 
-In addition to strings, you can also provide a dictionary of CSS properties and
-values for the `style` attribute:
+The `style` attribute has special handling to make it easy to combine multiple
+styles from different sources.  The simplest way is to provide a dictionary of
+CSS properties and values for the `style` attribute:
 
 ```python
 # Style attributes from dictionaries
@@ -155,6 +161,14 @@ styled = html(t"<p style={styles}>Important text</p>")
 # <p style="color: red; font-weight: bold; margin: 10px">Important text</p>
 ```
 
+Style attributes can also be merged to extend a base style:
+
+```python
+add_styles = {"font-weight": "bold"}
+para = html(t'<p style="color: red" style={add_styles}>Important text</p>')
+assert str(para) == '<p style="color: red; font-weight: bold">Important text</p>'
+```
+
 #### The `data` and `aria` Attributes
 
 The `data` and `aria` attributes also have special handling to convert
@@ -195,7 +209,7 @@ Special attributes likes `class` behave as expected when combined with
 spreading:
 
 ```python
-classes = ["btn", {"active": True}]
+classes = {"btn": True, "active": True}
 attrs = {"class": classes, "id": "act_now", "data": {"wow": "such-attr"}}
 button = html(t'<button {attrs}>Click me</button>')
 # <button class="btn active" id="act_now" data-wow="such-attr">Click me</button>
@@ -539,42 +553,6 @@ anywhere that expects an object with HTML representation. Converting a node to a
 string (via `str()` or `print()`) automatically renders it as HTML with proper
 escaping.
 
-#### The `classnames()` Helper
-
-The `classnames()` function provides a flexible way to build class name strings
-from various input types. It's particularly useful when you need to
-conditionally include classes:
-
-```python
-from tdom import classnames
-
-# Combine strings
-assert classnames("btn", "btn-primary") == "btn btn-primary"
-
-# Use dictionaries for conditional classes
-is_active = True
-is_disabled = False
-assert classnames("btn", {
-    "btn-active": is_active,
-    "btn-disabled": is_disabled
-}) == "btn btn-active"
-
-# Mix lists, dicts, and strings
-assert classnames(
-    "btn",
-    ["btn-large", "rounded"],
-    {"btn-primary": True, "btn-secondary": False},
-    None,  # Ignored
-    False  # Ignored
-) == "btn btn-large rounded btn-primary"
-
-# Nested lists are flattened
-assert classnames(["btn", ["btn-primary", ["active"]]]) == "btn btn-primary active"
-```
-
-This function is automatically used when processing `class` attributes in
-templates, so you can pass any of these input types directly in your t-strings.
-
 #### Utilities
 
 The `tdom` package includes several utility functions for working with

tdom/__init__.py

@@ -1,13 +1,11 @@
 from markupsafe import Markup, escape
 
-from .classnames import classnames
 from .nodes import Comment, DocumentType, Element, Fragment, Node, Text
 from .processor import html
 
 # We consider `Markup` and `escape` to be part of this module's public API
 
 __all__ = [
-    "classnames",
     "Comment",
     "DocumentType",
     "Element",