CSS Selectors

Target elements for set_content and set_style updates using CSS selectors or component IDs.

Targeting by Component ID (Preferred)

The most reliable way to target elements is by component_id:

# When creating content, always use component_id
content = Div(
    content="Hello",
    component_id="main-content",  # This becomes the element's id
)

# Target by id in set_content/set_style
set_content(widget_id, component_id="main-content", text="Updated!")
set_style(widget_id, component_id="main-content", styles={"color": "red"})

Why component_id is Preferred

Aspect	component_id	CSS Selector
Reliability	Always unique	May match multiple elements
Refactoring	Survives DOM changes	May break on restructure
Performance	Direct lookup	May scan document
Clarity	Self-documenting	Requires knowledge of DOM

Targeting by CSS Selector

When you don't have a component_id, use CSS selectors:

# By element type
set_style(widget_id, selector="h1", styles={"fontSize": "24px"})

# By class
set_style(widget_id, selector=".status-indicator", styles={"display": "block"})

# By attribute
set_content(widget_id, selector="[data-ticker='AAPL']", text="$185.00")

# Combined selectors
set_style(widget_id, selector=".toolbar button.primary", styles={"opacity": "0.5"})

# First/last/nth-child
set_style(widget_id, selector="ul li:first-child", styles={"fontWeight": "bold"})

Common Selector Patterns

Basic Selectors

Selector	Matches	Example
#myid	Element with id="myid"	#header
.myclass	Elements with class="myclass"	.card
div	All div elements	span
*	All elements	*

Combinators

Selector	Matches	Example
div.myclass	Divs with class="myclass"	button.primary
div > p	Direct child p of div	.card > .title
div p	Any descendant p of div	.sidebar a
div + p	p immediately after div	h1 + p
div ~ p	All p siblings after div	h1 ~ p

Attribute Selectors

Selector	Matches	Example
[data-x]	Elements with data-x attribute	[data-ticker]
[data-x="val"]	Elements where data-x="val"	[data-ticker="AAPL"]
[data-x^="pre"]	data-x starts with "pre"	[data-id^="user-"]
[data-x$="suf"]	data-x ends with "suf"	[href$=".pdf"]
[data-x*="sub"]	data-x contains "sub"	[class*="btn"]

Pseudo-Selectors

Selector	Matches	Example
:first-child	First child of parent	li:first-child
:last-child	Last child of parent	li:last-child
:nth-child(n)	Nth child of parent	tr:nth-child(2)
:nth-child(odd)	Odd children	tr:nth-child(odd)
:nth-child(even)	Even children	tr:nth-child(even)
:not(selector)	Elements not matching	div:not(.hidden)
:empty	Elements with no children	p:empty

Practical Examples

Updating Multiple Elements

# Update all prices
set_style(widget_id, selector=".price", styles={"color": "green"})

# Update all elements with data-status="stale"
set_style(widget_id, selector="[data-status='stale']", styles={"opacity": "0.5"})

Targeting by Data Attribute (Dynamic Content)

# Create content with data attributes
html = """
<div class="ticker-row" data-ticker="AAPL">
    <span class="symbol">AAPL</span>
    <span class="price">$185.00</span>
</div>
<div class="ticker-row" data-ticker="GOOGL">
    <span class="symbol">GOOGL</span>
    <span class="price">$142.00</span>
</div>
"""

# Update specific ticker
set_content(widget_id, selector="[data-ticker='AAPL'] .price", text="$186.50")

Table Row Styling

# Highlight header row
set_style(widget_id, selector="table tr:first-child", styles={
    "backgroundColor": "var(--bg-secondary)",
    "fontWeight": "bold",
})

# Zebra striping
set_style(widget_id, selector="table tr:nth-child(even)", styles={
    "backgroundColor": "var(--bg-tertiary)",
})

Best Practices

1. Prefer component_id Over Selectors

# ✓ Preferred - explicit and stable
set_content(widget_id, component_id="price-display", text="$100")

# ✓ OK for bulk updates
set_style(widget_id, selector=".price", styles={"color": "green"})

# ✗ Fragile - depends on DOM structure
set_style(widget_id, selector="div > div > span:nth-child(2)", styles=...)

2. Use Data Attributes for Dynamic Content

# Create with data attributes
content = Div(
    content='<span data-field="status">Unknown</span>',
    component_id="card",
)

# Update by data attribute
set_content(widget_id, selector="[data-field='status']", text="Active")

3. Avoid Overly Specific Selectors

# ✗ Bad - breaks if DOM changes
selector = "div.container > div.row:nth-child(2) > div.col > span.value"

# ✓ Good - targeted but flexible
selector = ".row .value"  # or use component_id

4. Test Selectors First

Use browser DevTools to test selectors:

1. Open DevTools (F12)
2. Go to Console
3. Type: document.querySelectorAll(".your-selector")
4. Verify it matches expected elements

Performance Considerations

• component_id → Direct getElementById (fastest)
• ID selector (#myid) → Very fast
• Class selector (.myclass) → Fast
• Complex selectors → Slower, may need traversal
• Universal selector (*) → Slowest

For frequent updates (animations, live data), prefer component_id or simple selectors.