Feature: Partial Pattern Matching (partial=True) for Callbacks

Description

Dash's pattern-matching callbacks currently require the pattern and component IDs to have exactly the same set of keys. This means a pattern like {"type": "btn"} will not match a component with ID {"type": "btn", "index": 1} — even though the component clearly has the key and value the pattern cares about.

This PR adds an opt-in partial=True flag on Input, Output, and State that relaxes the key-matching constraint: a pattern's keys only need to be a subset of the component's keys. Extra keys on the component are ignored.

Closes #3764

Usage

@app.callback(
    Output("output", "children"),
    Input({"type": "btn"}, "n_clicks", partial=True),
    prevent_initial_call=True,
)
def on_btn_click(n_clicks):
    return f"clicked {n_clicks}"

This fires for any component whose ID contains "type": "btn", regardless of other keys:

• {"type": "btn", "index": 1}
• {"type": "btn", "page": "home", "section": "main"}
• {"type": "btn", "index": 2, "tab": "first"}

Works with wildcards:

Input({"type": ALL}, "n_clicks", partial=True)    # collects all components with a "type" key
Input({"type": MATCH}, "n_clicks", partial=True)   # matches one-at-a-time on "type"

Design choices

• Per-dependency opt-in — not a new wildcard type, not a per-callback flag. Each Input/Output/State independently opts in, so partial and non-partial deps can coexist in the same callback.
• Extra keys are ignored — component keys not present in the pattern are treated as if they matched. This is the most permissive and intuitive behavior.
• Literal-only partial patterns are implicitly multi-valued — a pattern like {"type": "btn"} with partial=True can match multiple components, so the callback receives a list (same as ALL). Patterns with MATCH still resolve one-at-a-time.
• Zero-cost when unused — a hasPartialPatterns boolean guard on the callback graph ensures apps without partial=True pay no additional overhead. All new loops are skipped entirely.

Contributor Checklist

• I have broken down my PR scope into the following TODO tasks
  • Add partial attribute to DashDependency, Input, Output, State (Python)
  • Update _id_matches() for subset key matching when partial=True
  • Serialize partial flag via to_dict() and propagate through insert_callback()
  • Add addPartialPattern() and partialIdMatch() to frontend (dependencies.js)
  • Update computeGraphs() to detect partial flags and set hasPartialPatterns
  • Update getWatchedKeys(), getCallbackByOutput(), getUnfilteredLayoutCallbacks() with partial pattern loops
  • Add resolvePartialDeps() and update getCallbacksByInput() in dependencies_ts.ts
  • Update isMultiValued() for implicit multi-valued partial patterns
  • Fix requestedCallbacks.ts type compatibility for updated isMultiValued signature
  • Add hasPartialPatterns guard to all new loops for zero-cost when unused
• I have run the tests locally and they passed. (refer to testing section in contributing)
• I have added tests, or extended existing tests, to cover any new features or bugs fixed in this PR
  • 16 unit tests in tests/unit/test_partial_matching.py — subset matching, wildcards, serialization, bidirectional checks, negative cases
  • 6 integration tests in tests/integration/callbacks/test_partial_wildcards.py — basic partial match, ALL collection, literal filtering, mixed key sets

Optionals

• I have added entry in the CHANGELOG.md
• If this PR needs a follow-up in dash docs, community thread, I have mentioned the relevant URLs as follows
  • this GitHub #PR number updates the dash docs
  • here is the show and tell thread in Plotly Dash community

Performance

Partial matching requires cross-keyStr searches in several hot-path functions that were previously $O(1)$ hash lookups:

Where $K$ = number of distinct keyStr entries in the pattern index, $p$ = number of keys in the pattern (typically 1–5), and $N$ = number of components in a layout chunk.

hasPartialPatterns guard

To ensure zero cost for apps that don't use partial=True, computeGraphs() sets a hasPartialPatterns boolean on the graph in $O(D)$ time (where $D$ = total dependencies). Every new loop is guarded:

if (graphs.hasPartialPatterns) {
    for (const patKeyStr in graphs.inputPatterns) { ... }
}