/phoenix-tracing

Manual Instrumentation (Python)

Add custom spans using decorators or context managers for fine-grained tracing control.

Setup

pip install arize-phoenix-otel

from phoenix.otel import register
tracer_provider = register(project_name="my-app")
tracer = tracer_provider.get_tracer(__name__)

Quick Reference

Decorator Approach (Recommended)

Use for: Full function instrumentation, automatic I/O capture

@tracer.chain
def rag_pipeline(query: str) -> str:
    docs = retrieve_documents(query)
    ranked = rerank(docs, query)
    return generate_response(ranked, query)

@tracer.retriever
def retrieve_documents(query: str) -> list[dict]:
    results = vector_db.search(query, top_k=5)
    return [{"content": doc.text, "score": doc.score} for doc in results]

@tracer.tool
def get_weather(city: str) -> str:
    response = requests.get(f"https://api.weather.com/{city}")
    return response.json()["weather"]

Custom span names:

@tracer.chain(name="rag-pipeline-v2")
def my_workflow(query: str) -> str:
    return process(query)

Context Manager Approach

Use for: Partial function instrumentation, custom attributes, dynamic control

from opentelemetry.trace import Status, StatusCode
import json

def retrieve_with_metadata(query: str):
    with tracer.start_as_current_span(
        "vector_search",
        openinference_span_kind="retriever"
    ) as span:
        span.set_attribute("input.value", query)

        results = vector_db.search(query, top_k=5)

        documents = [
            {
                "document.id": doc.id,
                "document.content": doc.text,
                "document.score": doc.score
            }
            for doc in results
        ]
        span.set_attribute("retrieval.documents", json.dumps(documents))
        span.set_status(Status(StatusCode.OK))

        return documents

Capturing Input/Output

Always capture I/O for evaluation-ready spans.

Automatic I/O Capture (Decorators)

Decorators automatically capture input arguments and return values:

@tracer.chain
def handle_query(user_input: str) -> str:
    result = agent.generate(user_input)
    return result.text

# Automatically captures:
# - input.value: user_input
# - output.value: result.text
# - input.mime_type / output.mime_type: auto-detected

Manual I/O Capture (Context Manager)

Use set_input() and set_output() for simple I/O capture:

from opentelemetry.trace import Status, StatusCode

def handle_query(user_input: str) -> str:
    with tracer.start_as_current_span(
        "query.handler",
        openinference_span_kind="chain"
    ) as span:
        span.set_input(user_input)

        result = agent.generate(user_input)

        span.set_output(result.text)
        span.set_status(Status(StatusCode.OK))

        return result.text

What gets captured:

{
  "input.value": "What is 2+2?",
  "input.mime_type": "text/plain",
  "output.value": "2+2 equals 4.",
  "output.mime_type": "text/plain"
}

Why this matters:

• Phoenix evaluators require input.value and output.value
• Phoenix UI displays I/O prominently for debugging
• Enables exporting data for fine-tuning datasets

Custom I/O with Additional Metadata

Use set_attribute() for custom attributes alongside I/O:

def process_query(query: str):
    with tracer.start_as_current_span(
        "query.process",
        openinference_span_kind="chain"
    ) as span:
        # Standard I/O
        span.set_input(query)

        # Custom metadata
        span.set_attribute("input.length", len(query))

        result = llm.generate(query)

        # Standard output
        span.set_output(result.text)

        # Custom metadata
        span.set_attribute("output.tokens", result.usage.total_tokens)
        span.set_status(Status(StatusCode.OK))

        return result

See Also

• Span attributes: span-chain.md, span-retriever.md, span-tool.md, span-llm.md, span-agent.md, span-embedding.md, span-reranker.md, span-guardrail.md, span-evaluator.md
• Auto-instrumentation: instrumentation-auto-python.md for framework integrations
• API docs: https://docs.arize.com/phoenix/tracing/manual-instrumentation