Python Generators and Iterators for Streaming AI Responses

Why Streaming Matters for AI Applications

When an LLM generates a 2,000-token response, waiting for the full completion before showing anything creates a poor user experience. Streaming sends tokens as they are generated, reducing perceived latency from seconds to milliseconds. Python generators are the natural abstraction for this pattern — they produce values lazily, one at a time, without holding the entire response in memory.

Beyond user experience, generators enable memory-efficient processing of large datasets for embeddings, batch inference, and document chunking — all critical operations in AI pipelines.

Generator Basics for Token Streaming

A generator function uses yield instead of return. Each yield pauses the function and produces a value. The function resumes from where it paused on the next iteration.

flowchart TD
    START["Python Generators and Iterators for Streaming AI …"] --> A
    A["Why Streaming Matters for AI Applicatio…"]
    A --> B
    B["Generator Basics for Token Streaming"]
    B --> C
    C["Async Generators for Real LLM Streaming"]
    C --> D
    D["Pipeline Composition with Generators"]
    D --> E
    E["Memory-Efficient Batch Processing"]
    E --> F
    F["Generator-Based Streaming in FastAPI"]
    F --> G
    G["FAQ"]
    G --> DONE["Key Takeaways"]
    style START fill:#4f46e5,stroke:#4338ca,color:#fff
    style DONE fill:#059669,stroke:#047857,color:#fff

from typing import Generator

def stream_tokens(text: str, chunk_size: int = 4) -> Generator[str, None, None]:
    """Simulate token streaming by yielding chunks of text."""
    for i in range(0, len(text), chunk_size):
        yield text[i:i + chunk_size]

# Lazy evaluation - only one chunk in memory at a time
for token in stream_tokens("The agent analyzed the document carefully."):
    print(token, end="", flush=True)

Async Generators for Real LLM Streaming

Real LLM APIs stream over HTTP using server-sent events. Async generators handle this naturally.

import httpx
import json
from typing import AsyncGenerator

async def stream_chat_completion(
    messages: list[dict],
    model: str = "gpt-4o",
    api_key: str = "",
) -> AsyncGenerator[str, None]:
    async with httpx.AsyncClient() as client:
        async with client.stream(
            "POST",
            "https://api.openai.com/v1/chat/completions",
            headers={"Authorization": f"Bearer {api_key}"},
            json={"model": model, "messages": messages, "stream": True},
            timeout=60.0,
        ) as response:
            async for line in response.aiter_lines():
                if line.startswith("data: ") and line != "data: [DONE]":
                    chunk = json.loads(line[6:])
                    delta = chunk["choices"][0].get("delta", {})
                    if content := delta.get("content"):
                        yield content

# Usage
async def main():
    messages = [{"role": "user", "content": "Explain agentic AI"}]
    full_response = []
    async for token in stream_chat_completion(messages, api_key="sk-..."):
        print(token, end="", flush=True)
        full_response.append(token)

    complete = "".join(full_response)

Pipeline Composition with Generators

Generators compose into processing pipelines where each stage transforms the stream without buffering everything.

from typing import AsyncGenerator

async def chunk_by_sentence(
    token_stream: AsyncGenerator[str, None]
) -> AsyncGenerator[str, None]:
    buffer = ""
    async for token in token_stream:
        buffer += token
        while ". " in buffer:
            sentence, buffer = buffer.split(". ", 1)
            yield sentence.strip() + "."

async def add_citations(
    sentence_stream: AsyncGenerator[str, None],
    knowledge_base: dict,
) -> AsyncGenerator[str, None]:
    async for sentence in sentence_stream:
        # Check if sentence needs a citation
        for keyword, citation in knowledge_base.items():
            if keyword.lower() in sentence.lower():
                sentence += f" [{citation}]"
                break
        yield sentence

# Compose the pipeline
async def enriched_stream(messages, api_key, kb):
    raw_tokens = stream_chat_completion(messages, api_key=api_key)
    sentences = chunk_by_sentence(raw_tokens)
    enriched = add_citations(sentences, kb)
    async for sentence in enriched:
        yield sentence

Memory-Efficient Batch Processing

When generating embeddings for thousands of documents, generators prevent loading everything into memory at once.

from typing import Generator
from pathlib import Path

def read_documents(directory: Path) -> Generator[str, None, None]:
    for file_path in directory.glob("*.txt"):
        yield file_path.read_text()

def batch_items(items: Generator, batch_size: int = 32) -> Generator[list, None, None]:
    batch = []
    for item in items:
        batch.append(item)
        if len(batch) == batch_size:
            yield batch
            batch = []
    if batch:
        yield batch

async def embed_directory(directory: Path):
    documents = read_documents(directory)
    for batch in batch_items(documents, batch_size=32):
        embeddings = await embedding_api.embed(batch)
        await vector_db.upsert(embeddings)
        # Only 32 documents in memory at any time

Generator-Based Streaming in FastAPI

FastAPI supports StreamingResponse with generators for real-time AI output.

from fastapi import FastAPI
from fastapi.responses import StreamingResponse

app = FastAPI()

@app.post("/chat/stream")
async def chat_stream(request: ChatRequest):
    async def generate():
        async for token in stream_chat_completion(
            request.messages, api_key=settings.api_key
        ):
            yield f"data: {json.dumps({'token': token})}\n\n"
        yield "data: [DONE]\n\n"

    return StreamingResponse(generate(), media_type="text/event-stream")

FAQ

What is the difference between a generator and an iterator in Python?

Every generator is an iterator, but not every iterator is a generator. An iterator is any object with __iter__ and __next__ methods. A generator is a function that uses yield and automatically creates an iterator. Generators are more concise and handle state management automatically.

How do I handle errors in a streaming pipeline without losing partial results?

Wrap the consumer loop in a try/except and process whatever has been yielded so far. You can also use generator.throw() to inject exceptions into a running generator from the outside, allowing it to handle errors and potentially recover or yield a fallback value.

Can generators cause memory leaks if not fully consumed?

Yes. If you break out of a generator loop early, the generator object stays in memory with its full stack frame until garbage collected. Call generator.close() explicitly or use it inside a with statement via contextlib.closing to ensure cleanup.

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