Comma-Separated Output Parser

What it is A Comma-Separated Output Parser takes the text output from a language model and splits it into a list of items using commas as separators. It converts free-form text like "apple, banana, cherry" into ["apple", "banana", "cherry"].

Why it exists Many LLM outputs are plain text, but your code often needs structured data. A comma-separated parser provides a simple way to turn a list-like response into a Python list without complex processing.

Real-world analogy It’s like taking a grocery list written in one line—“milk, eggs, bread”—and turning it into separate items in a shopping app.

Minimal beginner example

import os
from dotenv import load_dotenv
from langchain_google_genai import ChatGoogleGenerativeAI
from langchain_core.output_parsers import CommaSeparatedListOutputParser
from langchain_core.prompts import PromptTemplate

load_dotenv()
api_key = os.getenv("GEMINI_API_KEY")

llm = ChatGoogleGenerativeAI(model="gemini-2.5-flash", api_key=api_key)

# Step 1: Prepare parser
parser = CommaSeparatedListOutputParser()

# Step 2: Prepare prompt
prompt = PromptTemplate(
    template="List 5 popular fruits, separated by commas.",
    input_variables=[]
)

# Step 3: Run LLM and parse output
response = llm.invoke(prompt.format())
parsed_output = parser.parse(response.content)

print(parsed_output)  # Example output: ['apple', 'banana', 'cherry', 'mango', 'grape']

Small LangChain workflow

1. Prepare a prompt asking for comma-separated items.

2. Send it to the LLM.

3. Parse the output with CommaSeparatedListOutputParser.

4. Use the list in chains, loops, or databases.

Common beginner mistakes

• LLM output contains extra text, numbers, or punctuation → parser may fail.

• Forgetting to trim whitespace from items (some parsers may leave extra spaces).

• Using this parser for complex nested data—only suitable for simple lists.

When to use this vs alternatives

• Use Comma-Separated Parser for simple flat lists.

• Use Pydantic Parser for structured objects with fields and types.

• Use Regex Parser if your output has custom patterns instead of simple commas.