Skip to content

Usage pattern

Usage Pattern#

Defining a custom prompt#

Defining a custom prompt is as simple as creating a format string

from llama_index.core import PromptTemplate

template = (
    "We have provided context information below. \n"
    "Given this information, please answer the question: {query_str}\n"
qa_template = PromptTemplate(template)

# you can create text prompt (for completion API)
prompt = qa_template.format(context_str=..., query_str=...)

# or easily convert to message prompts (for chat API)
messages = qa_template.format_messages(context_str=..., query_str=...)

Note: you may see references to legacy prompt subclasses such as QuestionAnswerPrompt, RefinePrompt. These have been deprecated (and now are type aliases of PromptTemplate). Now you can directly specify PromptTemplate(template) to construct custom prompts. But you still have to make sure the template string contains the expected parameters (e.g. {context_str} and {query_str}) when replacing a default question answer prompt.

You can also define a template from chat messages

from llama_index.core import ChatPromptTemplate
from llama_index.core.llms import ChatMessage, MessageRole

message_templates = [
    ChatMessage(content="You are an expert system.", role=MessageRole.SYSTEM),
        content="Generate a short story about {topic}",
chat_template = ChatPromptTemplate(message_templates=message_templates)

# you can create message prompts (for chat API)
messages = chat_template.format_messages(topic=...)

# or easily convert to text prompt (for completion API)
prompt = chat_template.format(topic=...)

Getting and Setting Custom Prompts#

Since LlamaIndex is a multi-step pipeline, it's important to identify the operation that you want to modify and pass in the custom prompt at the right place.

For instance, prompts are used in response synthesizer, retrievers, index construction, etc; some of these modules are nested in other modules (synthesizer is nested in query engine).

See this guide for full details on accessing/customizing prompts.

Commonly Used Prompts#

The most commonly used prompts will be the text_qa_template and the refine_template.

  • text_qa_template - used to get an initial answer to a query using retrieved nodes
  • refine_template - used when the retrieved text does not fit into a single LLM call with response_mode="compact" (the default), or when more than one node is retrieved using response_mode="refine". The answer from the first query is inserted as an existing_answer, and the LLM must update or repeat the existing answer based on the new context.

Accessing Prompts#

You can call get_prompts on many modules in LlamaIndex to get a flat list of prompts used within the module and nested submodules.

For instance, take a look at the following snippet.

query_engine = index.as_query_engine(response_mode="compact")
prompts_dict = query_engine.get_prompts()

You might get back the following keys:

['response_synthesizer:text_qa_template', 'response_synthesizer:refine_template']

Note that prompts are prefixed by their sub-modules as "namespaces".

Updating Prompts#

You can customize prompts on any module that implements get_prompts with the update_prompts function. Just pass in argument values with the keys equal to the keys you see in the prompt dictionary obtained through get_prompts.

e.g. regarding the example above, we might do the following

# shakespeare!
qa_prompt_tmpl_str = (
    "Context information is below.\n"
    "Given the context information and not prior knowledge, "
    "answer the query in the style of a Shakespeare play.\n"
    "Query: {query_str}\n"
    "Answer: "
qa_prompt_tmpl = PromptTemplate(qa_prompt_tmpl_str)

    {"response_synthesizer:text_qa_template": qa_prompt_tmpl}

Modify prompts used in query engine#

For query engines, you can also pass in custom prompts directly during query-time (i.e. for executing a query against an index and synthesizing the final response).

There are also two equivalent ways to override the prompts:

  1. via the high-level API
query_engine = index.as_query_engine(
    text_qa_template=custom_qa_prompt, refine_template=custom_refine_prompt
  1. via the low-level composition API
retriever = index.as_retriever()
synth = get_response_synthesizer(
    text_qa_template=custom_qa_prompt, refine_template=custom_refine_prompt
query_engine = RetrieverQueryEngine(retriever, response_synthesizer)

The two approaches above are equivalent, where 1 is essentially syntactic sugar for 2 and hides away the underlying complexity. You might want to use 1 to quickly modify some common parameters, and use 2 to have more granular control.

For more details on which classes use which prompts, please visit Query class references.

Check out the reference documentation for a full set of all prompts.

Modify prompts used in index construction#

Some indices use different types of prompts during construction (NOTE: the most common ones, VectorStoreIndex and SummaryIndex, don't use any).

For instance, TreeIndex uses a summary prompt to hierarchically summarize the nodes, and KeywordTableIndex uses a keyword extract prompt to extract keywords.

There are two equivalent ways to override the prompts:

  1. via the default nodes constructor
index = TreeIndex(nodes, summary_template=custom_prompt)
  1. via the documents constructor.
index = TreeIndex.from_documents(docs, summary_template=custom_prompt)

For more details on which index uses which prompts, please visit Index class references.

[Advanced] Advanced Prompt Capabilities#

In this section we show some advanced prompt capabilities in LlamaIndex.

Related Guides:

Partial Formatting#

Partially format a prompt, filling in some variables while leaving others to be filled in later.

from llama_index.core import PromptTemplate

prompt_tmpl_str = "{foo} {bar}"
prompt_tmpl = PromptTemplate(prompt_tmpl_str)
partial_prompt_tmpl = prompt_tmpl.partial_format(foo="abc")

fmt_str = partial_prompt_tmpl.format(bar="def")

Template Variable Mappings#

LlamaIndex prompt abstractions generally expect certain keys. E.g. our text_qa_prompt expects context_str for context and query_str for the user query.

But if you're trying to adapt a string template for use with LlamaIndex, it can be annoying to change out the template variables.

Instead, define template_var_mappings:

template_var_mappings = {"context_str": "my_context", "query_str": "my_query"}

prompt_tmpl = PromptTemplate(
    qa_prompt_tmpl_str, template_var_mappings=template_var_mappings

Function Mappings#

Pass in functions as template variables instead of fixed values.

This is quite advanced and powerful; allows you to do dynamic few-shot prompting, etc.

Here's an example of reformatting the context_str.

def format_context_fn(**kwargs):
    # format context with bullet points
    context_list = kwargs["context_str"].split("\n\n")
    fmtted_context = "\n\n".join([f"- {c}" for c in context_list])
    return fmtted_context

prompt_tmpl = PromptTemplate(
    qa_prompt_tmpl_str, function_mappings={"context_str": format_context_fn}

prompt_tmpl.format(context_str="context", query_str="query")