Partial prompt templates in LangChain - Step-by-Step Execution

Concept Flow - Partial prompt templates

Define base prompt with placeholders

↓

Create partial prompt with .partial()

↓

Fill some placeholders

↓

Use partial template to create full prompt

↓

Send full prompt to LLM

↓

Get response

Shows how to create a prompt template with placeholders, partially fill some, then complete and send to the language model.

Execution Sample

LangChain

from langchain.prompts import PromptTemplate

base_prompt = PromptTemplate(
    input_variables=["name", "task"],
    template="Hello {name}, please help me with {task}."
)

partial_prompt = base_prompt.partial(name="Alice")

full_prompt = partial_prompt.format(task="writing code")
print(full_prompt)

Defines a prompt with two variables, partially fills 'name' using .partial(), then completes with 'task' and prints the full prompt.

Execution Table

Step	Action	Input Variables	Partial Variables	Resulting Prompt
1	Define base prompt	name, task	none	"Hello {name}, please help me with {task}."
2	Call base_prompt.partial(name="Alice")	task	name=Alice	"Hello Alice, please help me with {task}."
3	Format partial prompt with task	task=writing code	name=Alice	"Hello Alice, please help me with writing code."
4	Print full prompt	none	none	Hello Alice, please help me with writing code.
5	End	none	none	Execution complete

💡 All placeholders filled, prompt fully formatted and printed.

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
base_prompt.template	"Hello {name}, please help me with {task}."	"Hello {name}, please help me with {task}."	"Hello {name}, please help me with {task}."	"Hello {name}, please help me with {task}."
partial_variables	{}	{"name": "Alice"}	{"name": "Alice"}	{"name": "Alice"}
full_prompt	N/A	N/A	"Hello Alice, please help me with writing code."	"Hello Alice, please help me with writing code."

Key Moments - 2 Insights

Why does the partial prompt still have {task} after creating PartialPromptTemplate?

Because only 'name' was filled as a partial variable at Step 2, 'task' remains as a placeholder until formatting at Step 3.

What happens if we try to format the partial prompt without providing 'task'?

An error occurs because 'task' is still required and not yet filled, as shown by the need to provide 'task' at Step 3.

Visual Quiz - 3 Questions

Look at the execution table at Step 2. What does the prompt look like?

A "Hello {name}, please help me with writing code."

B "Hello Alice, please help me with writing code."

C "Hello Alice, please help me with {task}."

D "Hello {name}, please help me with {task}."

💡 Hint

Check the 'Resulting Prompt' column at Step 2 in the execution table.

At which step is the full prompt finally printed?

A Step 2

B Step 4

C Step 3

D Step 5

💡 Hint

Look for the step where the prompt is printed in the 'Action' column.

If we changed partial_variables to include 'task' instead of 'name', what would happen at Step 3?

A The prompt would still have {name} placeholder.

B The prompt would be fully formatted with no placeholders.

C An error would occur because 'name' is missing.

D The prompt would be unchanged from base.

💡 Hint

Refer to variable_tracker and how partial variables fill placeholders.

Concept Snapshot

Partial Prompt Templates in Langchain:
- Define a PromptTemplate with placeholders.
- Create partial PromptTemplate using .partial() by filling some placeholders.
- Format partial prompt with remaining variables.
- Result is a fully formatted prompt ready for LLM.
- Helps reuse prompt parts and fill variables stepwise.

Full Transcript

This visual execution shows how to use Partial Prompt Templates in Langchain. First, a base prompt is defined with placeholders for 'name' and 'task'. Then, a partial prompt is created by calling base_prompt.partial(name="Alice"). The partial prompt still has the 'task' placeholder. Next, the partial prompt is formatted by providing the 'task' value 'writing code'. This produces the full prompt: 'Hello Alice, please help me with writing code.' Finally, the prompt is printed. The execution table tracks each step, showing how placeholders are filled progressively. The variable tracker shows how the template and variables change. Key moments clarify why placeholders remain until fully formatted. The quiz tests understanding of prompt states at each step. This method helps build prompts flexibly by filling variables in parts.

Practice

1. What is the main purpose of using PartialPromptTemplate in Langchain?

easy

A. To create reusable parts of prompts that can be filled later

B. To execute a prompt directly without variables

C. To store the final output of a prompt

D. To connect multiple language models together

Solution

1. Step 1: Understand the role of PartialPromptTemplate

   PartialPromptTemplate is designed to hold parts of a prompt with placeholders for variables.

2. Step 2: Recognize its use for reusability

   This allows you to reuse prompt pieces and fill variables later to form a complete prompt.

3. Final Answer:

   To create reusable parts of prompts that can be filled later -> Option A

4. Quick Check:

   PartialPromptTemplate = reusable prompt parts [OK]

Hint: Think reusable prompt pieces filled later [OK]

Common Mistakes:

• Confusing it with final prompt execution
• Thinking it stores output instead of template
• Assuming it connects models directly

2. Which of the following is the correct way to create a PartialPromptTemplate with a variable named name?

easy

A. PartialPromptTemplate(template="Hello {name}")

B. PartialPromptTemplate(template="Hello {name}", variables=["name"])

C. PartialPromptTemplate(template="Hello {name}", inputs=["name"])

D. PartialPromptTemplate(template="Hello {name}", input_variables=["name"])

Solution

1. Step 1: Check the required parameters for PartialPromptTemplate

   It requires a template string and a list named input_variables specifying variable names.

2. Step 2: Match the correct syntax

   PartialPromptTemplate(template="Hello {name}", input_variables=["name"]) correctly uses input_variables=["name"] to declare the variable.

3. Final Answer:

   PartialPromptTemplate(template="Hello {name}", input_variables=["name"]) -> Option D

4. Quick Check:

   Use input_variables list to declare variables [OK]

Hint: Remember input_variables param holds variable names [OK]

Common Mistakes:

• Using wrong parameter names like variables or inputs
• Omitting input_variables list
• Not matching variable names in template and list

3. Given the following code, what will be the output of full_prompt.format(name="Alice")?

from langchain.prompts import PartialPromptTemplate, PromptTemplate
partial = PartialPromptTemplate(template="Hello {name}", input_variables=["name"])
full_prompt = PromptTemplate(template="{greeting}, welcome!", input_variables=["greeting"])
full_prompt = full_prompt.partial(greeting=partial)

medium

A. "{greeting}, welcome!"

B. "Hello Alice, welcome!"

C. "Hello {name}, welcome!"

D. Error: missing variable 'name'

Solution

1. Step 1: Understand partial prompt substitution

   The partial prompt replaces the greeting variable in full_prompt with the partial template.

2. Step 2: Format the full prompt with name="Alice"

   Calling full_prompt.format(name="Alice") fills {name} in partial, producing "Hello Alice", then inserts it into full prompt, resulting in "Hello Alice, welcome!".

3. Final Answer:

   "Hello Alice, welcome!" -> Option B

4. Quick Check:

   Partial fills greeting, then full prompt formats [OK]

Hint: Partial fills variables inside main prompt [OK]

Common Mistakes:

• Expecting raw template string without substitution
• Confusing variable names and placeholders
• Missing that partial is nested inside full prompt

4. What is the error in the following code snippet?

partial = PartialPromptTemplate(template="Hi {user}", input_variables=["name"])

medium

A. Variable name in template and input_variables do not match

B. Missing import statement for PartialPromptTemplate

C. Template string must not contain variables

D. input_variables should be a string, not a list

Solution

1. Step 1: Compare template variables and input_variables list

   The template uses {user} but input_variables list contains "name".

2. Step 2: Identify mismatch causes error

   Variables must match exactly; mismatch causes runtime error when formatting.

3. Final Answer:

   Variable name in template and input_variables do not match -> Option A

4. Quick Check:

   Variable names must match in template and input_variables [OK]

Hint: Check variable names match exactly in template and list [OK]

Common Mistakes:

• Assuming variable names can differ
• Ignoring case sensitivity
• Thinking input_variables can be a string

5. You want to build a prompt that greets a user and mentions their favorite color using partial prompt templates. Which approach correctly combines two partial templates greet and color into a full prompt?

hard

A. Create two PartialPromptTemplates but combine by concatenating their templates as strings manually

B. Create one PartialPromptTemplate with all variables: PartialPromptTemplate(template="Hello {name}. Your favorite color is {color}.", input_variables=["name", "color"])

C. Create greet = PartialPromptTemplate(template="Hello {name}", input_variables=["name"]) and color = PartialPromptTemplate(template="Your favorite color is {color}", input_variables=["color"]), then combine with full = PromptTemplate(template="{greeting}. {color_info}.", input_variables=["greeting", "color_info"]) and use full.partial(greeting=greet, color_info=color)

D. Use PromptTemplate only with variables name and color without partial templates

Solution

1. Step 1: Define two partial templates for greeting and color

   Each partial holds a reusable piece with its own variables.

2. Step 2: Combine partials into a full prompt using placeholders

   The full prompt uses placeholders for each partial, then partial() method fills them with partial templates.

3. Step 3: This approach keeps prompts modular and variables scoped

   It allows filling variables later and keeps code organized.

4. Final Answer:

   Create greet = PartialPromptTemplate(template="Hello {name}", input_variables=["name"]) and color = PartialPromptTemplate(template="Your favorite color is {color}", input_variables=["color"]), then combine with full = PromptTemplate(template="{greeting}. {color_info}.", input_variables=["greeting", "color_info"]) and use full.partial(greeting=greet, color_info=color) -> Option C

5. Quick Check:

   Combine partials via placeholders and partial() method [OK]

Hint: Use partial() to nest partial templates inside full prompt [OK]

Common Mistakes:

• Trying to concatenate templates as strings manually
• Using one partial for all variables losing modularity
• Ignoring partial() method for combining templates