PromptTemplate basics in LangChain - Step-by-Step Execution

Concept Flow - PromptTemplate basics

Define template string with placeholders

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Create PromptTemplate object

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Provide values for placeholders

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Call format() method

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Get final prompt string with values inserted

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Use prompt string in language model or output

This flow shows how you define a template with placeholders, create a PromptTemplate, fill in values, and get the final prompt string.

Execution Sample

LangChain

from langchain import PromptTemplate

template = "Hello, {name}! Today is {day}."
prompt = PromptTemplate(template=template, input_variables=["name", "day"])
final_prompt = prompt.format(name="Alice", day="Monday")
print(final_prompt)

This code creates a prompt template with placeholders for name and day, then fills them with values and prints the final prompt.

Execution Table

Step	Action	Input	Output	Notes
1	Define template string	"Hello, {name}! Today is {day}."	Template string stored	Placeholders {name} and {day} noted
2	Create PromptTemplate object	template string, input_variables=["name", "day"]	PromptTemplate instance created	Ready to format with values
3	Call format() with values	name="Alice", day="Monday"	Formatted string generated	Placeholders replaced with values
4	Print final prompt	Formatted string	Hello, Alice! Today is Monday.	Final prompt ready for use
5	End	-	-	Process complete

💡 All placeholders replaced, final prompt string generated and printed

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
template	undefined	"Hello, {name}! Today is {day}."	"Hello, {name}! Today is {day}."	"Hello, {name}! Today is {day}."	"Hello, {name}! Today is {day}."
prompt	undefined	undefined	PromptTemplate object	PromptTemplate object	PromptTemplate object
final_prompt	undefined	undefined	undefined	"Hello, Alice! Today is Monday."	"Hello, Alice! Today is Monday."

Key Moments - 3 Insights

Why do we need to specify input_variables when creating PromptTemplate?

Input_variables tell PromptTemplate which placeholders to expect. Without them, format() won't know what values to replace. See execution_table step 2 where input_variables are set.

What happens if we call format() without providing all required variables?

Format() will raise an error because it can't replace all placeholders. In execution_table step 3, all variables are provided to avoid this error.

Can we reuse the same PromptTemplate object with different values?

Yes, you can call format() multiple times with different inputs to get different prompt strings. The prompt object stays the same as shown in variable_tracker.

Visual Quiz - 3 Questions

Look at the execution_table at step 3. What is the output after calling format()?

A "Hello, Alice! Today is Monday."

B "Hello, {name}! Today is {day}."

C An error because variables are missing

D "Hello, Bob! Today is Tuesday."

💡 Hint

Check the Output column in execution_table row with Step 3

According to variable_tracker, what is the value of 'final_prompt' after step 2?

A PromptTemplate object

B "Hello, Alice! Today is Monday."

C undefined

D "Hello, {name}! Today is {day}."

💡 Hint

Look at the 'final_prompt' row and 'After Step 2' column in variable_tracker

If you forget to include 'day' in input_variables when creating PromptTemplate, what will happen when calling format()?

A Format will succeed and ignore missing placeholders

B Format will raise an error due to missing variable

C PromptTemplate will automatically add missing variables

D The output will be empty string

💡 Hint

Refer to key_moments about input_variables and format() behavior

Concept Snapshot

PromptTemplate basics:
- Define a template string with placeholders like {name}
- Create PromptTemplate(template, input_variables=[...])
- Call format() with values to replace placeholders
- Result is a final prompt string
- Useful for dynamic prompt creation in LangChain

Full Transcript

PromptTemplate basics in LangChain involve creating a template string with placeholders such as {name} and {day}. You then create a PromptTemplate object by passing the template string and a list of input variables that correspond to the placeholders. When you want to generate the final prompt, you call the format() method on the PromptTemplate object, providing values for each placeholder. The format() method returns the prompt string with all placeholders replaced by the provided values. This process allows you to reuse the same template with different inputs easily. It is important to specify all input variables when creating the PromptTemplate to avoid errors during formatting.