What is LangChain - Visual Explanation

Concept Flow - What is LangChain

User Input Text

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LangChain Processes Input

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Calls Language Model

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Processes Model Output

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Returns Final Answer

LangChain takes user text, sends it to a language model, processes the response, and returns an answer.

Execution Sample

LangChain

from langchain import LLMChain, PromptTemplate

prompt = PromptTemplate(input_variables=["name"], template="Hello {name}!")
chain = LLMChain(llm=llm, prompt=prompt)
result = chain.run("Alice")

This code creates a simple LangChain that greets a user by name.

Execution Table

Step	Action	Input	Output	Notes
1	Create PromptTemplate	name='Alice'	Template with variable {name}	Prepare prompt with placeholder
2	Create LLMChain	llm, prompt	Chain object ready	Chain links prompt and model
3	Run chain	Input: 'Alice'	Prompt: 'Hello Alice!'	Input fills prompt variable
4	Call LLM	Prompt: 'Hello Alice!'	Model generates response	Model processes prompt
5	Process output	Model response	Final result string	Chain returns answer

💡 Chain run completes after processing model output and returning final result.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	Final
prompt	None	PromptTemplate object	PromptTemplate object	PromptTemplate object	PromptTemplate object	PromptTemplate object
chain	None	None	LLMChain object	LLMChain object	LLMChain object	LLMChain object
result	None	None	None	None	Model response string	Final result string

Key Moments - 2 Insights

How does LangChain use the prompt template with variables?

LangChain replaces variables like {name} in the prompt template with actual input values during the run step, as shown in execution_table step 3.

What happens between running the chain and getting the final result?

After running the chain, LangChain sends the filled prompt to the language model, gets the model's response, then processes and returns it, as seen in steps 4 and 5.

Visual Quiz - 3 Questions

Look at the execution table, what is the output at step 3?

A Prompt: 'Hello Alice!'

B Model generates response

C Final result string

D Chain object ready

💡 Hint

Check the 'Output' column for step 3 in the execution_table.

At which step does LangChain call the language model?

A Step 2

B Step 4

C Step 3

D Step 5

💡 Hint

Look for the step mentioning 'Call LLM' in the execution_table.

If the input name changes to 'Bob', which part of the execution_table changes?

A Step 1: Create PromptTemplate

B Step 4: Call LLM

C Step 3: Run chain input and prompt

D Step 5: Process output

💡 Hint

Focus on where the input variable is inserted into the prompt in the execution_table.

Concept Snapshot

LangChain connects your text input to a language model using prompts.
You create a PromptTemplate with variables.
LLMChain links the prompt and model.
Run the chain with input to get model output.
It helps build apps using language models easily.

Full Transcript

LangChain is a tool that helps you talk to language models by using prompts with variables. You first make a prompt template that has placeholders like {name}. Then you create a chain that connects this prompt with a language model. When you run the chain with your input, LangChain fills the prompt with your input, sends it to the model, and returns the model's answer. This process makes it easy to build applications that use language models.