Build an LLM Web App in Python from Scratch: Part 1 (Local CLI)

Ever thought about sprinkling some AI magic into your web app? This is Part 1 of our journey from a basic command-line tool to a full-blown AI web app. Today, we're building a "Human-in-the-Loop" (HITL) chatbot. Think of it as an AI that politely asks for your "OK" before it says anything. We'll use a tiny, super-simple tool called PocketFlow – it's just 100 lines of code!

So, You Want to Add AI to Your Web App?

Adding AI (especially those smart Large Language Models or LLMs) to web apps can make them super powerful. Imagine smart chatbots, instant content creation, or even coding help. But hold on, it's not just "plug and play." You'll bump into questions like:

• Where does the AI brain actually live? In the user's browser? On your server?

• How do you handle AI tasks that need multiple steps?

• How can users tell the AI what to do or give feedback?

• What's the deal with remembering conversation history?

• Most importantly: How do you stop the AI from saying something totally weird or wrong?

This tutorial series is your guide to building LLM web apps in Python, and we'll tackle these questions head-on! Here's our 4-part adventure:

• Part 1 (That's Today!): We'll build a basic Human-in-the-Loop (HITL) chatbot that runs in your computer's command line. We'll use PocketFlow to nail down the core logic, no fancy UI to distract us.

• Part 2: Let's get this HITL bot on the web! We'll build a user interface using Streamlit (or maybe Flask), and learn how to manage user interactions smoothly.

• Part 3: Time for real-time action! We'll upgrade our web app with WebSockets (using FastAPI) for instant messages and a slicker chat feel.

• Part 4: What about AI tasks that take a while? We'll set up background processing and use Server-Sent Events (SSE) with FastAPI. This lets us show users live progress updates, making our app feel truly pro.

To make this journey smooth, we'll use PocketFlow. It's not just for simple AI calls; PocketFlow helps you build complex AI workflows that you can actually control. And because it's so small (seriously, just 100 lines for the core framework), you'll always know what's going on. No black boxes!

The secret sauce to control and quality? A pattern called Human-in-the-Loop (HITL). With PocketFlow, HITL is easy. It means your AI is like a helpful assistant: it drafts stuff, but you (the human co-pilot) get the final say. Approve, edit, or reject – you're in control before anything goes live. Total quality control, phew!

You can try out the code for the command-line HITL bot discussed in this part at: PocketFlow Command-Line HITL Example.

Your First HITL App: The "Are You Sure?" Bot

Let's build the most basic HITL app: a chatbot where you approve every single AI response. Imagine it's an AI trying to tell jokes. AI can be funny, but sometimes its jokes might be... well, not funny, or maybe even a little weird or inappropriate. That's where you come in!

The Basic Idea (It's Super Simple!)

Think of a tiny script. The AI suggests a joke, and you, the human, give it a thumbs-up or thumbs-down before it's told. That's HITL in action! If the AI suggests, "Why did the chicken cross the playground? To get to the other slide!" and you think it's a winner, you say "yes!" If it suggests something that makes no sense, or isn't right for your audience, you say "nope!"

user_topic_request = "Tell me a joke about cats."
ai_suggested_joke = call_llm_for_joke(user_topic_request)

# You get to approve!
approval = input(f"AI suggests: '{ai_suggested_joke}'. Tell this joke? (y/n): ")

if approval.lower() == 'y':
    print(f"To Audience: {ai_suggested_joke}")
else:
    print("Human said: Nope! That joke isn't hitting the mark.")

That's the core! AI suggests a joke, human (input()) approves, then it's (maybe) told. This simple check is crucial for joke quality and appropriateness. Everything else we build is about making this core idea robust for more complex apps. Visually, it's a little loop, especially for our joke bot:

Let's See It Go!

You: Tell me a programming joke.
AI suggests: 'Why do programmers prefer dark mode? Because light attracts bugs!'
Approve? (y/n): y
To Audience: Why do programmers prefer dark mode? Because light attracts bugs!

You: Tell me a joke about vegetables.
AI suggests: 'Why did the tomato turn red? Because it saw the salad dressing! But also, all vegetables are boring.'
Approve? (y/n): n
Human said: Nope! That joke isn't hitting the mark. (And the comment about vegetables is a bit much!)
Regenerating... (Imagine we have logic for this)
AI suggests: 'What do you call a sad strawberry? A blueberry!'
Approve? (y/n): y
To Audience: What do you call a sad strawberry? A blueberry!

See? The human caught that slightly off-kilter joke and the unnecessary comment. That's HITL making sure the AI comedian doesn't bomb too hard!

Why Just Plain Python Gets Tangled Fast

When you try to build something more than a quick demo with plain Python while loops and if statements, things can turn into a bowl of spaghetti code REAL quick. The main headaches are:

1. 🍝 Spaghetti Code: Add features like conversation history, letting users edit, or trying different AI prompts, and your simple loop becomes a monster of nested if/else blocks. It's tough to read and a nightmare to fix.

2. 🧱 Not Very Mix-and-Match: Your logic for getting input, calling the AI, getting approval, and sending the response all gets jumbled together. Want to test just one piece? Or reuse your "AI calling" bit in another app? Good luck untangling that!

3. 🕸️ Hard to Change or Grow: Want to add a new step, like checking for bad words before the human sees it? Or offer three ways to react instead of just "yes/no"? In plain Python, these changes mean carefully rewiring everything, and you'll probably break something.

These problems make it super hard to build AI workflows that are robust and ready for real users.

Enter PocketFlow - Think of it as LEGO blocks for your AI workflows.

PocketFlow: HITL Workflows, Super Simple!

Trying to build complex AI steps with plain Python is like building a giant LEGO castle without instructions – you'll end up with a wobbly mess. PocketFlow is your friendly instruction manual and a set of perfectly fitting LEGO bricks. It helps you build AI workflows in just 100 lines of actual framework code!

Imagine PocketFlow as running a little workshop:

• You have Nodes: These are your specialist workers, each good at one job.

• You have a Shared Store: This is like a central whiteboard where everyone shares notes.

• You have a Flow: This is the manager who tells the workers what to do and in what order.

The Node Pattern: Your Specialist Workers

In PocketFlow, each main task is a Node. A Node is like a specialist worker who's a pro at one specific thing.

Here's what a Node looks like in PocketFlow:

class Node:
    def prep(self, shared): pass
    def exec(self, prep_res): pass
    def post(self, shared, prep_res, exec_res): pass
    def run(self,shared): p=self.prep(shared); e=self.exec(p); return self.post(shared,p,e)

Don't worry if __init__ or self look weird; they're just Python things! The important bit is the prep -> exec -> post cycle:

1. prep(shared): "Hey, I'm about to start. What info do I need from the shared whiteboard?"

2. exec(data_from_prep): "Okay, I have my info. Now I'll do my main job!" (Like calling an AI).

3. post(shared, prep_res, exec_res): "Job's done! I'll write my results back to the shared whiteboard and tell the manager (the Flow) what happened by returning a simple signal (like a keyword, e.g., "done" or "needs_approval")."

The Shared Store: The Central Whiteboard (Just a Python Dictionary!)

This is just a plain old Python dictionary (let's call it shared_store). All our Node workers can read from it and write to it. It's how they pass info—like the user's question, the AI's draft answer, or conversation history—to each other.

For a math problem, it might start like this:

shared_store = {
    "number_to_process": 0,
    "intermediate_result": None,
    "final_answer": None
}

As Nodes do their work, they'll update this shared_store.

The Flow: The Workshop Manager

A Flow object is like the manager of your workshop. You tell it which Node kicks things off. When you run a Flow:

1. It runs that first Node.

2. The Node finishes and returns a signal (just a text string, like "user_approves" or "try_again").

3. The Flow checks a little map on that Node (called successors) that says: "If the signal is "user_approves", go to the SendResponseNode next. If it's "try_again", go back to the GenerateAINode."

You build this map with simple connections:

• node_a >> node_b: This is a shortcut. If node_a finishes and gives the usual "all good" signal (PocketFlow calls this "default"), then node_b runs next.

• node_a - "custom_signal" >> node_c: This means if node_a finishes and shouts "custom_signal", then node_c is up.

The Flow keeps this going: run a Node, get its signal, find the next Node. If it gets a signal and can't find a next step, the flow for that path just ends. Easy!
This lets you make workflows that branch off in different directions based on what happens. Like a choose-your-own-adventure for your AI!

Here's how tiny the Flow manager class actually is in PocketFlow:

class Flow:
    def __init__(self, start_node): self.start_node = start_node
    def run(self, shared_store):
        current_node = self.start_node
        while current_node:
            signal = current_node.run(shared_store)
            current_node = current_node.successors.get(signal)

That's it! It just needs a start_node and then it keeps running nodes and following their signals until there's no next step defined for a signal.

Tiny Math Example: PocketFlow in Action!

Let's build a super-tiny workflow: take a number, add 5, then multiply by 2.

Worker 1: The Adder Node
This Node's job is to add 5.

class AddFive(Node):
    def prep(self, shared):
        return shared.get("number_to_process", 0)

    def exec(self, current_number):
        return current_number + 5

    def post(self, shared, prep_res, addition_result):
        shared["intermediate_result"] = addition_result
        print(f"AddFive Node: Added 5, result is {addition_result}")
        return "default" # Signal "all good, continue"

1. prep: Grabs "number_to_process" from shared_store.

2. exec: Adds 5.

3. post: Saves the new number as "intermediate_result" and says "default" (meaning "continue to the next step in line").

Worker 2: The Multiplier Node
This Node's job is to multiply by 2.

class MultiplyByTwo(Node):
    def prep(self, shared):
        return shared["intermediate_result"]

    def exec(self, current_number):
        return current_number * 2

    def post(self, shared, prep_res, multiplication_result):
        shared["final_answer"] = multiplication_result
        print(f"MultiplyByTwo Node: Multiplied, final answer is {multiplication_result}")
        return "done" # Signal "all finished with this path"

1. prep: Grabs "intermediate_result".

2. exec: Multiplies by 2.

3. post: Saves it as "final_answer" and says "done".

Connecting the Workers (The Assembly Line):
Now, let's tell PocketFlow how these Nodes connect.

# First, make our specialist worker Nodes
adder_node = AddFive()
multiplier_node = MultiplyByTwo()

adder_node >> multiplier_node

# Create the Flow manager
math_flow = Flow(start_node=adder_node)

# Let's get some data for them to work on
shared_store_for_math = {"number_to_process": 10}
print(f"\nStarting math game with: {shared_store_for_math}")

# Run the flow!
math_flow.run(shared_store_for_math)

print(f"Math game finished. Whiteboard looks like: {shared_store_for_math}")

And if you run this, you'd see:

Starting math game with: {'number_to_process': 10}
AddFive Node: Added 5, result is 15
MultiplyByTwo Node: Multiplied, final answer is 30
Math game finished. Whiteboard looks like: {'number_to_process': 10, 'intermediate_result': 15, 'final_answer': 30}

See? Each Node (worker) is simple. The shared_store (whiteboard) carries the number through. The Flow (manager) made sure AddFive ran, then MultiplyByTwo, because of the "default" signal. If MultiplyByTwo had other signals for different outcomes, the flow could branch off!

Now we know how PocketFlow uses Nodes, a Shared Store, and a Flow to handle steps and pass data. Let's use these exact same ideas for our HITL approval chatbot!

Building Your HITL "Approval Bot" Workflow

Alright, enough theory! Let's get our hands dirty and build that HITL workflow with PocketFlow for our joke bot. This time, we'll start by thinking about what information our Nodes will need to share.

First, Design the Shared Store (Our Whiteboard)

For our interactive joke generator, the shared_store dictionary (our central whiteboard) needs to keep track of a few key things as the conversation flows:

• topic: What kind_of_joke the user wants (e.g., "cats", "programming"). This will be filled in by our first Node.

• current_joke: The latest joke the AI cooked up. This will be updated by the joke generation Node.

• disliked_jokes: A list of jokes about the current topic that the user already said "no" to. This helps the AI avoid telling the same bad joke twice. It will be updated by our feedback Node.

• user_feedback: The user's latest decision (e.g., "approve" or "disapprove"). Also updated by the feedback Node.

Here's a peek at what it might look like while the bot is running:

shared_store = {
    "topic": "dogs",
    "current_joke": "What do you call a dog magician? A labracadabrador!",
    "disliked_jokes": ["Why did the dog cross the road? To get to the barking lot!"],
    "user_feedback": None
}

Each Node we design will read the information it needs from this shared_store and write its results back here for other Nodes to use. This way, everyone's on the same page!

The Three Core Nodes: Our Specialist Joke Crafters

We'll use three main Nodes for our joke-making machine:

1. GetTopicNode: Asks the user what they want a joke about.

2. GenerateJokeNode: Cooks up a joke using the AI.

3. GetFeedbackNode: Asks the user if the joke was a hit or a miss.

Let's build them one by one!

1. GetTopicNode - The Idea Catcher 🎤

This Node's only job is to ask the user for a joke topic.

class GetTopicNode(Node):
    def prep(self, shared):
        shared["topic"] = None
        shared["current_joke"] = None
        shared["disliked_jokes"] = [] 
        shared["user_feedback"] = None
        return None

First, the prep method. Think of this as setting the table before a meal. It just cleans up any old information from a previous joke in our shared whiteboard so we start fresh for the new topic.

    def exec(self, _prep_res): 
        return input("What topic shall I jest about today? ")

Next, exec does the main work: it simply asks the user for a topic using input() and returns whatever they type.

    def post(self, shared, _prep_res, topic_input):
        shared["topic"] = topic_input
        print(f"Alright, a joke about '{topic_input}'! Coming right up...")
        return "generate_joke"

Finally, post takes the topic_input from exec, saves it to our shared whiteboard under the key "topic", prints a little message, and then returns the signal "generate_joke". This signal tells the Flow manager, "Okay, we have a topic, now go to the node that generates jokes!"

2. GenerateJokeNode - The AI Comedy Chef 🤖

This Node grabs the topic (and any jokes the user didn't like about it) and tells the AI to whip up a new joke.

class GenerateJokeNode(Node):
    def prep(self, shared):
        topic = shared.get("topic", "something random")
        disliked = shared.get("disliked_jokes", [])

        prompt = f"Tell me a short, funny joke about: {topic}."
        if disliked:
            avoid_these = "; ".join(disliked)
            prompt += f" Please try to make it different from these: [{avoid_these}]."
        return prompt

In prep, this Node looks at the shared whiteboard for the "topic" and any "disliked_jokes". It then crafts a prompt (a set of instructions) for our AI. If there are disliked jokes, it cleverly tells the AI, "Hey, avoid jokes like these ones the user didn't like!"

    def exec(self, joke_prompt):
        return call_llm(joke_prompt)

Then, exec is where the AI magic would happen. We take the joke_prompt from prep and send it to our call_llm function. This function would be responsible for talking to a real AI service (like OpenAI, Anthropic, etc.) and returning its response.

    def post(self, shared, _prep_res, ai_joke):
        shared["current_joke"] = ai_joke
        print(f"🤖 AI Suggests: {ai_joke}")
        return "get_feedback"

And in post, we save the ai_joke to our shared whiteboard as "current_joke", print it out for the user to see, and then return the signal "get_feedback". This tells the Flow manager, "Joke's ready! Go to the node that gets the user's opinion."

3. GetFeedbackNode - The Joke Judge 🤔

This Node shows the joke and asks the user: thumbs up or thumbs down? Based on their answer, it decides if we should try another joke on the same topic or if we're done.

class GetFeedbackNode(Node):
    def prep(self, shared):
        return None

prep is super simple here. GenerateJokeNode already showed the joke, so there's nothing to set up. We just pass None.

    def exec(self, _prep_res):
        while True: 
            decision = input("Did you like this joke? (yes/no): ").strip().lower()
            if decision in ["yes", "y", "no", "n"]:
                return decision 
            print("Hmm, I didn't catch that. Please type 'yes' or 'no'.")

exec asks the user if they liked the joke. It waits for a clear "yes" (or "y") or "no" (or "n") before moving on.

    def post(self, shared, _prep_res, user_decision):
        if user_decision in ["yes", "y"]:
            print("🎉 Hooray! Glad you liked it!")
            shared["user_feedback"] = "approve"
            return "joke_approved"
        else: 
            print("😅 Oops! My circuits must be crossed. Let me try again...")
            shared["user_feedback"] = "disapprove"
            current_joke = shared.get("current_joke")
            if current_joke: 
                shared.setdefault("disliked_jokes", []).append(current_joke)
            return "regenerate_joke"

Finally, post looks at the user_decision:

• If it's a "yes", we celebrate, store "approve" in shared["user_feedback"], and return the signal "joke_approved". This means we're done with this joke topic.

• If it's a "no", we apologize, store "disapprove", add the failed current_joke to our shared["disliked_jokes"] list (so GenerateJokeNode knows not to repeat it), and return the signal "regenerate_joke". This tells the Flow manager: "Back to the joke drawing board (the GenerateJokeNode) for the same topic!"

Connecting the Flow: Drawing the Joke Map & Running It! 🗺️

Now we tell PocketFlow how to get from one Node to another using the signals we just defined, and then we'll run it.

# 1. Make our specialist Node workers
get_topic_node = GetTopicNode()
generate_joke_node = GenerateJokeNode()
get_feedback_node = GetFeedbackNode()

# 2. Draw the paths for the Flow manager
get_topic_node - "generate_joke" >> generate_joke_node
generate_joke_node - "get_feedback" >> get_feedback_node
get_feedback_node - "regenerate_joke" >> generate_joke_node

# 3. Let's Run Our HITL Joke Bot!
shared = {
    "topic": None, "current_joke": None,
    "disliked_jokes": [], "user_feedback": None
}
hitl_joke_flow = Flow(start_node=get_topic_node)
hitl_joke_flow.run(shared)

print(f"\n🎤 Joke session over! Here's what happened: {shared}")

This is like drawing a map for our Flow manager and then telling it to start the journey:

1. We create our three Node workers.

2. We use the node - "signal" >> next_node pattern to define the paths.

3. We set up our shared whiteboard.

4. We create our Flow manager (using the Flow class we saw earlier), telling it to kick things off with get_topic_node.

5. We call hitl_joke_flow.run(shared). The Flow manager now takes over! It runs the Nodes, listens for their signals, and follows the map. The shared dictionary gets updated live.

6. When the flow naturally ends (because "joke_approved" has no next step), the run method finishes, and we print out the final state of our whiteboard.

And that's it! You've built a Human-in-the-Loop chatbot using PocketFlow. Each piece is small, understandable, and they all work together to create a flexible workflow where the human is always in control.

From CLI to Web App: What's Next & Key Takeaways

You've built a cool command-line bot where you're the boss! But most people don't hang out in command prompts, right? They use web apps! The great news is that the HITL logic you've built with PocketFlow is the engine that can power a web UI too.

The Journey Ahead:

• Part 2 (Web UI): We'd take this exact HITL flow and hook it up to something like Streamlit or Flask. Instead of input(), users would click buttons (st.button("👍 Approve")). The underlying PocketFlow logic remains largely the same!

• Part 3 (Real-Time): Time for real-time action! We'll upgrade our web app with WebSockets (using FastAPI) for instant messages and a slicker chat feel. The core HITL decision points (AI drafts, human approves) are still there.

• Part 4 (Background Tasks & Progress Updates): What about AI tasks that take a while? We'll set up background processing and use Server-Sent Events (SSE) with FastAPI. This lets us show users live progress updates, making our app feel truly pro. Your PocketFlow Flow is a key part of each user's session.

In Part 2, we'll take our HITL bot to the web using Streamlit! This means building a proper user interface where users can interact with buttons and see responses. We'll even explore how such a UI could handle more than just text, like displaying images, making our AI interactions richer.

Want to try out this exact command-line HITL bot yourself? You can find the complete, runnable code in the PocketFlow cookbook here: PocketFlow Command-Line HITL Example. Go ahead, build your own HITL apps!