app.py

import streamlit as st
from transformers import AutoTokenizer, AutoModelForSequenceClassification
import shap
import torch
import numpy as np
import matplotlib.pyplot as plt
import pandas as pd

# Load model and tokenizer with caching
@st.cache_resource
def load_model():
    tokenizer = AutoTokenizer.from_pretrained("nlptown/bert-base-multilingual-uncased-sentiment")
    model = AutoModelForSequenceClassification.from_pretrained("nlptown/bert-base-multilingual-uncased-sentiment")
    return tokenizer, model

tokenizer, model = load_model()

# Define prediction function
def predict(texts):
    processed_texts = []
    for text in texts:
        processed_texts.append(text if not isinstance(text, list)
                               else tokenizer.convert_tokens_to_string(text))
    
    inputs = tokenizer(
        processed_texts, 
        return_tensors="pt", 
        padding=True, 
        truncation=True, 
        max_length=512,
        add_special_tokens=True
    )
    
    with torch.no_grad():
        outputs = model(**inputs)
    
    return torch.nn.functional.softmax(outputs.logits, dim=-1).numpy()

# Initialize SHAP components
output_names = [model.config.id2label[i] for i in range(5)]
masker = shap.maskers.Text(tokenizer=tokenizer, mask_token=tokenizer.mask_token, collapse_mask_token=True)
explainer = shap.Explainer(predict, masker, output_names=output_names)

# Streamlit UI
st.title("🎯 BERT Sentiment Analysis with SHAP")
st.markdown("""
**How it works:**
1. Enter text in the box below  
2. See predicted sentiment (1-5 stars)  
3. View confidence scores and word-level explanations  
""")

text_input = st.text_area("Input Text", placeholder="Enter text to analyze...", height=100)

if st.button("Analyze Sentiment"):
    if text_input.strip():
        with st.spinner("Analyzing..."):
            # Get predictions
            probabilities = predict([text_input])[0]
            predicted_class = np.argmax(probabilities)
            
            # Display results
            st.subheader("📊 Results")
            cols = st.columns(2)
            cols[0].metric("Predicted Sentiment", output_names[predicted_class])
            with cols[1]:
                st.markdown("**Confidence Scores**")
                for label, score in zip(output_names, probabilities):
                    st.progress(float(score), text=f"{label}: {score:.1%}")
            
            # Generate SHAP explanations
            st.subheader("🔍 Explanation")
            st.markdown("""
            **Feature importance (word-level impacts)**  
            🔴 Higher positive values → Increases sentiment  
            🔵 Lower negative values → Decreases sentiment
            """)

            # Get SHAP values for the input text
            shap_values = explainer([text_input])
            
            # Create tabs for each sentiment class
            tabs = st.tabs(output_names)
            for i, tab in enumerate(tabs):
                with tab:
                    # Extract the values and corresponding tokens for our single example.
                    # shap_values is of shape (1, num_tokens, num_classes)
                    values = shap_values.values[0, :, i]  # SHAP values for class i
                    tokens = shap_values.data[0]          # Tokenized words

                    # Create a DataFrame to sort and plot the tokens by importance
                    df = pd.DataFrame({"token": tokens, "shap_value": values})
                    # Sort tokens by the absolute SHAP value (smallest at the bottom for horizontal bar plot)
                    df = df.sort_values("shap_value", key=lambda x: np.abs(x), ascending=True)

                    # Create a horizontal bar plot
                    fig, ax = plt.subplots(figsize=(8, max(4, len(tokens) * 0.3)))
                    ax.barh(df["token"], df["shap_value"], color='skyblue')
                    ax.set_xlabel("SHAP value")
                    ax.set_title(f"SHAP bar plot for class '{output_names[i]}'")
                    st.pyplot(fig)
                    plt.close(fig)

    else:
        st.warning("Please enter some text to analyze")

st.markdown("---")
st.markdown("Example texts to try:")
examples = st.columns(4)
example_texts = [
    "This product exceeded all my expectations!",
    "Terrible customer service experience.",
    "The movie was okay, nothing special.",
    "You are kinda cool"
]

for col, text in zip(examples, example_texts):
    with col:
        if st.button(text, use_container_width=True):
            st.session_state.last_input = text

if 'last_input' in st.session_state:
    text_input = st.text_area("", value=st.session_state.last_input, height=100)