app/model.py

"""Functions for model training, evaluation, and inference."""

from __future__ import annotations

import warnings
from typing import TYPE_CHECKING, Literal, Sequence

import numpy as np
from joblib import Memory
from sklearn.exceptions import ConvergenceWarning
from sklearn.feature_extraction.text import CountVectorizer, HashingVectorizer, TfidfVectorizer
from sklearn.linear_model import LogisticRegression
from sklearn.model_selection import RandomizedSearchCV, cross_val_score, train_test_split
from sklearn.pipeline import Pipeline

from app.constants import CACHE_DIR
from app.data import tokenize

if TYPE_CHECKING:
    from sklearn.base import BaseEstimator, TransformerMixin

__all__ = ["train_model", "evaluate_model", "infer_model"]


def _identity(x: list[str]) -> list[str]:
    """Identity function for use in vectorizers.

    Args:
        x: Input data

    Returns:
        Unchanged input data
    """
    return x


def _get_vectorizer(
    name: Literal["tfidf", "count", "hashing"],
    n_features: int,
    min_df: int = 5,
) -> TransformerMixin:
    """Get the appropriate vectorizer.

    Args:
        name: Type of vectorizer
        n_features: Maximum number of features
        min_df: Minimum document frequency (ignored for hashing)

    Returns:
        Vectorizer instance

    Raises:
        ValueError: If the vectorizer is not recognized
    """
    shared_params = {
        "ngram_range": (1, 2),  # unigrams and bigrams
        # disable text processing
        "tokenizer": _identity,
        "preprocessor": _identity,
        "lowercase": False,
        "token_pattern": None,
    }

    match name:
        case "tfidf":
            return TfidfVectorizer(
                max_features=n_features,
                min_df=min_df,
                **shared_params,
            )
        case "count":
            return CountVectorizer(
                max_features=n_features,
                min_df=min_df,
                **shared_params,
            )
        case "hashing":
            if n_features < 2**15:
                warnings.warn(
                    "HashingVectorizer may perform poorly with small n_features, default is 2^20.",
                    stacklevel=2,
                )

            return HashingVectorizer(
                n_features=n_features,
                **shared_params,
            )
        case _:
            msg = f"Unknown vectorizer: {name}"
            raise ValueError(msg)


def train_model(
    token_data: Sequence[Sequence[str]],
    label_data: list[int],
    vectorizer: Literal["tfidf", "count", "hashing"],
    max_features: int,
    min_df: int = 5,
    cv: int = 5,
    n_jobs: int = 4,
    seed: int = 42,
) -> tuple[BaseEstimator, float]:
    """Train the sentiment analysis model.

    Args:
        token_data: Tokenized text data
        label_data: Label data
        vectorizer: Which vectorizer to use
        max_features: Maximum number of features
        min_df: Minimum document frequency (ignored for hashing)
        cv: Number of cross-validation folds
        n_jobs: Number of parallel jobs
        seed: Random seed (None for random seed)

    Returns:
        Trained model and accuracy

    Raises:
        ValueError: If the vectorizer is not recognized
    """
    rs = None if seed == -1 else seed

    # Split the data into training and testing sets
    text_train, text_test, label_train, label_test = train_test_split(
        token_data,
        label_data,
        test_size=0.2,
        random_state=rs,
    )

    # Create the model pipeline
    vectorizer = _get_vectorizer(vectorizer, max_features, min_df)
    classifier = LogisticRegression(max_iter=1000, random_state=rs)
    model = Pipeline(
        [("vectorizer", vectorizer), ("classifier", classifier)],
        memory=Memory(CACHE_DIR, verbose=0),
    )
    param_dist = {"classifier__C": np.logspace(-4, 4, 20)}

    # Perform randomized search for hyperparameter tuning
    search = RandomizedSearchCV(
        model,
        param_dist,
        cv=cv,
        random_state=rs,
        n_jobs=n_jobs,
        scoring="accuracy",
        n_iter=10,
        verbose=2,
    )

    with warnings.catch_warnings():
        warnings.filterwarnings("once", category=ConvergenceWarning)
        warnings.filterwarnings("ignore", category=UserWarning, message="Persisting input arguments took")

        search.fit(text_train, label_train)

    final_model = search.best_estimator_
    return final_model, final_model.score(text_test, label_test)


def evaluate_model(
    model: BaseEstimator,
    token_data: Sequence[Sequence[str]],
    label_data: list[int],
    cv: int = 5,
    n_jobs: int = 4,
) -> tuple[float, float]:
    """Evaluate the model using cross-validation.

    Args:
        model: Trained model
        token_data: Tokenized text data
        label_data: Label data
        cv: Number of cross-validation folds
        n_jobs: Number of parallel jobs

    Returns:
        Mean accuracy and standard deviation
    """
    with warnings.catch_warnings():
        warnings.filterwarnings("ignore", category=UserWarning, message="Persisting input arguments took")

        # Perform cross-validation to evaluate the model
        scores = cross_val_score(
            model,
            token_data,
            label_data,
            cv=cv,
            scoring="accuracy",
            n_jobs=n_jobs,
            verbose=2,
        )

    return scores.mean(), scores.std()


def infer_model(
    model: BaseEstimator,
    text_data: list[str],
    batch_size: int = 32,
    n_jobs: int = 4,
) -> list[int]:
    """Predict the sentiment of the provided text documents.

    Args:
        model: Trained model
        text_data: Text data
        batch_size: Batch size for tokenization
        n_jobs: Number of parallel jobs

    Returns:
        Predicted sentiments
    """
    tokens = tokenize(
        text_data,
        batch_size=batch_size,
        n_jobs=n_jobs,
        show_progress=False,
    )
    return model.predict(tokens)