Classto

Classto is a Python library for building lightweight, browser-based tools to manually classify images into custom categories - ideal for preparing datasets or sorting visual content.

With just a few lines of Python, Classto spins up a local web interface built on Flask and styled with Tailwind CSS to let you quickly review, label, and organize images - right from your browser.

Interface Previews

   

Classto in Light and Dark Mode

 

Features

• Three Operational Modes: Local Folder, Static URL, or Dynamic Database Hooks.
• One-Click Classification: Fast, keyboard-friendly browser UI.
• Flexible Data Management: Moves local files, logs to CSV, or streams directly from cloud/DB pipelines.
• Smart Suffixing: Optionally add unique filename suffixes to avoid naming conflicts.
• Dark Mode Toggle: Easy on the eyes during long labeling sessions.

Installation

You can install Classto via pip:

pip install classto

Quickstart

Classto adapts to your workflow. You can use it in exactly one of the following three modes at a time:

1. Local Folder Mode

Reads images from a local directory, moves them into per-label subfolders, and optionally creates a CSV log.

import classto as ct

labeler = ct.ImageLabeler(
    classes=["Cat", "Dog"],
    image_folder="images",     # Mandatory path to your images
    delete_button=True,        # Shows a button to delete/skip images
    suffix=True,               # Add unique suffix to avoid conflicts
    log_to_csv=True            # Saves results to a local CSV file
)

labeler.launch()

Then open your browser at http://127.0.0.1:5000.

Local Folder Architecture

Place your images in a folder (e.g. images/) relative to your script:

project/
├── images/
│   ├── cat1.jpg
│   ├── cat2.jpg
│   ├── dog1.jpg
│   └── dog2.jpg
├── app.py

After classification, images are moved to:

project/
├── classified/
│   ├── Cat/
│   │   ├── cat1__K8dLs.jpg
│   │   └── cat2__a7JkL.jpg
│   ├── Dog/
│   │   ├── dog1__Xy4Tz.jpg
│   │   └── dog2__Zx9Pm.jpg
│   └── labels-20260522-163400Z.csv

2. Static URL Mode

Streams images directly from a list of web URLs. Classifications are tracked via a local CSV file.

import classto as ct

urls = [
    "https://example.com/image1.jpg",
    "https://example.com/image2.png"
]

labeler = ct.ImageLabeler(
    classes=["Product", "Background Only"],
    urls=urls,                     # Mandatory for URL Mode
    log_to_csv=True,               # Keeps track of URLs in a CSV file
    shuffle=True
)

labeler.launch()

3. Dynamic Hook Mode (Database Integration)

Integrate seamlessly with databases (e.g., MongoDB, Cosmos DB) or cloud storages. Images are streamed on-demand via custom callback hooks without loading datasets into memory or enforcing local file moves.

import classto as ct

# Define your custom database connection logic
def my_next_hook():
    # Fetch next document from DB. Must return {"id": str, "url": str} or None
    doc = db.images.find_one({"labeled": False})
    return {"id": str(doc["_id"]), "url": doc["image_url"]} if doc else None

def my_label_hook(image_id, label):
    # Save the label back to your database
    db.images.update_one({"_id": ObjectId(image_id)}, {"$set": {"label": label, "labeled": True}})

def my_stats_hook():
    # Update live session progress badges in the UI
    return {
        "total_remaining": db.images.count_documents({"labeled": False}),
        "total_labeled": db.images.count_documents({"labeled": True})
    }

def my_delete_hook(image_id):
    # Optional: Action when the delete/skip button is pressed
    db.images.update_one({"_id": ObjectId(image_id)}, {"$set": {"is_deleted": True, "labeled": True}})

labeler = ct.ImageLabeler(
    classes=["Valid", "Corrupted"],
    delete_button=True,
    on_next=my_next_hook,           # Mandatory for Hook Mode
    on_label=my_label_hook,         # Mandatory for Hook Mode
    on_get_stats=my_stats_hook,     # Optional: Renders live stats UI badges
    on_delete=my_delete_hook,       # Mandatory if delete_button=True in Hook Mode
    log_to_csv=False                # Optional: Set to True for a parallel local CSV backup
)

labeler.launch()

Parameters

• classes (List[str]): A list of categories for classification (e.g. ["Dog", "Cat"]).
• image_folder (Optional[str]): Path to the folder containing local images. Required for Folder Mode. Defaults to None.
• urls (Optional[List[str]]): A list of image URLs to stream. Required for URL Mode. Defaults to None.
• delete_button (bool): If True, shows a delete button to remove or skip images. Defaults to False.
• shuffle (bool): If True, images are presented in a random order (applies to Folder and URL mode only). Defaults to False.
• suffix (bool): If True, appends a random 5-character suffix to local filenames to prevent overwriting. Defaults to False.
• log_to_csv (bool): If True, logs classifications into a local CSV file. Works as a primary tracker for URL mode or as a secondary backup across all modes. Defaults to False.
• log_path (Optional[str]): Custom directory path where the CSV log file should be written.
• log_file_name (Optional[str]): Custom file name for the CSV log. If omitted, a UTC-timestamped name is automatically generated.
• on_next (Optional[Callable]): Hook returning {"id": str, "url": str} or None. Required for Hook Mode.
• on_label (Optional[Callable]): Hook accepting (image_id: str, label: str). Required for Hook Mode.
• on_get_stats (Optional[Callable]): Hook returning {"total_remaining": int, "total_labeled": int} to power the UI counter.
• on_delete (Optional[Callable]): Hook accepting (image_id: str). Required if delete_button=True in Hook Mode.

CSV Logging Format

If log_to_csv=True is enabled, data is written into a CSV file containing the following structure:

original_filename	new_filename	label	timestamp
img01.jpg	img01__4Fg7T.jpg	Cat	2026-05-22T16:34:00+00:00
img02.jpg	img02__8Hv2f.jpg	Dog	2026-05-22T16:34:32+00:00

• original_filename: The local filename (Folder Mode) or the custom database image_id (Hook Mode).
• new_filename: The new name after suffixing (Folder Mode) or the streamed image_url (Hook Mode/URL Mode).
• label: The category selected during classification (or DELETED).
• timestamp: Execution time in ISO 8601 format (UTC).