Pillow

Pillow is the maintained fork of PIL - the standard library for opening, converting, resizing, cropping, and saving images in Python pipelines, web upload handlers, and ML preprocessing.

Recipe

from PIL import Image
 
with Image.open("photo.jpg") as img:
    img = ImageOps.exif_transpose(img)
    img.thumbnail((800, 800))
    img.save("preview.webp", format="WEBP", quality=85)

When to reach for this:

• User avatar/thumbnail generation in web apps
• Batch converting PNG/JPEG/WEBP for CDNs
• Preprocessing images before PyTorch/torchvision training
• Reading dimensions and metadata without loading full decode into UI

Working Example

from __future__ import annotations
 
from io import BytesIO
from pathlib import Path
 
from PIL import Image, ImageOps, UnidentifiedImageError
 
 
MAX_BYTES = 5 * 1024 * 1024
ALLOWED = {"JPEG", "PNG", "WEBP"}
 
 
def make_thumbnail(data: bytes, max_edge: int = 256) -> bytes:
    if len(data) > MAX_BYTES:
        raise ValueError("file too large")
    try:
        with Image.open(BytesIO(data)) as img:
            if img.format not in ALLOWED:
                raise ValueError(f"unsupported format {img.format}")
            img = ImageOps.exif_transpose(img)
            img = img.convert("RGB")
            img.thumbnail((max_edge, max_edge))
            out = BytesIO()
            img.save(out, format="WEBP", quality=80, method=6)
            return out.getvalue()
    except UnidentifiedImageError as exc:
        raise ValueError("not an image") from exc
 
 
if __name__ == "__main__":
    sample = Path("photo.jpg").read_bytes()
    Path("thumb.webp").write_bytes(make_thumbnail(sample))

What this demonstrates:

• Size cap before decode mitigates decompression bombs
• exif_transpose fixes rotated phone photos
• convert("RGB") normalizes modes before WEBP export
• In-memory BytesIO pipeline for API handlers
• Explicit allowed formats whitelist

Deep Dive

How It Works

• Image.open - Lazy load; decode happens on first pixel access.
• Modes - RGB, RGBA, L (grayscale), P (palette) - convert before formats that do not support alpha.
• Transforms - resize, thumbnail, crop, rotate, filter module for blur/sharpen.
• Saving - Format from extension or format= kwarg; quality settings vary per codec.

Common Operations

Python Notes

# Verify dimensions without full decode (where supported)
with Image.open(path) as img:
    width, height = img.size

Gotchas

• Decompression bombs - Tiny PNG expands to gigapixel. Fix: max bytes + max dimension checks pre-decode.
• Ignoring EXIF orientation - Thumbnails appear sideways. Fix: ImageOps.exif_transpose.
• RGBA to JPEG - Save fails or drops alpha oddly. Fix: convert("RGB") on white background first.
• Not closing files on Windows - File lock on overwrite. Fix: context managers.
• Trusting client Content-Type - Attackers spoof MIME. Fix: validate img.format after open.
• LANCZOS on huge images - Memory spikes. Fix: thumbnail first, then optional resize.

Alternatives

FAQs

Related

• PyTorch / Deep Learning Skill - training image pipelines
• Input Validation - upload security
• FastAPI Basics - file upload routes
• File and Directory Automation - batch processing

Stack versions: This page was written for Python 3.14.0 (stable 3.14, maintenance 3.13), FastAPI 0.115+, Django 5.2, Flask 3.1, Pydantic 2, PyTorch 2.6+, pandas 2.2+, Polars 1.x, ruff 0.9+, and uv 0.6+.