Pillow で元画像を保持したままフォルダ内の画像を一括リサイズする

scale = min(300 / width, 300 / height, 1)
new_width = int(width * scale)
new_height = int(height * scale)

from pathlib import Path
from PIL import Image

SOURCE = Path("outputs") / "image_resize_demo"
IMAGES = {
    "landscape.png": ((800, 600), (220, 80, 80)),
    "portrait.png": ((600, 800), (80, 120, 220)),
    "small.png": ((200, 100), (80, 180, 100)),
    "wide.png": ((1200, 300), (180, 120, 60)),
}

SOURCE.parent.mkdir(parents=True, exist_ok=True)
SOURCE.mkdir()  # Stop if this synthetic source already exists.

for filename, (size, color) in IMAGES.items():
    image = Image.new("RGB", size, color)
    target = SOURCE / filename
    image.save(target, format="PNG")

python create_resize_demo.py

from pathlib import Path
from PIL import Image, ImageOps

SOURCE = Path("outputs") / "image_resize_demo"
OUTPUT_DIR = Path("outputs") / "image_resize_result"
MAX_WIDTH = 300
MAX_HEIGHT = 300
SUPPORTED = {".png", ".jpg", ".jpeg", ".webp"}


def target_size(width: int, height: int) -> tuple[int, int]:
    scale = min(MAX_WIDTH / width, MAX_HEIGHT / height, 1)
    new_width = max(1, int(width * scale))
    new_height = max(1, int(height * scale))
    return new_width, new_height


def main() -> None:
    if not SOURCE.is_dir():
        raise FileNotFoundError(f"Source folder not found: {SOURCE}")
    if MAX_WIDTH < 1 or MAX_HEIGHT < 1:
        raise ValueError("Maximum dimensions must be positive integers.")

    source_resolved = SOURCE.resolve()
    output_resolved = OUTPUT_DIR.resolve()
    if source_resolved == output_resolved:
        raise ValueError("SOURCE and OUTPUT_DIR must be different.")

    OUTPUT_DIR.parent.mkdir(parents=True, exist_ok=True)
    OUTPUT_DIR.mkdir()  # Refuse to reuse an existing destination.

    candidates = sorted(
        path for path in SOURCE.iterdir()
        if path.is_file() and path.suffix.lower() in SUPPORTED
    )

    resized_count = 0
    unchanged_size_count = 0

    for source_path in candidates:
        output_path = OUTPUT_DIR / source_path.name
        if output_path.exists():
            raise FileExistsError(f"Output already exists: {output_path}")

        with Image.open(source_path) as opened:
            image = ImageOps.exif_transpose(opened)
            image.load()
            old_size = image.size
            new_size = target_size(*old_size)

            if new_size == old_size:
                result = image.copy()
                unchanged_size_count += 1
            else:
                result = image.resize(new_size, Image.Resampling.LANCZOS)
                resized_count += 1

            save_options = {}
            if source_path.suffix.lower() in {".jpg", ".jpeg"}:
                save_options["quality"] = 90
            result.save(output_path, **save_options)
            result.close()

        print(f"{source_path.name}: {old_size[0]}x{old_size[1]} -> {new_size[0]}x{new_size[1]}")

    print(f"Processed {len(candidates)} images.")
    print(f"Resized: {resized_count}; already within limit: {unchanged_size_count}.")
    print(f"Output folder: {OUTPUT_DIR.as_posix()}")


if __name__ == "__main__":
    main()

landscape.png: 800x600 -> 300x225
portrait.png: 600x800 -> 225x300
small.png: 200x100 -> 200x100
wide.png: 1200x300 -> 300x75
Processed 4 images.
Resized: 3; already within limit: 1.
Output folder: outputs/image_resize_result

from pathlib import Path
from PIL import Image

SOURCE = Path("outputs") / "image_resize_demo"
OUTPUT_DIR = Path("outputs") / "image_resize_result"

for source_path in sorted(SOURCE.glob("*.png")):
    output_path = OUTPUT_DIR / source_path.name
    with Image.open(source_path) as original, Image.open(output_path) as resized:
        print(
            f"{source_path.name}: "
            f"original={original.size}, output={resized.size}"
        )