Resize an image to a chosen width

Before you begin

Python 3.12 or newer
Pillow installed in the same Python environment

What this script changesCreates new image files or a new frame directory. Inputs and outputs are limited to 20 million pixels per image; animated tasks also cap frame count. Ordinary transforms process the first frame and normalize EXIF orientation. Color profiles and original metadata are not preserved reliably; use source copies for archival work.

Set up your example

Save the complete code as image-resize-width.py in a working folder.
Save the provided sample files in that same folder; run commands from that folder.
Install Pillow: python -m pip install Pillow
Generate the original fixtures once: python "make_samples.py". Choose an empty folder so the fixture files do not exist.

Sample files in this download:

make_samples.py

Download the ZIP, extract it, and open a terminal inside the extracted folder. The source file is image-resize-width.py.

Run the script

python "image-resize-width.py" --input "sample.png" --width 80 --output "resized.png"

If your system uses python3 instead of python, use that command for Python examples. Keep paths containing spaces in quotes. Required inputs and optional settings are described in the code and setup steps.

What to expect

Resize one image while maintaining its aspect ratio. The 160 by 100 fixture becomes 80 by 50.

The complete script

#!/usr/bin/env python3
'Resize an image to a chosen width'
import argparse
import json
from pathlib import Path

def write_json(path, value):
    data = json.dumps(value, indent=2, ensure_ascii=False, allow_nan=False) + "\n"
    with Path(path).open("x", encoding="utf-8", newline="") as handle:
        handle.write(data)

import math

try:
    from PIL import Image, ImageOps, ImageEnhance, ImageFilter, ImageDraw, ImageFont, ImageChops
except ImportError:
    raise SystemExit("Install Pillow first: python -m pip install Pillow")
MAX_PIXELS=20_000_000
def check_size(size):
    if min(size)<1 or size[0]*size[1]>MAX_PIXELS:
        raise ValueError("Image dimensions must be positive and at most 20 million pixels.")
def read_image(path):
    with Image.open(path) as original:
        check_size(original.size)
        return ImageOps.exif_transpose(original).convert("RGBA")
def save_image(path,image,format="PNG"):
    check_size(image.size)
    with Path(path).open("xb") as handle:
        image.save(handle,format=format)

def main():
    parser = argparse.ArgumentParser(description='Resize an image to a chosen width')
    parser.add_argument("--input",required=True,help="Local source image")
    parser.add_argument("--output",required=True,help="New output file or directory")
    parser.add_argument("--width",type=int,default=800)
    args = parser.parse_args()
    image=read_image(args.input)
    if args.width<1: raise ValueError("Width must be positive.")
    size=(args.width,max(1,round(image.height*args.width/image.width)))
    check_size(size)
    save_image(args.output,image.resize(size,Image.Resampling.LANCZOS))

if __name__ == "__main__":
    try:
        main()
    except (OSError, ValueError, KeyError, TypeError, ArithmeticError, IndexError, LookupError) as exc:
        raise SystemExit(str(exc))

Open source as plain text · Download source and sample files

How it works

Pillow reads the local image; the source is left unchanged and existing destinations are refused.
Height is calculated from the width ratio and rounded to an integer; the pixel cap is checked before resizing.
The 160 by 100 fixture becomes 80 by 50.

Make it your own

--width

Use --width 1200 for a larger display copy.

Adapt the processing

Edit main's size calculation to constrain height instead of width.

Change one setting at a time, run it again on sample data, and compare the output. The customization guide walks through this process.

If something goes wrong

The requested output exceeds the pixel cap.

Choose a smaller width; the tool does not disable Pillow's image protections.

The destination already exists.

Choose a new --output path. The script deliberately refuses to replace an existing result.

Reference & next steps

These official references explain the underlying tools. The Scriptage example and guide are original instructional content. See verification notes for what has been checked and the limits of those checks.

Copies a template for your own notes. It does not send anything.