A blog and website by Peter Bengtsson

How to string pad a string in Python with a variable

19 October 2021 1 comment   Python

I just have to write this down because that's the rule; if I find myself googling something basic like this more than once, it's worth blogging about.

Suppose you have a string and you want to pad with empty spaces. You have 2 options:

>>> s = "peter"
>>> s.ljust(10)
'peter     '
>>> f"{s:<10}"
'peter     '

The f-string notation is often more convenient because it can be combined with other formatting directives.
But, suppose the number 10 isn't hardcoded like that. Suppose it's a variable:

>>> s = "peter"
>>> width = 11
>>> s.ljust(width)
'peter      '
>>> f"{s:<width}"
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: Invalid format specifier

Well, the way you need to do it with f-string formatting, when it's a variable like that is this syntax:

>>> f"{s:<{width}}"
'peter      '

How to bulk-insert Firestore documents in a Firebase Cloud function

23 September 2021 0 comments   Node, Firebase, JavaScript

You can't batch-add/bulk-insert documents in the Firebase Web SDK. But you can with the Firebase Admin Node SDK. Like, in a Firebase Cloud Function. Here's an example of how to do that:

const firestore = admin.firestore();
let batch = firestore.batch();
let counter = 0;
let totalCounter = 0;
const promises = [];
for (const thing of MANY_MANY_THINGS) {
  const docRef = firestore.collection("MY_COLLECTION").doc();
  batch.set(docRef, {
    favNumber: 0,
  if (counter >= 500) {
    console.log(`Committing batch of ${counter}`);
    totalCounter += counter;
    counter = 0;
    batch = firestore.batch();
if (counter) {
  console.log(`Committing batch of ${counter}`);
  totalCounter += counter;
await Promise.all(promises);
console.log(`Committed total of ${totalCounter}`);

I'm using this in a Cloud HTTP function where I can submit a large amount of data and have each one fill up a collection.

I'm a GitHubber now

21 September 2021 1 comment   Work, GitHub

Starting today, I'm a Hubber. Meaning, I work for GitHub. I'll be joining the GitHub Docs team to help technical writers document all the various products that GitHub have. Since I haven't actually started coding anything yet, I don't want to claim I know how it works or exactly what I'll be working but on, but I do know that the site at hand is and I've previously taken a lot of inspiration from this site when building the MDN rewrite.

GitHub profile

If you are a Hubber, too, and reading this; Hi! Let's be friends! I'm a friendly guy. Please ping me and say hi.
I'll be working from home here in Mount Pleasant, South Carolina. I have 2 young kids; Tucker and Charlotte, who mean the world to me. And my backbone-of-life wife Ashley. My hobbies are coding, swimming, golf, and cooking.

TypeScript generic async function wrapper function

12 September 2021 0 comments   JavaScript

I find this so fiddly! I love TypeScript and will continue to use it if there's a choice. But I just wanted to write a simple async function wrapper and I had to Google for it and nothing was quite right. Here's my simple solution, as an example:

function wrappedAsyncFunction<T>(
    fn: (...args: any[]) => Promise<T>
  ): (...args: any[]) => Promise<T> {
    return async function(...args: any[]) {
      try {
        return await fn(...args);
      } catch(error) {
        console.warn("FYI, an error happened:", error);
        throw error;
      } finally {


Here's a Playground demo

What I use it for is to wrap my Firebase Cloud Functions so that if any error happens, I can send that error to Rollbar. In particular, here's an example of it in use:

diff --git a/functions/src/cleanup-thumbnails.ts b/functions/src/cleanup-thumbnails.ts
index 46bdb34..a3e8d54 100644
--- a/functions/src/cleanup-thumbnails.ts
+++ b/functions/src/cleanup-thumbnails.ts
@@ -2,6 +2,8 @@ import * as admin from "firebase-admin";
 import * as functions from "firebase-functions";
 import { logger } from "firebase-functions";

+import { wrappedLogError } from "./rollbar-logger";
 const OLD_DAYS = 30 * 6; // 6 months
 // const ADDITIONAL_DAYS_BACK = 15;
@@ -9,7 +11,7 @@ const PREFIX = "thumbnails";

 export const scheduledCleanupThumbnails = functions.pubsub
   .schedule("every 24 hours")
-  .onRun(async () => {
+  .onRun(wrappedLogError(async () => {
     logger.debug("Running scheduledCleanupThumbnails");


And my wrappedLogError looks like this:

export function wrappedLogError<T>(
  fn: (...args: any[]) => Promise<T>
): (...args: any[]) => Promise<T> {
  return async function(...args: any[]) {
    try {
      return await fn(...args);
    } catch (error) {
      throw error;

I'm not sure it's the best or correct way to do it, but it seems to work. Perhaps there's a more correct solution but for now I'll ship this because it seems to work fine.

From photo of ingredients, to your shopping list

10 September 2021 0 comments   Web development, That's Groce!, Firebase

Today I launched a really cool new feature to That's Groce!: Ability to upload photos of ingredients and have the food words automatically suggested to be added to your shopping list.

It's best explained with this 26-second video.

The general idea

Food words found
The idea is that you know you're going to cook that Vegetarian Curry Lasagna on page 123 in Jamie's Summer Cookbook. Either you read the ingredients and type in each ingredient you're going to need to buy (because you know what's in your pantry and fridge) or just take a photo of the whole ingredient listing. Now, when you're in the store and wonder: "I remember I need red peppers, but how many was it again?!".
But not only that, once you've taken a photo of the list of ingredients, it can help you populate your shopping list.

How it works in That's Groce! is that your photo is turned into a block of text, and from that text certain "food words" are extracted and all else is ignored. The food words are based on a database of 3,600+ English food words that I've gathered and also manually curated. There are lots of caveats. The 3,600+ food words are not perfect and there are surprisingly many combinations and plural vs. singular that can make the list incomplete. But, that's where you can help! If you find that there's a word it correctly scanned but didn't suggest, you can type in your own suggestions for everyone to benefit from. If you type in something it didn't manage to spot, I'll review that and add it to the global database.

Food-word recognition

The word recognition is done using Google Cloud Vision API which is a powerful machine-learning-based service from Google Cloud that is stunningly accurate. Tidy camera photos of cookbooks with good light is nearly perfect, but it can also do an impressive job with photos of handwritten recipes, like this:

Handwritten recipe photo

One thing you will find is that it's often hard to only take a photo of the actual list of ingredients. Often, a chunk of the cooking instructions or the recipe "back story" gets into the photo frame. These words aren't actually ingredients and can lead to surprising food word suggestions that you definitely don't need to buy. It's not perfect but at least you'll have a visual memory of what you're cooking as you're standing there in the grocery store.


Understandably, it's nearly impossible for the app to know what you have in your pantry/fridge/freezer/spice rack. But a lot of recipes spell out exactly everything you need, not for buying, but for cooking. E.g. 1 teaspoon salt. But salt (or pepper or sugar or butter) is the kind of foodstuff you probably always have at home. So then it doesn't make sense to suggest that you put "salt" on the shopping list. To solve for that, you can override your own preferred options of special keywords. For example, I've added "salt", "pepper", "sugar, "table salt", "black pepper" as words that can always be ignored. You can also train the system to set up aliases. For example, a lot of recipes call for "lemon zest" but what you actually purchase is a lemon and then grate it yourself on the grater. So you can add special keywords that act as aliases for other words. Here's one example:


Help out!

Foodwords database sample
Over the last couple of days, I've been snapping lots of photos from lots of different cookbooks, and every time I begin to think the database is getting good, I stumble on some new food word. For example, I recently tested a photo of a recipe that called for "Jackfruit". What even is that?! Anyway, it is inevitable that certain words are missing. But that's where we can help each other. If you test it out and you notice that it correctly scanned the text but the word wasn't suggested, click the "Suggest" button and type in your suggestions. Together we can, one food word at a time, eradicate misses.

There's still some work to be done to make the database even stronger by setting up clever aliases for everyone. For example, a lot of recipes call for "(some number) cloves garlic" but it can easily get confused for the spice "cloves" and the root vegetable "garlic". So, perhaps we can train it to recognize "cloves garlic" to actually just mean "garlic".

Also, the database is currently only in (primarily American) English. The platform would support other languages but I would definitely need a hand to seed it with more words in other languages.

Try it out

If you haven't set up an account yet (it's still free!), to test it, you can go to, click "Get started without signing in", go into your newly created shopping list, and press the "Photos" button. Try uploading some snapped photos from your cookbooks. Please please let me know what you think!

TypeScript function keyword arguments like Python

08 September 2021 0 comments   Python, JavaScript

To do this in Python:

def print_person(name="peter", dob=1979):

# prints: name=peter   dob=1979

# prints: name=Tucker  dob=1979

# prints: name=peter   dob=2013

# TypeError: print_person() got an unexpected keyword argument 'sex' TypeScript:

function printPerson({
  name = "peter",
  dob = 1979
}: { name?: string; dob?: number } = {}) {

// prints: name=peter  dob=1979

// prints: name=peter  dob=1979

printPerson({ name: "Tucker" });
// prints: name=Tucker dob=1979

printPerson({ dob: 2013 });
// prints: name=peter  dob=2013

printPerson({ gender: "boy" })
// Error: Object literal may only specify known properties, and 'gender' 

Here's a Playground copy of it.

It's not a perfect "transpose" across the two languages but it's sufficiently similar.
The trick is that last = {} at the end of the function signature in TypeScript which makes it possible to omit keys in the passed-in object.

By the way, the pure JavaScript version of this is:

function printPerson({ name = "peter", dob = 1979 } = {}) {

But, unlike Python and TypeScript, you get no warnings or errors if you'd do printPerson({ gender: "boy" }); with the JavaScript version.