Math factoid of the day: 60

Thursday 16 June 2022

60 shows up in lots of places. It’s the smallest number divisible by 1 through 6, and perhaps because of that, it’s the basis of our timekeeping and angular measurements.

Of course the angles in an equilateral triangle are 60 degrees. But 60 also appears in solid geometry. There are four Archimedean solids (regular polyhedra made with any mixture of regular polygons) with 60 vertices. You can use Nat Alison’s beautiful polyhedra viewer to explore them:

400 walks

Tuesday 14 June 2022

Yesterday I did my 400th pandemic walk. These started as a way to get exercise during lockdown with my son Nat, as I wrote about in Pandemic walks (Feb 2021) and 300 walks (Sept 2021).

Now I’ve done 400 walks starting and ending at the same point, totaling 2318 miles:

All the streets I've walked

The cadence of these walks has slowed quite a bit. Not the pace of the walking itself, but the number of walks in a week. Nat is only with us one day a week now, so I don’t have to be out for his sake. I have biking and swimming as exercise options now, and my toes were getting a bit aggravated by walking.

Quantifying the walks was a fun project, and motivated me to get out and go, but it also pushed me to go farther and farther: in February, all the walks were longer than 7 miles, which might not have been wise. The problem with a statistic is the unreasonable expectation that it will constantly increase.

I’m not sure what will happen now. I’m going to continue walking as an at least occasional exercise, but maybe with a different motivator? Or maybe not: it’s hard to drop a project once you start it...

Now I’m going on a bike ride, and I’m not going to measure my speed.

Adding a dunder to an object

Sunday 5 June 2022

We had a tricky debugging need at work: we wanted to track how an attribute on an object was changing. Here’s the unusual solution we used.

The __setattr__ special method (dunder) is called when an attribute is changed on an object. But like all special methods, it’s not found on the object, only on the class. We didn’t want to track changes to all objects of the class because it would produce too much noise in the logs.

So we wanted a per-object special method. The way we came up with was to define a new class with the special method. The class is derived from the object’s existing class, so that everything would work the way it should. Then we changed the object’s class.

Changing an object’s class sounds kind of impossible, but since in Python everything happens at run-time, you can just assign a new class to obj.__class__, and now that is the object’s class.

Here’s the code, simplified:

>>> class SomeObject:
...     ...

>>> class Nothing:
...     """Just to get a nice repr for Nothing."""
...     def __repr__(self):
...         return "<Nothing>"

>>> obj = SomeObject()
>>> obj.attr = "first"
>>> obj.attr

>>> def spy_on_changes(obj):
...     """Tweak an object to show attributes changing."""
...     class Wrapper(obj.__class__):
...         def __setattr__(self, name, value):
...             old = getattr(self, name, Nothing())
...             print(f"Spy: {name}{old!r} -> {value!r}")
...             return super().__setattr__(name, value)
...     obj.__class__ = Wrapper

>>> spy_on_changes(obj)
>>> obj.attr = "second"
Spy: attr: 'first' -> 'second'

>>> obj.attr

>>> obj.another = 'foo'
Spy: another: <Nothing> -> 'foo'

One more detail: the Nothing class lets us use repr() to show an object but also get a nice message if there wasn’t a value before.

The real code was more involved, and showed what code was changing the attribute. This is extreme, but helped us debug a problem. As I said in Machete-mode Debugging, Python’s dynamic nature can get us into trouble, so we might as well use it to get us out of trouble.

Custom search keywords

Thursday 26 May 2022

You can define custom search keywords in your browser to make common searches easier. How to do it isn’t well documented, so I’m showing you how here.

As an example, I have a custom keyword “pypi”, so I can search PyPI for a package name by typing “pypi some name” in my browser’s address bar, and it takes me directly to name.

You can create your own custom keywords to have shortcuts to all kinds of searches. You provide a URL with a placeholder %s in it. When you type the keyword in the address bar, the rest of the entry is plugged into the placeholder, and you jump to the resulting URL.

Handy keywords

These are some of the custom search keywords I use. You can use them as-is, adapt them as you need, or create your own. Instructions for creating keywords are further down the page.

py: Search Python docs
pep: Find a PEP by number
pypi: Find a PyPI package
gh: GitHub code search
gpy: Google search for Python things, but not Monty Python
xlate: Translate text, auto-detecting the language

How to define a keyword in Firefox

Firefox defines custom keywords as bookmarks:

  1. Select Bookmarks - Manage Bookmarks from the menu to open the Library dialog.
  2. In the left-hand sidebar, select Quick Searches. You’ll see a handful of pre-defined searches.
  3. Using the gear drop-down at the top of the dialog, select Add Bookmark...
  4. Enter the URL of the search in the URL field. Use %s as the placeholder.
  5. Put the keyword you want to use in the Keyword field.
  6. You can give the bookmark a name to make it easier to find it later if you need to.
  7. Save the bookmark, and close the Library.
  8. Now you can use your keyword in the address bar.

How to define a keyword in Chrome

Chrome defines custom keywords as search engines in Settings:

  1. Select Preferences or type chrome://settings in the address bar.
  2. In the left-hand sidebar, select Search engine and then Manage search engines and site search.
  3. Scroll down to Site search.
  4. Click Add.
  5. Put a description in the Search engine field.
  6. Enter the URL in the URL field, with %s as a placeholder.
  7. Put the keyword in the Shortcut field.
  8. Click Add, then close the Settings tab.
  9. Now you can use your keyword in the address bar.

Cairo in Jupyter, better

Sunday 15 May 2022

I finally came up with a way I like to create PyCairo drawings in a Jupyter notebook.

A few years ago I wrote here about how to draw Cairo SVG in a Jupyter notebook. That worked, but wasn’t as convenient as I wanted. Now I have a module that manages the PyCairo contexts for me. It automatically handles the displaying of SVG and PNG directly in the notebook, or lets me write them to a file.

The module is

The code looks like this (with a sample drawing copied from the PyCairo docs):

from drawing import cairo_context

def demo():
    with cairo_context(200, 200, format="svg") as context:
        x, y, x1, y1 = 0.1, 0.5, 0.4, 0.9
        x2, y2, x3, y3 = 0.6, 0.1, 0.9, 0.5
        context.scale(200, 200)
        context.move_to(x, y)
        context.curve_to(x1, y1, x2, y2, x3, y3)
        context.set_source_rgba(1, 0.2, 0.2, 0.6)
        context.move_to(x, y)
        context.line_to(x1, y1)
        context.move_to(x2, y2)
        context.line_to(x3, y3)
    return context


Using demo() in a notebook cell will draw the SVG. Nice.

The key to making this work is Jupyter’s special methods _repr_svg_, _repr_png_, and a little _repr_html_ thrown in also.

The code is at I created it so that I could play around with Truchet tiles:

A multi-scale Truchet tiling

Twitter tidbits

Sunday 8 May 2022

Lately I’ve been posting Python tidbits on Twitter. It’s been fun finding things that people might not yet know, fitting them into a tweet, and giving them some energy.

People ask how I make them. I tried a few “make a code screenshot” sites at first, but wanted more control over how the images looked. Now I create the code shots in vim with a tweaked color scheme (shades-of-purple). This lets me do things like highlight specific words I want to draw attention to. Then I use the Mac screenshot tool to grab an image:

An example of purpled code

The font is Recursive Mono SemiCasual. Recursive is a lovely and lively variable font with a number of axes to play with, especially the Casual axis, which I’ve set to halfway between Linear and Casual. You can get it now on Google Fonts. I’d like to try using Semi-Mono, but vim only understands monospaced fonts.

A sample of Recursive Mono, in Linear, SemiCasual, and Casual

Some people have asked about publishing these tidbits here on also, but I’m not sure it’s worth the effort. Would it be helpful to have a page like this that collected them per-month or something? Or one tweet per page? Or just let them live on Twitter?