Home Page - Philipp K. Janert on Philipp K. Janert, Ph.D.

Filesystem operations in Python can feel clumsy, because the relevant operations are distributed over so many different modules in the standard library: some are in os (itself very large and bloated), some in os.path, then there is shutil, and you may also

I recently discovered a set of useful packages and commands that facilitate some common file and buffer handling scenarios.

I am sure this is standard procedure for many others, but I only discovered this idea now: a temporary safe mode when doing extensive filesystem operations on the (notoriously unforgiving) Unix platform.

Wouldn’t it be nice to have an environment in which to see the effect of a regular expression on a set of target strings?

After realizing that the 2024 polyfill.io supply chain attack affected this website, I upgraded from Mathjax 3 to Mathjax 4.

For a brief time, starting after 01 June 2026 (when there was no apparent problem) and 18 June 2026 (when the problem first came to my attention), visitors to this site were faced with a spurious “Sign-In” pop-up, identifying itself as “polyfill.io”.

The Schrödinger equation in a wedge potential is one of the “standard” problems in Quantum Mechanics — standard, but not routine, as its solution involves the Airy equation, which is likely to be unfamiliar, at least until this precise moment.

Gmail has its own ideas how to do things.

I occasionally fantasize about an “ideal math curriculum” for Physics graduate students, based on my experience, in school and out.

Besides the Normal Distribution Function, I occasionally need the Airy Function Ai(x): it arises in perturbation theory and some other contexts.

I recently got interested in singular perturbation theory , and to get help with the algebra, I turned to SymPy.

Formal perturbation theory, and in particular singular perturbation theory, are standard topics in applied mathematics, but seem to be largely unknown outside that specific community.

It turns out, Caltech has made the Feynman Lectures available online on their website.

Occasionally, I like to drink hot chocolate — that is, real hot chocolate, from pure cocoa powder and whole milk; not some (sugared, flavored) drink mix.

I always thought that the “types” that are created using Go’s type keyword were, in some vague sense, new, unique, original; but in any case separate and distinct types.

What does it take to stand up a minimal REST service in Go, using only facilities provided by the standard library?

Der alte Dubslav von Stechlin wird gerne als Selbstportrait Fontane’s betrachtet.

How does one actually read a plain text file in Go?

I am currently working (again) on a larger writing project in Latex.

Erich Kästners Mutterkomplex ist bekannt, hinreichend dokumentiert, und sowieso nicht zu übersehen.

The Token Bucket (or Leaking Bucket) algorithm is a mechanism to rate-limit the average traffic in a stream, while allowing for a certain amount of burstiness as well.

The functional equation of the logarithm is well-known: \[ \log(xy) = \log(x) + \log(y) \] But why is this so?

When I discovered the first edition of Carl D.

The proof that $\sqrt{2}$ is irrational is part of the standard high-school curriculum.

Let a be a Go slice.

Bernhard von Lepel ist für uns hauptsächlich interessant durch seine Freundschaft mit Theodor Fontane, und das meiste, das wir über ihn wissen, wissen wir durch Fontane.

Absence of Evidence is not Evidence of Absence.

Here is the situation: an application maintains an in-memory cache.

The so-called “birthday paradox” concerns the fact that even in surprisingly small groups the probability for at least two people to share their birthdate is surprisingly high.

Having done extensive programming in C, I am not particularly spoiled when it comes to idiosyncrasies of a language’s “string” type.

Go request handlers typically include a “context” value as their first argument: func handler( ctx context.Context, ... ) { ... } In my experience, this convention is typically fastidiously followed, but then nothing is ever done with that ctx argum…

Every once in a while, I need to evaluate the normal distribution function $\Phi(x)$: $$ \Phi(x) = \frac{1}{\sqrt{2 \pi}} \int_{-\infty}^x \!

I have never seen a software project in which the limiting factor was the speed with which programmers could type.

The default highlighting of the Emacs “region” (the part of a buffer between “point” and “mark” — selected text, if any, in other words) is, out of the box, barely visible, at least against a light background.

I wanted to explore some of the new features in Emacs 29 (in particular Eglot).

As it turns out, Emacs actually allows for different cursor shapes.

Imagine a parking lot, consisting of a long, linear strip of slots.

A little while ago, I read (and quite enjoyed) “Learning eBPF” by Liz Rice.

A while back, I looked at the Diamond-Square Algorithm for terrain generation.

The extended Berkeley Packet Filter, or eBPF for short, is a plugin architecture for the Linux kernel.

Imagine a bunch of wood chips randomly distributed on a surface.

A tool to display directory entries, sorted by size, together with their cumulative contribution to the total.

After seeing positive references to it on Linux Weekly News and elsewhere, I bought a copy of “Mastering Emacs” by Mickey Petersen.

On 08 April 2024, I drove up to Erie, PA, to see the total eclipse of the sun.

Go is weird.

For every function, data type, or other such code artifact, there are two bits of code: the part that defines and implements it, and the one that uses it.

I originally prepared this video as a guide for a relative of mine, but then was surprised how well it came out.

In response to a previous post, very insightfully, Steve Singer, pointed out that it is helpful to distinguish between three concepts, when talking about “interfaces” in general: Data types Capabilities Facades

I was recently reminded of the minimal set of project management artifacts and activities that are, in one way or another, simply indispensable.

The interface construct in the Go language is one of its most immediately visible features.