Adam Johnson (original) (raw)
Python: sharing common tests in unittest
2025-05-30
A neat testing pattern is writing common tests in a base class and then applying them to multiple objects through subclassing. Doing so can help you test smarter and cover more code with less boilerplate.
Python: a quick cProfile recipe with pstats
2025-05-20
Python comes with two built-in profilers for measuring the performance of your code: cProfile and profile. They have the same API, but cProfile is a C extension, while profile is implemented in Python. You nearly always want to use cProfile, as it’s faster and doesn’t skew measurements as much.
pre-commit: install with uv
2025-05-07
pre-commit is my favourite Git-integrated “run things on commit” tool. It acts as a kind of package manager, installing tools as necessary from their Git repositories. This makes it fairly easy to set up: all you need to install is pre-commit itself, and it takes things from there.
Python type hints: mixin classes
2025-05-01
In Python, a mixin class is a class that is not intended to be used directly, but instead “mixed in” to other classes through multiple inheritance. Mixins are not really a language feature but more of a conventional pattern allowed by Python’s multiple inheritance rules. Unfortunately, adding type hints to mixin classes can be tricky because they implicitly require subclasses to fit a certain shape.
Python: fuss-free use of Homebrew libraries for package dependencies
2025-01-10
Some Python packages require native libraries to be installed on your system when you install them. For example, mysqlclient requires libssl and libcrypto. If those libraries are missing at install time, clang, the C compiler, fails with a message like:
Python: test for unraisable exceptions with unittest.mock
2025-01-07
A few days ago, I blogged about debugging unraisable exceptions with Rich. Here’s a sequel on testing that some block of code doesn’t trigger any unraisable exceptions.
Python: debug unraisable exceptions with Rich
2025-01-02
Take this Python class:
Python: spy for changes with sys.monitoring
2024-12-30
Python 3.12 introduced sys.monitoring, a new framework for “monitoring” tools like debuggers and profilers to hook into. It provides fine-grained control so tools can listen only to certain events on specific lines of code. The framework came from PEP 669, thanks to Mark Shannon of the Faster CPython team.
Python: create temporary files and directories in unittest
2024-12-30
Sometimes, tests need temporary files or directories. You can do this in Python’s unittest with the standard library tempfile module. Let’s look at some recipes to do so within individual tests and setUp().
Python: my new uv setup for development
2024-09-18
uv is a new, fast Python packaging tool. Since the August 20 update, uv can now manage both versions of Python and Python-powered tools.
Python: profile total memory allocated with tracemalloc
2024-08-30
tracemalloc is Python’s standard library module for tracking memory allocations. It has many bells and whistles for detailed analysis, allowing you to slice allocations by file and line or compare snapshots. But for simple purposes, displaying the total memory allocated is sufficient, which is what this recipe does:
Python: Fail in three characters with 1/0
2024-06-18
Here’s a code snippet I often type:
Python: Import by string with pkgutil.resolve_name()
2024-06-17
Django and other frameworks often allow you to configure classes, functions, or modules to import using strings. To do the same in your own code, use pkgutil.resolve_name() from the standard library (added in Python 3.9):
Python: Mock an inner import
2024-05-17
Sometimes, you want to test a function that uses an inner import and mock that imported object. For example, say you wanted to mock dig() in this example:
Python: Show all subclasses of a class
2024-05-10
class.__subclasses__() returns a list of the direct subclasses of a given class:
Python: Diffing unit tests to keep a copy-pasted code in sync
2024-04-26
Copy-paste-tweaking library code feels like a dirty but inevitable programming practice. Often driven by deadlines or other constraints, it seems all projects end up with something copy-pasted in and tweaked for one specific use case.
Python: Make line number paths with inspect
2024-04-25
Many terminals and text editors support what I’ll call “line number paths” of the form <filename>:<lineno>. Opening that path, whether by clicking or passing to a CLI, opens the given file at that line.
My appearance on The Python Show
2023-11-17
Earlier this week, I made another podcast appearance on The Python Show, episode 22: Git and Django with Adam Johnson. As fellow authors, Mike and I talked a lot about the writing process, on topics like:
My appearance on the PyBites Podcast
2023-11-13
Last week, I made another podcast appearance on the PyBites Podcast, episode 139: Maximizing Your Developer Experience (DX) with Adam Johnson. We talked about various topics, including:
My appearance on The Real Python Podcast 179
2023-11-09
I had the pleasure of returning to The Real Python podcast in last week’s episode 179, Improving Your Git Developer Experience in Python. It was great to catch up with host Christopher Bailey and chat about:
GitHub Actions: Faster Python runs with cached virtual environments
2023-11-02
Most projects I work on use Python, good ol’ Pip, and pip-tools. Below is a pattern I’ve used to speed up the GitHub Actions workflow runs on several such projects. On larger projects with many dependencies, it can save tens of seconds per run.
Introducing flake8-logging
2023-09-07
The Python standard library’s logging module is a go-to for adding observability to applications. Many tools also integrated with it or enhance its capabilities, such as structlog and Sentry.
Python: Profile a section of code with cProfile
2023-07-23
When trying to improve a slow function or module, it’s always a good idea to profile it. Here’s a snippet for quickly profiling a section of code with Python’s cProfile module, in two flavours. It’s adapted from the cProfile documentation’s Profile example. I have used versions of this snippet over the years to narrow in on performance issues.
Python type hints: How to pass Any for unused parameters in tests
2023-07-15
When you create a function to match an interface, it often needs to accept parameters that it doesn’t use. Once you introduce type hints, testing such functions can become a little irksome as Mypy will require all arguments to have the correct types. Your tests can end up creating unused objects only to match the tested function’s signature. Here’s a technique to avoid that work.
Python type hints: modernized error messages in Mypy 1.4.0
2023-06-26
Mypy 1.4.0 was released last week (2023-06-20). I’m happy to see it includes three improvements that modernize error messages with newer type hint syntax: