Python

In multi-page web applications, a common workflow is where a user: Loads a specific page or clicks on some button that triggers a long-running task. On the server side, a background worker picks up the task and starts processing it asynchronously. The page shouldn’t reload while the task is running. The backend then communicates the status of the long-running task in real-time. Once the task is finished, the client needs to display a success or an error message depending on the final status of the finished task. The de facto tool for handling situations where real-time bidirectional communication is necessary is WebSocket1. However, in the case above, you can see that the communication is mostly unidirectional where the client initiates some action in the server and then the server continuously pushes data to the client during the lifespan of the background job. ...

I’ve always had a vague idea about what Unix domain sockets are from my experience working with Docker for the past couple of years. However, lately, I’m spending more time in embedded edge environments and had to explore Unix domain sockets in a bit more detail. This is a rough documentation of what I’ve explored to gain some insights. The dry definition Unix domain sockets (UDS) are similar to TCP sockets in a way that they allow two processes to communicate with each other, but there are some core differences. While TCP sockets are used for communication over a network, Unix domain sockets are used for communication between processes running on the same computer. ...

While working on a multithreaded socket server in an embedded environment, I realized that the default behavior of Python’s socketserver.ThreadingTCPServer requires some extra work if you want to shut down the server gracefully in the presence of an interruption signal. The intended behavior here is that whenever any of SIGHUP, SIGINT, SIGTERM, or SIGQUIT signals are sent to the server, it should: Acknowledge the signal and log a message to the output console of the server. Notify all the connected clients that the server is going offline. Give the clients enough time (specified by a timeout parameter) to close the requests. Close all the client requests and then shut down the server after the timeout exceeds. Here’s a quick implementation of a multithreaded echo server and see what happens when you send SIGINT to shut down the server: ...

I was working on a project where I needed to poll multiple data sources and consume the incoming data points in a single thread. In this particular case, the two data streams were coming from two different Redis lists. The correct way to consume them would be to write two separate consumers and spin them up as different processes. However, in this scenario, I needed a simple way to poll and consume data from one data source, wait for a bit, then poll and consume from another data source, and keep doing this indefinitely. That way I could get away with doing the whole workflow in a single thread without the overhead of managing multiple processes. ...

Consider this iterable: it = (1, 2, 3, 0, 4, 5, 6, 7) Let’s say you want to build another iterable that includes only the numbers that appear starting from the element 0. Usually, I’d do this: # This returns (0, 4, 5, 6, 7). from_zero = tuple(elem for idx, elem in enumerate(it) if idx >= it.index(0)) While this is quite terse and does the job, it won’t work with a generator. There’s an even more generic and terser way to do the same thing with itertools.dropwhile function. Here’s how to do it: ...

I needed to write a socket server in Python that would allow me to intermittently pause the server loop for a while, run something else, then get back to the previous request-handling phase; repeating this iteration until the heat death of the universe. Initially, I opted for the low-level socket module to write something quick and dirty. However, the implementation got hairy pretty quickly. While the socket module gives you plenty of control over how you can tune the server’s behavior, writing a server with robust signal and error handling can be quite a bit of boilerplate work. ...

Back in the days when I was working as a data analyst, I used to spend hours inside Jupyter notebooks exploring, wrangling, and plotting data to gain insights. However, as I shifted my career gear towards backend software development, my usage of interactive exploratory tools dwindled. Nowadays, I spend the majority of my time working on a fairly large Django monolith accompanied by a fleet of microservices. Although I love my text editor and terminal emulators, I miss the ability to just start a Jupyter Notebook server and run code snippets interactively. While Django allows you to open up a shell environment and run code snippets interactively, it still isn’t as flexible as a notebook. ...

I was working with a table that had a similar (simplified) structure like this: | uuid | file_path | |----------------------------------|---------------------------| | b8658dfc3e80446c92f7303edf31dcbd | media/private/file_1.pdf | | 3d750874a9df47388569a23c559a4561 | media/private/file_2.csv | | d177b7f7d8b046768ab65857451a0354 | media/private/file_3.txt | | df45742175d7451dad59761f15653d9d | media/private/image_1.png | | a542966fc193470dab84351c15523042 | media/private/image_2.jpg | Let’s say the above table is represented by the following Django model: from django.db import models class FileCabinet(models.Model): uuid = models.UUIDField( primary_key=True, default=uuid.uuid4, editable=False ) file_path = models.FileField(upload_to="files/") I needed to extract the file names with their extensions from the file_path column and create new paths by adding the prefix dir/ before each file name. This would involve stripping everything before the file name from a file path and adding the prefix, resulting in a list of new file paths like this: ['dir/file_1.pdf', ..., 'dir/image_2.jpg']. ...

At my workplace, I was writing a script to download multiple files from different S3 buckets. The script relied on Django ORM, so I couldn’t use Python’s async paradigm to speed up the process. Instead, I opted for boto3 to download the files and concurrent.futures.ThreadPoolExecutor to spin up multiple threads and make the requests concurrently. However, since the script was expected to be long-running, I needed to display progress bars to show the state of execution. It’s quite easy to do with tqdm when you’re just looping over a list of file paths and downloading the contents synchronously: ...

Django has a Model.objects.bulk_update method that allows you to update multiple objects in a single pass. While this method is a great way to speed up the update process, oftentimes it’s not fast enough. Recently, at my workplace, I found myself writing a script to update half a million user records and it was taking quite a bit of time to mutate them even after leveraging bulk update. So I wanted to see if I could use multiprocessing with .bulk_update to quicken the process even more. Turns out, yep I can! ...

I’ve just migrated from Ubuntu to macOS for work and am still in the process of setting up the machine. I’ve been a lifelong Linux user and this is the first time I’ve picked up an OS that’s not just another flavor of Debian. Primarily, I work with Python, NodeJS, and a tiny bit of Go. Previously, any time I had to install these language runtimes, I’d execute a bespoke script that’d install: ...

TIL that you can specify update_fields while saving a Django model to generate a leaner underlying SQL query. This yields better performance while updating multiple objects in a tight loop. To test that, I’m opening an IPython shell with python manage.py shell -i ipython command and creating a few user objects with the following lines: In [1]: from django.contrib.auth import User In [2]: for i in range(1000): ...: fname, lname = f'foo_{i}', f'bar_{i}' ...: User.objects.create( ...: first_name=fname, last_name=lname, username=f'{fname}-{lname}') ...: Here’s the underlying query Django generates when you’re trying to save a single object: ...

At my workplace, while working on a Lambda1 function, I noticed that my Python logs weren’t appearing on the corresponding Cloudwatch2 log dashboard. At first, I thought that the function wasn’t picking up the correct log level from the environment variables. We were using serverless3 framework and GitLab CI to deploy the function, so my first line of investigation involved checking for missing environment variables in those config files. However, I quickly realized that the environment variables were being propagated to the Lambda function as expected. So, the issue had to be coming from somewhere else. After perusing through some docs, I discovered from the source code of Lambda Python Runtime Interface Client4 that AWS Lambda Python runtime pre-configures5 a logging handler that modifies the format of the log message, and also adds some metadata to the record if available. What’s not pre-configured though is the log level. This means that no matter the type of log message you try to send, it won’t print anything. ...

Python makes it freakishly easy to load the whole content of any file into memory and process it afterward. This is one of the first things that’s taught to people who are new to the language. While the following snippet might be frowned upon by many, it’s definitely not uncommon: # src.py with open("foo.csv", "r") as f: # Load the whole content of the file as a string in memory and return it. f_content = f.read() # ...do your processing here. ... Adopting this pattern as the default way of handling files isn’t the most terrible thing in the world for sure. Also, this is often the preferred way of dealing with image files or blobs. However, overzealously loading file content is only okay as long as the file size is smaller than the volatile memory of the working system. ...

While working with GitHub webhooks, I discovered a common pattern1 a webhook receiver can adopt to verify that the incoming webhooks are indeed arriving from GitHub; not from some miscreant trying to carry out a man-in-the-middle attack. After some amount of digging, I found that it’s quite a common practice that many other webhook services employ as well. Also, check out how Sentry does it here2. Moreover, GitHub’s documentation demonstrates the pattern in Ruby. So I thought it’d be a good idea to translate that into Python in a more platform-agnostic manner. The core idea of the pattern goes as follows: ...

While going through the documentation of Python’s sqlite31 module, I noticed that it’s quite API-driven, where different parts of the module are explained in a prescriptive manner. I, however, learn better from examples, recipes, and narratives. Although a few good recipes already exist in the docs, I thought I’d also enlist some of the examples I tried out while grokking them. Executing individual statements To execute individual statements, you’ll need to use the cursor_obj.execute(statement) primitive. ...

TIL from this1 video that Python’s urllib.parse.urlparse2 is quite slow at parsing URLs. I’ve always used urlparse to destructure URLs and didn’t know that there’s a faster alternative to this in the standard library. The official documentation also recommends the alternative function. The urlparse function splits a supplied URL into multiple seperate components and returns a ParseResult object. Consider this example: In [1]: from urllib.parse import urlparse In [2]: url = "https://httpbin.org/get?q=hello&r=22" In [3]: urlparse(url) Out[3]: ParseResult( scheme='https', netloc='httpbin.org', path='/get', params='', query='q=hello&r=22', fragment='' ) You can see how the function disassembles the URL and builds a ParseResult object with the URL components. Along with this, the urlparse function can also parse an obscure type of URL that you’ll most likely never need. If you notice closely in the previous example, you’ll see that there’s a params argument in the ParseResult object. This params argument gets parsed whether you need it or not and that adds some overhead. The params field will be populated if you have a URL like this: ...

Over the years, I’ve used Python’s contextlib.ExitStack in a few interesting ways. The official documentation1 advertises it as a way to manage multiple context managers and has a couple of examples of how to leverage it. However, neither in the docs nor in GitHub code search2 I could find examples of some of the maybe unusual ways I’ve used it in the past. So, I thought I’d document them here. ...

While reading the second version of Brian Okken’s pytest book1, I came across this neat trick to compose multiple levels of fixtures. Suppose, you want to create a fixture that returns some canned data from a database. Now, let’s say that invoking the fixture multiple times is expensive, and to avoid that you want to run it only once per test session. However, you still want to clear all the database states after each test function runs. Otherwise, a test might inadvertently get coupled with another test that runs before it via the fixture’s shared state. Let’s demonstrate this: ...

I was reading Ned Bachelder’s blog “Why your mock doesn’t work”1 and it triggered an epiphany in me about a testing pattern that I’ve been using for a while without being aware that there might be an aphorism on the practice. Patch where the object is used; not where it’s defined. To understand it, consider the example below. Here, you have a module containing a function that fetches data from some fictitious database. ...