Journey's End

Datetime Display in Django

Suppose you have instruments in different timezones taking readings. You might have the following Django data model:

class Instrument(models.Model):
    timezone_utc_offset_hours = models.FloatField(...)
    ...

class Reading(models.Model):
    instrument = models.ForeignKey(Instrument, ...)
    timestamp = models.DateTimeField(auto_now_add=True)
    ...

Furthermore suppose settings.py contains the following:

...
USE_TZ = True
TIME_ZONE = "Europe/Paris"
...

It would be convenient to be able to display the timestamp for Readings in the timezone of the instrument. To do this, we need to convert the UTC timestamps stored in the database into local time of each instrument:

import datetime

class Instrument(models.Model):
    ...
    @property
    def timezone(self) -> datetime.timezone:
        utc_offset = datetime.timedelta(hours=self.timezone_utc_offset_hours)
        return datetime.timezone(utc_offset)

class Reading(models.Model):
    ...

    @property
    def local_timestamp(self) -> datetime.datetime:
        return self.timestamp.astimezone(self.instrument.timezone)

However if you were to display local_timestamp in a template, e.g. {{a_reading.local_timestamp | time}}, you will get the equivalent time in Paris, not the time where the instrument is located. This is because, when USE_TZ is set, TIME_ZONE in settings controls the default time zone used to display datetimes.

This time zone conversion can be made explicit by using a custom time format that includes the time zone offset: {{a_reading.local_timestamp | time:"H:i O"}}. This will produce something like

16:52 +0200

regardless of the value of timezone_utc_offset_hours due to automatic conversion to TIME_ZONE. In order to display the instrument-local time, we have to disable this automatic conversion using the localtime template tag:

{% load tz %}
...
{% localtime off %}
...
{{ a_reading.local_timestamp | time:"H:i O" }}
...
{% endlocaltime %}

With this change the display time is now in the instrument's timezone:

00:52 +1000

Template Tag Scoping

Note that due to the way templates work, {% localtime off %}...{% endlocaltime %} must be inside the same block as the time display. If specified at the template level, e.g.

{% extends "base.html" %}
{% load tz %}
{% localtime off %}
{% block page-content %}
...
    {{ a_reading.local_timestamp | time:"H:i O" }}
...
{% endblock %}
{% endlocaltime %}

It will have no effect inside the page-content block!

Adventures in Compressed DNG

I have been working with industrial cameras and frame-grabbers that simply gives me a dump of their sensor data. To make that data useful it needs to undergo demosaicing, ideally using one of the modern demosaic algorithms like RCD rather than simple interpolation or the rather outdated VNG. Unfortunately the …

Python One-liner for Local QR Code Decoding

Because KeepassXC lacks the ability to decode QR code for TOTP setup and not all MFA implementations show the required secret in text format, I use the following snipet to read the QR code information locally:

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Unexpected Latency in Tinyproxy

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LogLevel Info …

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Installing python package arcgis in conda on Apple Silicon

In order to install the arcgis package in conda, you have to specify the architecture osx-64 otherwise the package will not be found. This works because MacOS will capable of running x86_64 code on Apple Silicon processors - that is how efficient they are!

conda install -c esri/osx-64 arcgis

In …

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Backups are a great idea, and encrypted backups are a even better idea. When backing up iOS devices on macOS, it is possible to set an encryption key to protect local files. There is however no straightforward GUI method to verify the encryption key.

Enter mvt:

conda create -n mvt …

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Due to unexpected power loss or disconnection of the underlying storage, the filesystem inside Veracrypt volumes is desired to prevent future data loss.

On macOS I tried using 

diskutil verifyDisk /dev/diskXXX

This didn't  work for some reason, most likely because of some interaction with veracrypt. Same results via Disk …

TigerVNC and Systemd

A quick systemd service definition file for tigervnc on Ubuntu 20.04

[Unit]
Description=tigervnc

[Service]
Type=forking
ExecStart=/usr/bin/vncserver -localhost no -depth 24 -geometry 1920x1080 -noreset
User=steve

[Install]
WantedBy=multi-user.target

Key components:

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Kopia: Handling Directories That No Longer Exist

Scenario

1. There exists a policy that snapshots directory ABC 
2. Directory ABC now no longer exists
3. You want to keep existing snapshots
4. You want to stop kopia from attempting to take any more snapshots

In order to achieve (4):

1. Set the policy to manual only
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The main issues:

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I wrote an alternative lecroy.py that should be a drop-in replacement. It  handles 16 bit waveforms and exports a LecroyBinaryWaveform …

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Method Forwarding in Objective-C Pitfall

A common pitfall of trying to use message forwarding in Objective-C is forgetting to implement

- (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector

in addition to

- (void)forwardInvocation:(NSInvocation *)anInvocation

Another problem is that sometimes calling objects will use

- (BOOL)respondsToSelector:(SEL)aSelector

to determine if your proxy object can respond to a particular …

Permission Denied When Programming Arduino Micro/SparkFun Pro Micro

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The usual solutions to this is to add oneself to the dialup group, or mess around in …

Notes on Migrating a Linux Install

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Thinkpad Mute Buttons and Xubuntu

I recently acquired a used Lenovo X220 for use as a linux laptop, and needed the following in my openbox configuration XML to make the hardware speaker and mic mute buttons work:

  <!-- Modified for X220 -->
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    <action name="Execute">
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    </action>
  </keybind>
  <keybind …

Simple FM Receiver with GNU Radio and RTL-SDR

Introduction