Python string formatting: f-strings to the rescue

Just discovered f-strings for Python string interpolation – much, much better than any of the other formatting techniques such as .format or %.

Example:

print(f'{name} is {age} years old')
Jon is 21 years old

I’ve written a load of examples and notes in an interactive Jupiter notebook, mainly for my own reference:

https://gitlab.com/cds/pythonlearning/blob/master/fstring.ipynb

macOS, Visual Studio Code, Python 3.7.5, OpenCV4

It took a few attempts to get a compatible Python and OpenCV library running under Visual Studio Code on macOS Catalina using a virtual environment. I made a video to show how I got this going – this post just adds some more details.

There is also an excellent tutorial from Microsoft:

Getting Started with Python in VS Code

Note: virtual machine rendering problem

Visual Studio Code running on a virtual machine may have problems rendering the interface. This seems to be related to the underlying Electron framework and GPU acceleration. I made a quick video to show how I got around this:

Fix rendering problems for Visual Studio Code running on a virtual machine

Install Python 3.7.5

A virgin Mac comes with Python 2.7 installed – this is not recommended and V3.7.5 works with OpenCV4 on a Mac. V3.8 does not work at the time of writing (although since I started writing this post it looks like it now does). Download the installer from the main python website by selecting Downloads, Mac OS X, and then selecting the 64-bit installer:

Run the installer – I used all default settings.

Install Visual Studio Code

Download the installer from Visual Studio Code and immediately move the downloaded file to the Applications folder. (This is the actual application, not an installer). Try to run once – macOS will refuse due to security:

Close the message, open System Preferences, and select the Security and Privacy settings. Then select “Open Anyway” to allow VSC.

Visual Studio Code should now start:

Configure Python

Open a folder by selecting Open folder and then add a new file. Save the file using the .py extension:

Visual Studio Code immediately offers to install the Python extension, select Install:

On a virgin Mac there will now be a prompt to install command line developer tools, so click Install if prompted and allow the installation to complete before returning to Visual Studio Code.

The status bar will show the selected interpret if everything has gone well:

Install the linter (pylint): this helps analyse the code for bugs and style issues. It also might not work first time but we can fix shortly…

If the terminal window suggests upgrading pip, the Python package manager, then go for it by running the following in the terminal window:

python3 -m pip install --upgrade pip

Make a virtual environment

A virtual environment is a self-contained directory tree that contains a Python installation for a particular version of Python

https://docs.python.org/3/tutorial/venv.html

Each project can use its own virtual environment to ensure any modules it requires don’t clash with modules in other projects.

From the terminal create a virtual environment:

python3 -m venv .venv

Visual Studio Code will detect this new environment and offer to select it for the current project folder – select Yes:

Because this is a new Python environment you may need to install the linter again:

Now – the bit that confused me… the project is now using the .venv virtual environment:

However, the terminal session has so far only created the environment, it has not activated it for itself. The shell identifier says:

jon@Jons-MacBook-Pro Python %

There are two ways to fix this. First, using the source command in the terminal window:

source .venv/bin/activate

Second, by creating a new Terminal session using the command palette. (Select View, ten Command Palette):

Now the terminal shows that it’s using the virtual environment:

Install OpenCV

At last we can install OpenCV. Using the terminal session in the virtual environment we can first search for OpenCV packages:

python3 -m pip search opencv  

When called with -m module-name, the given module is located on the Python module path and executed as a script

https://docs.python.org/3/using/cmdline.html

We see results like this:

opencv-utils (0.0.2) – OpenCV Utilities
ctypes-opencv (0.8.0) – ctypes-opencv – A Python wrapper for OpenCV using ctypes
opencv-wrapper (0.2.3) – A Python wrapper for OpenCV.
opencv-cython (0.4) – An alternative OpenCV wrapper
dajngo-opencv (0.3) – Django Opencv integratio
opencv-python (4.1.2.30) – Wrapper package for OpenCV python bindings

For this test I’m using opencv-python. The details on version 4.1.2.30 can be found on the Python Package Index site. Interestingly this version was only released a few hours ago and says it supports Python 3.8 😬 I guess I’ll try this on a virtual machine first to check it’s all ok!

Install OpenCV using pip:

python3 -m pip install opencv-python

Write some code and fix the linter

First test: import the OpenCV module and print the library version.

import cv2
print('Using OpenCV version {0}'.format(cv2.__version__))

After running this output is shown in the terminal:

But – there’s a problem. In the editor the linter is suggesting that cv2 is not a known module:

This has been seen before on the pylint GitHub issues page. For me, the solution is to edit the .vscode settings. Using ⇧⌘E (shift+command+E) to view the explorer page, expand the .vscode file and click settings.json:

Add a comma to the end of the line of the existing setting, then add the following new setting:

"python.linting.pylintArgs": ["--generate-members"]

My settings now look like this:

And now the red squiggle has gone from cv2.__version__ 😀

All that remains is to learn Python and OpenCV which will surely lead to great things!

Hope this helps.

Windows system timer granularity

While running one of my apps on a Windows 10 VM I noticed that the timing was much different to that seen on the host PC. After lots of digging I finally found that the granularity of the system timer on the VM was around 16ms versus around 0.5ms on the host PC. My app is using some 1-5 millisecond sleeps but when the granularity is 16ms then 1ms becomes 16! (The actual granularity is 15.6ms due to a default 64Hz timer frequency).

Some cool resources on the web related to this:

Solved my problems by setting the granularity to the minimum supported by the PC; this setting remains in place until the application exits. So it just seems that my VM doesn’t have anything running that would otherwise cause the timer to run more quickly than the default (of 64Hz), whereas my development PC must have all sorts that are running the timer flat out; probably one reason my battery goes down more quickly than expected!

To query and change the granularity I used theses methods via C#:

I then wrote a little wrapper class to let me play with the timings using the .Net TimeSpan. Note: this is a frustrating struct to use because it really doesn’t want to use fractions of a millisecond without more than a bit of persuasion, specifically because FromMilliseconds will only consider the requested value to the nearest millisecond.

/// <summary>
/// Utility to query the timer resolution
/// </summary>
class TimerResolution
{
    [DllImport("ntdll.dll", SetLastError = true)]
    private static extern int NtQueryTimerResolution(out int MinimumResolution, out int MaximumResolution, out int CurrentResolution);


    [DllImport("ntdll.dll", SetLastError = true)]
    private static extern int NtSetTimerResolution(int DesiredResolution, bool SetResolution, out int CurrentResolution);


    private static TimeSpan TimeSpanFrom100nsUnits(int valueIn100nsUnits)
    {
        var nanoseconds = (double)valueIn100nsUnits * 100.0;
        var seconds = nanoseconds / 1000000000.0;
        var ticks = seconds * System.Diagnostics.Stopwatch.Frequency;
        var timeSpan = TimeSpan.FromTicks((long)ticks);
        return timeSpan;
    }


    private static (TimeSpan min, TimeSpan max, TimeSpan cur) Query()
    {
        NtQueryTimerResolution(out var min, out var max, out var cur);
        return (min: TimeSpanFrom100nsUnits(min), max: TimeSpanFrom100nsUnits(max), cur: TimeSpanFrom100nsUnits(cur));
    }


    /// <summary>Gets the minimum timer resolution</summary>
    public static TimeSpan MinResolution => Query().min;


    /// <summary>Gets the maximum timer resolution</summary>
    public static TimeSpan MaxResolution => Query().max;


    /// <summary>Gets/sets the current timer resolution</summary>
    public static TimeSpan CurrentResolution
    {
        get { return Query().cur; }

        set
        {
            var valueInSeconds = value.TotalMilliseconds / 1000.0;
            var valueInNanoseconds = valueInSeconds * 1000000000.0;
            var valueIn100Nanoseconds = (int)(valueInNanoseconds / 100.0);
            NtSetTimerResolution(DesiredResolution: valueIn100Nanoseconds, SetResolution: true, out _);
        }
    }
}

A little test app on my VM produced these results…

Minimum resolution:   15.6ms
Maximum resolution:   0.5ms
Current resolution:   15.6ms

Attempt to change to 2ms
Current resolution:   00:00:00.0020000
DateTime granularity: 00:00:00.0020970
Sleep 0:              00:00:00.0000009
Sleep 1:              00:00:00.0020053

Attempt to change to 5ms
Current resolution:   00:00:00.0050000
DateTime granularity: 00:00:00.0050328
Sleep 0:              00:00:00.0000012
Sleep 1:              00:00:00.0049719

Attempt to change to 0.5ms
Current resolution:   00:00:00.0005000
DateTime granularity: 00:00:00.0005471
Sleep 0:              00:00:00.0000008
Sleep 1:              00:00:00.0011774

Attempt to change to 15.6ms
Current resolution:   00:00:00.0156250
DateTime granularity: 00:00:00.0156280
Sleep 0:              00:00:00.0000011
Sleep 1:              00:00:00.0155707

Scanning receipts from iPhone to OneNote (App Store)

There are many ways to scan a document, such as a receipt, and import it into OneNote. I’m now using the App Store version of OneNote on Windows10 and one of the (many) limiations is the inability to resize large images without having to cut them out, edit, and paste back in.

I’ve tried a bunch of scanning apps on the iPhone and one of the main issues is finding something than can scan, including auto-detection of document borders, adjust brightness and contrast, and send to OneNote. The Adobe Scanner app for the iPhone is awesome but the PDF appears in OneNote as an icon and doesn’t appear to want to change into a readable document!

So for now the fastest way I’ve found is to use the Microsoft Office Lens iPhone app – this does a great job of scanning and although I can’t change the image resolution I can set it as a simple black and white image and send directly to OneNote.

First, run Office Lens.

1

 

Then let it find your document – it helps to have some natural contrast between the edges of the document and the background. 

Make sure that the Document option is selected:

Display the filters after scanning the document – just slide your finger up to view them:

I’m choosing the black and white option as it seems to help keep the file size down and also makes the receipt more readable:

After applying the filter click on the Done button at the bottom and go through the export options:

Enter a title for the note and choose which section on OneNote to save it to:

 

It would be a great improvement if the Office Lens app could have a quality or file size option to reduce the amount of data stored in ON. There are already suggestions for this on the Office Lens feedback hub going back to 2015 – not sure if anyone’s listening tho!

Visual Studio 2013 to 2015, MSB3884, Could not find rule set file

After loading a VS2013 C# project in VS2015 I got the dreaded MSB3884 warning (Could not find rule set file). A little annoying since I wasn’t knowingly using code analysis (but I might now).

Here’s what I did to fix the warning:

  1. On the project properties go to the Code Analysis section.
  2. Select the Browse option on the list of rule sets:
    W10_Test.png
  3. Browse to the VS2015 rule sets folder:
    C:\Program Files (x86)\Microsoft Visual Studio 14.0\Team Tools\Static Analysis Tools\Rule Sets
  4. Pick a rule set.. the default used by new projects is:
    MinimumRecommendedRules.ruleset
  5. The new rule set is applied:
    W10_Test.png

Then rebuild – problems gone !