Knowing that an error occurred is the first step to preventing and solving errors. There are many types of errors that PyScript applications will experience. Network failures, resources not being available and programming mistakes are just a few problems to manage.

I try to cover several important items when writing PyScript applications:

  • Program Structure
  • Example Application Template
  • JavaScript Debuggers
  • Python Debuggers
  • JavaScript Error Management
  • Python Error Management
  • Uncaught Python Exceptions
  • Strange PyScript Error Messages
  • PyScript Error Handling Feature Improvements

Program structure

I do not recommend mixing HTML with Python. Python error messages often contain a line number where the error occurred. That line number is relative to the start of the Python code and not the actual line number when mixed with HTML.

Separate the HTML from the Python code to make testing easier. By splitting the Python code into multiple files, you can test the Python code independently of the browser. Only the parts that interface with the browser need to be tested with the browser.

Load your main Python source file:

Load additional Python source files:

And import the required functions from those files:

Load required Python packages:

By structuring your application in a consistent manner and with functionality split into different modules, you will improve your ability to detect and solve problems that occur. I prefer that a customer has a useful error box with detailed information that he can report over “hey – your program does not work and there are no error messages“.

If you use separate development and QA teams, your QA team will appreciate the time you spent structuring the source code and providing informative messages for all error conditions. Your development team will appreciate bug reports that include source code file and line details.

Example Template to use for Applications

This example includes the file error-handler.js that I will explain later in this article.

Notice the onerror=scriptLoadFailure('pyscript.js') added to the script tag for loading pyscript.js. The function scriptLoadFailure() is located in error-handler.js. That function will catch network problems loading the PyScript JavaScript file, which I see once in a while developing.

JavaScript Debuggers

Most web browsers include a very good HTML, CSS, and JavaScript debugger. I will not cover how to use the web browser debugger as there are many excellent resources on the Internet.

Shortcut to open the debugger: CTRL + Shift + J.

Python Debuggers

At this time, there are no Python debuggers for PyScript. I write my Python code as independent layers. One that interfaces with the browser and another layer that can be executed independently. That way I can use traditional code testing and debugging tools such as Visual Studio Code or PyCharm.

A great tool to get into the habit of running against your code often is Pylint. This is a static code analyzer that can catch many mistakes and ensure code compliance. I create Makefiles that run Pylint every time I deploy my application for testing. There are a number of other useful tools listed on the Pylint page.

JavaScript Error Management

PyScript depends upon JavaScript to load and interface with the web browser. Catching network and JavaScript errors will be your first defense. Next, write solid Python code that catches its own errors. How you structure the main components (HTML, CSS, JavaScript, Python) will also impact your ability to manage errors.

There are several types of errors that you can catch. I wrote handlers for these error types and placed them in the file error-handler.js.

To catch errors, add an event listener for the error event.

The error event is fired on a Window object when a resource failed to load or couldn’t be used — for example, if a script has an execution error.

This handler displays an alert dialog box with source file information::

To catch script load failures, such as pyscript.js not loading, add an onerror=scriptLoadFailure('pyscript.js') to the script tag. Example:

The function scriptLoadFailure()then displays an alert dialog box.

To catch unhandled promise rejections, add an event listener for the unhandledrejection event.

The following code is the error-handler.jsthat I use for my applications during development:

Python Error Management

Since there are no Python debuggers available for the browser, you must be creative. Python errors often result in a poorly formed pink error message from PyScript. Then you must use the browser debugger to analyze the actual error. A better strategy is to use exception handling that catches errors for your entire application.

I put my application code into the old fashioned main()function and wrap that with a try/except block.

If you see the filename as <exec>, that means the file specified by the PyScript tag. For some reason, that is not preserved by the PyScript loader.

Example template with error handler:

Uncaught Python Exceptions


Pyodide parses the entire Python source file before execution. The Exception SyntaxError can only be caught in the following cases:

  • eval()
  • exec()
  • import

Strange PyScript Error Messages

Missing Python Source File

If you declare a Python file via the <py-script src="file"> tag and the file does not exist, PyScript displays a strange error:

Complete error message:

PyScript Error Handling Feature Improvements

  1. If a Python source file is specified via the <py-script src="file"> , the property exception_traceback.tb_frame.f_code.co_filenamefilename has the value <exec> instead of the filename specified by the src attribute.


If you implement one or more of my strategies for PyScript application development, you might find that you will have an improved development and testing experience. PyScript is new and will become very popular. As that popularity grows, more options will become available for IDEs, debuggers, and libraries.

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Photography Credit

I write free articles about technology. Recently, I learned about which provides free images. The image in this article is courtesy of Pixabay at Pexels.

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