May 15th, 2019

Andre Roberge: Friendlier tracebacks

Python, by admin.

When beginners run programs that generate Python tracebacks, they are almost always confused by what the information shown and have no clue as to what this all means. More experienced programmers can sometimes extract enough information directly from tracebacks to figure out what what wrong, but they will often have to resort to inserting a few print calls and running their program again to truly figure out what went wrong and how to fix it. (A minority of programmers might eschew using print calls and use a debugger instead.)

In order to make tracebacks more useful for them, some advanced programmers have designed tools to add more information so that simply looking at the enhanced traceback might be sufficient to diagnose properly the problem.  These tools include better_exchook, infi.traceback, rich-traceback, stackprinter, as well as the beautiful better-exceptions, and many others including Python’s own cgitb module.  While there is no doubt that the additional information provided by these tools is useful for advanced programmers, it would likely only add to their confusion if it were used by beginners.

Friendly-traceback, a project I have been working on, and mentioned briefly in an addendum of my last post, aims at improving the situation for beginners.

A quick example


Have a look at the following Python traceback that might be generated by code written by a beginner.

Compare this with the following when using Friendly-traceback’s own REPL

The only thing that is shown in exactly the same way is the line showing the exception name an a message. Everything else can be made available in other languages as shown below with a French translation.



Currently, Friendly-traceback can offer a more helpful message than a normal Python traceback in approximately 50 different cases, most of which are cases of either TypeError or SyntaxError. My goal is to eventually include explanations for all standard Python exceptions, and include as many sub-cases as possible.

As shown above, it is possible to translate the information in any language. Currently, only English and French are included; inclusion of other languages will require the help of volunteers.

If you write programs with your own custom exceptions, it is possible to write them so that they could be interpreted correctly by Friendly-traceback.  I have done so in another project, AvantPy, which I mentioned previously here. AvantPy includes 10 custom exceptions.

Both AvantPy and Friendly-traceback are available from Pypi. Contributors are most definitely welcome.

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