Wednesday, December 25, 2019

Xmas present from Thonny

Today, a new version (3.2.5) of Thonny has been released. It incorporates support for Friendly-traceback (which needs to be installed separately). Currently, the download link on Thonny's homepage still links to version 3.2.4. The latest version can be found on Github.

Thonny is a fantastic IDE for beginners, especially those learning in a classroom environment, as it offers many useful tools that can be used effectively by teachers to demonstrate some programming concepts.  Thonny is the work of Aivar Annamaa, who is apparently recognized as an excellent lecturer -- which does not suprise me given the thoughtful design of Thonny. He has been interviewed about Thonny on PythonPodcast.

Real Python has a fairly comprehensive review here.

Saturday, December 14, 2019

A Tiny Python Exception Oddity

Today, while working on Friendly-traceback (improved documentation !) as I have been doing a lot recently, I came into an odd SyntaxError case:

  • The inconsistent behaviour is so tiny, that I doubt most people would notice - including myself before working on Friendly-traceback.
  • This is SyntaxError that is not picked up by flake8; however, pylint does pick it up.
  • By Python, I mean CPython.  After trying to figure out why this case was different, I downloaded Pypy and saw that Pypy did not show the odd behaviour.
  • To understand the origin of this different behaviour, one needs to look at some obscure inner parts of the CPython interpreter.
  • This would likely going to be found totally irrelevant by 99.999% of Python programmers. If you are not the type of person who is annoyed by tiny oddities, you probably do not want to read any further.
You have been warned.

Normal behaviour

When Python finds a SyntaxError, it flags its location.  Let's have a look at a simple case, using CPython 3.7.

Notice how it indicates where it found the error, as shown by the red arrow: this happened when it reached a token that was inconsistent with the code entered so far. According to my experience until today, this seemed to be always the case.  Note that using CPython 3.6 yields exactly the same behaviour, and unhelpful error message.

Before discussing the case with a different behaviour, let's make a detour and look at Pypy's handling of the same case.

Same location indicated, but a much more helpful error message, even though this is version 3.6.  This improved error message was discussed in this Pypy blog post.  I strongly suspect that this is what lead to this improved error message in CPython 3.8.

Same error message as Pypy ... but the exact location of the error, previously indicated by ^, no longer appears - which could be unfortunate when nested parenthesis (including square and curly brackets) are present.

What about Friendly-traceback you ask? I thought you never would! ;-)  

Well, here's the information when using CPython 3.7.

The line about not having enough information from Python refers to the unhelpful message ("invalid syntax"). Hopefully you will agree that the information given by Friendly-traceback would be generally more useful, and especially more so for beginners.   

But enough about this case. It is time to look at the odd behaviour one.

Odd case

Consider the following:

Having a variable declared both as a global and nonlocal variable is not allowed.  Let see what happens when this is executed by Pypy.

So, pypy processed the file passed the nonlocal statement and flagged the location where it encountered a statement which was inconsistent with everything that had been read so far: it thus flagged that as the location of the error.

Now, what happens with CPython:

The location flagged is one line earlier. The nonlocal statement is flagged as problematic but, reading the code up to that point, there is no indication that a global statement was encountered before.

Note that, changing the order of the two statements does not change the result: pypy shows the beginning of the second statement (line 6) as the problem, whereas CPython always shows the line before.

Why does it matter to me?

If you go back to the first case I discussed, with the unmatched parenthesis, in Friendly-traceback, I rely on the location of the error shown by Python to indicate where the problem arose and, when appropriate, I look *back* to also show where the potential problem started.  Unfortunately, I cannot do that in this case with CPython.

Why is this case handled differently by CPython?

While I have some general idea of how the CPython interpreter works, I absolutely do not understand well enough to claim with absolute certainty how this situation arise.  Please, feel free to leave a comment to correct the description below if it is incorrect.

 My understanding is the following:

After breaking down a file into tokens, parsing it according to the rules of the Python grammar, an abstract syntax tree (AST) is constructed if no syntax error is found.  The nonlocal/global problem noted is not picked up by CPython up to that point - which also explains why flake8 would not find it as it relies on the AST, and does not actually executes the code.  (I'm a bit curious as to how Pylint does ... I'll probably have to look into it when I have more time).

Using the AST, a control flow graph is created and various "frames" are created with links (GOTOs, under a different name...) joining different parts.  It is at that point that relationships between variables in different frames is examined in details.  Pictorially, this can be represented as follows:

(This image was taken from this blog post by Eli Bendersky)  In terms of the actual code, it is in the CPython symtable.c file. At that point, errors are not found by scanning lines of code linearly, but rather by visiting nodes in the AST in some deterministic fashion ... which leads to the oddity mentioned previously: CPython consistently shows the first of two statements as the source of the problem, whereas Pypy (which relies on some other method) shows the second, which is consistent with the way it shows the location of all SyntaxError messages.


For Friendly-traceback, this likely means that for such cases, and unlike the mismatched parenthesis case, I will not attempt to figure out which two lines are problematic, and will simply expand slightly on the terse one liner given by Python (and in a way that can be translated into languages other than English).

Sunday, December 08, 2019

pydeps: a very useful program

A few weeks ago, I was doing some refactoring of Friendly-traceback and had some minor difficulty in avoiding the creation of circular imports.  For some reason (age perhaps), I could not visualize the file structure properly.  Enter pydeps.  After I used it to generate a graph for all the files internal to Friendly-traceback, I was able to use that graph to figure out a better way to structure my program.

Today, as I stared at that graph, after including it in the newly styled documentation, I noticed that the "version" file I had created early on, was really redundant since its content (a single variable) could easily be incorporated in the Public API file.

So, one less file to deal with!

I think I am going to use pydeps a lot more from now on when I want to try to understand the how projects are structured, as I do find this type of graph very useful.

Thursday, December 05, 2019

Significant changes for some error messages in Python 3.8

As I work on including more exceptions in Friendly-traceback, I am mostly pleasantly surprised by generally more precise error messages.  For example, in Python 3.7, the following

__debug__ = 1

would yield "SyntaxError: assignment to keyword" which likely would baffle almost everyone looking up the list of Python keywords.   In Python 3.8, that message has been replaced by the more precise: "SyntaxError: cannot assign to __debug__". Much better, in my opinion, even though one may be surprised to learn about this constant.

However, today as I was working on adding another case, I came accross the following:

This change is ... unexpected.  And the "helpful hint", is not so helpful in this case.  However, I can guess as to how it came about.  It will be a challenge to provide a "friendly" explanation that does not lead the users looking for an incorrect solution to their problem.

Edit: Based on my own (limited) experience working on Friendly-traceback, I do realize that trying to provide helpful hints to assist programmers in fixing code that raises exceptions is a very difficult problem.  My task, with Friendly-traceback, is generally made much easier thanks to accurate and usually helpful error messages provided by Python. So, please do not read this post and conclude that I am dismissive of the efforts of the Python core developers in this area. 

Tuesday, December 03, 2019

Friendly-traceback, Real Python, Pycon, and more

After an interruption that lasted a few months, I've finally been able to return to programming, more specifically working mostly on Friendly-traceback. For those that do not know Friendly-traceback: it aims to replace the sometimes obscure traceback generated by Python with something easier to understand. Furthermore, Friendly-traceback is designed with support for languages other than English so that, in theory, beginners (who are the main target audience for Friendly-traceback) could benefit no matter what their native language is ... provided someone would have done the translation into that language, of course.

As of now, 75 different cases have been tested; you can find them in the documentation.  [If you have suggestions for improvements, please do not hesitate to let me know.]

Recently, a post by Real Python on SyntaxError has given me added impetus to work on Friendly-traceback. I'm happy to report that, other than the cases mentioned dealing with misspelled or missing keywords, all of the other examples mentioned in that post can be analyzed by Friendly-traceback with an appropriate explanation provided. Note that these are not hard-coded examples from that post, so that any similar cases should be correctly identified.

Friendly-traceback works with Python 3.6, 3.7 and 3.8.  As I included support for 3.8, I found that some error messages given by Python changed in this newer version, and were generally improved. However, this meant that I had to change a few things to support all three versions.

Working on Friendly-traceback, and on AvantPy, has been so far a fun learning experience for me. I was hoping and looking forward to submit a talk proposal dealing with both these project to the Pycon Education Summit, as I thought that both projects would be of interest to Python educators. However, the call for proposals is focused on people's experience with actual teaching case studies about how teachers and Python programmers have implemented Python instruction in their schools, communities, and other places of learning ... So, definitely no interest in talks about tools like those I create. I certainly do understand the reason for this choice, but I cannot help but feeling disappointed as I was definitely hoping to get an opportunity to give a talk on these projects, and exchange ideas with interested people afterwards.

I did submit a proposal for a reasonably advanced and more technical talk dealing with import hooks and exception hooks, to share what I have learned (while working on Friendly-traceback and AvantPy) with the Pycon crowd. The last time I gave a talk at Pycon was in 2009 and the "competition" to have a talk accepted was much less than what it is now.  Giving a talk is the only way that I can justify taking a leave from my day job to attend Pycon, something I really miss.

Back to Real Python ... I remember purchasing some books from them some time in 2014, and, more recently, I did the same for the "course" on virtual environments. I had never bothered with virtual environments until recently and thought that, if I actually paid to get some proper tutorial, I would have no excuse not to start using virtual environments properly.  The "course" that I bought was well put together.  Compared to standard books, I find it a bit overpriced for the amount of material included. 

As a pure Python hobbyist, I appreciate the material Real Python make freely available, but do find their membership price rather steep.  However, I did note that their tutorial writers could get free access to their entire collection ... 

;-) Perhaps I should offer to write tutorials on 1) using import hooks; 2) using exception hooks; 3) designing libraries with support for translations in a way that they "play well together" -- all topics I had to figure out on my own.  While there are tutorials about translation support, I found that all of them give the same gettext-based approach of defining a global function named "_" which works very well for isolated packages, but can fail spectacularly in some corner cases as I found out while developing Friendly-traceback and AvantPy. 

However, writing clear tutorials takes a lot of time and effort, and is not as fun to me as writing code. So, I think that, for now, I'll just go back to add support for more Python exceptions in Friendly-traceback - and hope that I will have soon to focus my entire free time in putting together material for a Pycon talk.

Tuesday, June 04, 2019

Abolishing SyntaxError: invalid syntax ...

... and other cryptic messages.

Do you remember when you first started programming (possibly with Python) and encountered an error message that completely baffled you? For some reason, perhaps because you were required to complete a formal course or because you were naturally persistent, you didn't let such messages discourage you entirely and you persevered. And now, whenever you see such cryptic error messages, you can almost immediately decipher them and figure out what causes them and fix the problem.

Congratulations, you are part of an elite group! Even a large number of people who claim that they can program are almost certainly less capable than you are.

Given your good fortune, would you mind donating 5 to 10 minutes of your time to help countless beginners that are struggling in trying to understand Python error messages?  All you need to do is:

  1. Glance through of exceptions on this page and use your experience to find a case not covered. Note that this excludes SyntaxError cases, some of which are included here, but would require more of your time.
  2. Fire up your favourite Python REPL and write some simple code that generates an exception not already covered.  Perhaps, something like ValueError: could not convert string to float: 'a'
  3. Create a new issue with the error message as the title, including the code that generated the exception in the description of the issue together with a simple explanation (in a couple of sentences) of what the error message means.  Imagine that you are writing an explanation for the twelve year old child of your best friend who has expressed some interest in learning how to program.  This simple explanation is the most important part ... however, do not worry about getting it absolutely perfect as it will likely be improved upon based on feedback from future "real beginners".
  4. Go back to whatever you were doing before, knowing that the few minutes you have invested will cumulatively save many hours to future generation of programmers that encounter the exception you wrote about.
It should go without saying that contributions that require more time and effort that what is described above are also very welcome!  If you feel particularly ambitious, you can certainly improve the existing code that currently analyses cases of SyntaxError: invalid syntax, which currently handles only a few cases, and should be seen more as a prototype/proof-of-concept.

Future plans for friendly-traceback

Friendly-traceback is being written so that it could be easily incorporated into editors or IDEs that are designed for beginners. I intend to ensure that it can be easily added to Python's IDLE, as well as Mu and Thonny. I hasten to add that Thonny already includes an excellent tool (its "Assistant") which provides amazing feedback to beginners in some cases. Thonny's assistant uses a complementary approach to that of Friendly-traceback and it is quite likely that a future version of Friendly-traceback will include and expand upon the type of analysis performed by Thonny's assistant to help beginners. However, for the moment, the current development of Friendly-traceback is focused on breadth of coverage (i.e. increasing the number of exceptions included), providing a single most-likely explanation for each exception, rather than looking at multiple possible causes for a given exception as is done by Thonny's Assistant.

Friday, May 24, 2019

Avant-IDLE: an experiment

[Edit: this post has an embedded video, which is not visible for PlanetPython readers.]

This is a follow-up from a previous post where I showed just a screenshot made after one hacking session.  A week later, much has changed.

As noted at the end of the video, the code is not (yet) available in a public repository.  Among other things, I need to figure out what license I can use and if I can reuse all of the content from Python's version, or if I need to remove certain assets, etc.

Avant-IDLE makes use of two projects that I started recently and mentioned before on this blog:

Friendly-traceback (code, documentation) and AvantPy (code, documentation). AvantPy itself depends on Friendly-traceback.

Both Friendly-traceback and AvantPy welcome contributions.  Publishing this video about AvantPy is a way to bring more attention to these other two projects.

Going forward with Avant-IDLE

As for Avant-IDLE, if I can go forward with it:

  • I would like to translate its menus.
  • I would like to add syntax coloring for each individual dialect: this would likely mean some significant change from the way that syntax coloring is done in IDLE.
  • I would like to have friendly tracebacks make use of color, like iPython does.
  • I would like to add line numbers to the file editor.
  • etc.
I might have to remove the debugger, as it makes little sense to have someone entering code in their native language and suddenly have to deal with standard Python since that is the code that is truly executed.  I think that, by the time someone is ready to use a debugger, they are definitely ready to use Python itself.

In an ideal world, save for the "dialect converter" demonstrated in the video, I would like for the rest of Avant-IDLE to be in a single window, with files in tabs and the interpreter below, like Al Sweigart described:

However, that last idea might be something simply too ambitious. After all, Al Sweigart, who is far more competent and productive than I am, never managed to get that project off the ground. The more the code base from Avant-IDLE diverges from that of IDLE itself, the more difficult it will become to incorporate code changes made by Python's core developers to improve IDLE.

Making the code public

Even if I figure out all I need to do to make Avant-IDLE public, I am somewhat hesitant in doing so.

There is no doubt in my mind that I can continue working and improving both Friendly-traceback and AvantPy.  I truly believe that Friendly-traceback could be very helpful for Python beginners.  It could be integrated in other beginner-friendly editors, like Mu or Thonny - two fantastic projects.

I think that AvantPy could be useful for beginners as well ... but I do admit that it is a bit quirky.

When it comes to Avant-IDLE itself, as the saying goes, I don't want to bite more than I can chew, and find out that I created something which I cannot maintain. I do realize my limitations: I am just a hobbyist with no formal training in programming and who likes to do quirky experiments in his spare time. Publishing code on Github and/or making it available at Pypi automatically raises people's expectations, and demands on one's time.

Still, if this video can inspire you to create something useful for beginners, it will have been worthwhile.  Even more so if it inspires you to contribute to either Friendly-traceback or AvantPy, ;-)


If you have any constructive criticism, or suggestions to offer, please feel free to do so, either on this blog, or by email.

Friday, May 17, 2019

Idle hacking - part 1

  1. File test_py.pyfr.  The pyfr extension indicates that this is written in the "French Python" dialect. Since this is the main file we'll run, it will also automatically set the language to French. This is all done by AvantPy.
  2. "afficher" is the French version of "print"
  3. "import" is the normal Python keyword; we can mix and match keywords from a given dialect with the normal Python ones.  Here, we are importing a module whose base name is "upper".
  4. The constant "a" is defined.
  5. File "upper.pyupper"; the only file whose base name is "upper" - thus, the one that will be imported. The "pyupper" extension indicates that the dialect is an "upper case" version of Python - designed for testing.
  6. The code that will be run when the file is imported.
  7. Output from both files.
  8. "a" is defined.
  9. We made a syntax mistake. However, Friendly-traceback is there to help us understand what went wrong ... picking up the default language to use (French) from that first file that was run.

All this was run with a crudely hacked version of Idle. I'm still confused with the communications done with the RPCServer and the TPCServer, having never looked at similar code before.  So, I haven't been able to make it do all that I wanted in this first hacking session. Hopefully, there will be more to come ...

Wednesday, May 15, 2019

Friendlier tracebacks

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.

Friday, April 05, 2019

AvantPy needs you

Update: Since this blog post was published, and before Pycoder's weekly mentioned it bringing in quite a few visitors, I had decided to carve out the friendlier tracebacks into a separate project. If you are interested in this idea, you can have a look at the design document and feel free to comment.  Work on AvantPy itself has been paused until friendly-traceback's API is completely implemented.  The API itself is easy to do - the tedious work is adding in an explanation for every possible Python exception, and translating each of them ...

Imagine that you are either learning Python or helping someone that is learning Python. It is almost a certainty that you will see some Python tracebacks, often much more complicated to decipher than

Imagine if you could see something like the following instead:

Or, if you speak French:

Friendlier tracebacks, translated into various languages, is only one of many things that AvantPy can do to help beginners learn programming.

AvantPy is very much in its infancy. You can think of it as a proof-of-concept that can be built upon.

PSF's request for proposal

Yesterday, on the PSF's blog, some information about a request for proposal has been posted.
I have jotted down some thoughts about AvantPy in this context. If you wish to add your own thoughts on this, you can do so here. I do not wish to get any direct funding from the PSF for this project. However, some potential collaborators might need some financial support depending on their goals.

What is AvantPy

Here are some useful links if you want to find out more about AvantPy.

Contributors to the AvantPy project are very much welcome.