Thursday, October 27, 2022

Better NameError messages for Python

Python 3.11 is barely out and already the 3.12 alpha has some improvements for NameError messages. I suspect that these will be backported to 3.11 in time for the next release. 

On Ideas Python Discussion, Pamela Fox suggested that it might be useful to consider potential missing import when a NameError was raised. Thus, instead of having

>>> stream = io.StringIO()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'io' is not defined. Did you mean 'id'?

one would see

>>> stream = io.StringIO()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'io' is not defined. Did you mean 'id'? Or did you forget to import 'io'?

Of course, something like this was already done by friendly/friendly-traceback (aka Friendly). However, in this particular case, the information provided by Friendly contained too much information; this has been since fixed.

To no one's surprise, Pablo Galindo Salgado came up with a version of this for Python, where names found in sys.stdlib_module_names were considered and potentially added, with the result as initially suggested by Pamela Fox. Pamela then made a second suggestion to see if names of popular third-party libraries could also be considered. This, for now, appears to be out of scope for Python.

This set the stage for a friendly (pun intended) competition...

I decided to revise what I had done for Friendly in such cases and found some room for improvements. First, let's look at a couple of examples (with screenshots) of the new behaviour for Python.

As we can see with the first example, Python first makes suggestions about potential typos ('io' instead of 'id') followed by the suggestion about a missing import. Note that 'id' is a builtin who is never used with a dotted attribute.

The second example suggest a missing import only. However, as I am using Windows, this module does not exist.

Can Friendly do better?  Note that Friendly can be used with Python 3.6+ (including Python 3.12), all of which would show the same output. I've chosen to use Python 3.10 for this example, as I will explain near the end of this post.

The message included in the Python traceback does not include the additional hint about a missing import in this case. However, Friendly adds it on its own.  Note that it does not suggest 'id' as a potential typo. But what if we had made such a typo?

Here, Friendly does make the suggestion about a potential typo.  What about the second example given above?

Friendly also uses sys.stdlib_module_names initially, but also check with importlib.util.find_spec() to see if the module can be located.

It can also find potentially relevant third-party modules that are installed, but not yet imported.

Using importlib.util.find_spec() allows us to implement Pamela Fox's suggestion about suggesting third-party modules that are installed.

However, we can do even better with some dedicated code. To demonstrate this, I need to use the latest addition to the "friendly-traceback family" - which I have only tested with Python 3.10 so far.

I'll likely have more to say about friendly_pandas in the near future.

Final thoughts

For those excited about the improved traceback with Python 3.11 and PEP 657: Fine-grained error locations in tracebacks, but cannot yet install Python 3.11, please note that Friendly can already something similar, if not better with any Python version 3.6.1+

I say "better" because, unlike Python's traceback, the information is not limited to a single line of code:

Tuesday, October 18, 2022

pandas' SettingWithCopyWarning: did I get it right?

 I am just beginning to learn pandas and am looking to provide some automated help. From what I read, it appears that SettingWithCopyWarning is something that confuse many people. Is the following correct?

In [2]:
df = pd.DataFrame([[10, 20, 30], [40, 50., 60]],
In [3]:
df.loc["b"]["x"] = 99
A value is trying to be set on a copy of a slice from a DataFrame

See the caveats in the documentation:
In [4]:
# What is SettingWithCopyWarning ?
Pandas occasionally emits a SettingWithCopyWarning when you use       
'chained indexing', either directly or indirectly,and you then attempt
to assign a value to the result. By 'direct chained indexing', we mean
that your code contains something like:                               

...[index_1][index_2] = ...                                           

During the first extraction using [index_1], pandas found that the    
series to be created contained values of different types. It          
automatically created a new series converting all values to a common  
type. The second indexing, [index_2] was then done a this copy instead
of the original dataframe. Thus, the assigment was not done on the    
original dataframe, which caused Pandas to emit this warning.         

An 'indirect chained indexing' essentially amount to the same problem 
except that the second indexing is not done on the same line as that  
which was done to extract the first series.                           
In [5]:
# Can I get more specific information for what I just did?
You used direct chained indexing of a dataframe which made a copy of  
the original content of the dataframe. If you try to assign a value to
that copy, the original dataframe will not be modified. Instead of    
doing a direct chained indexing                                       

df.loc["b"]["x"] ...                                                  


df.loc["b", "x"] ...                                                  
In [6]:
# What about if I tried to use indirect chaining. 
# There are two possibilities
series = df.loc["b"]
series["x"] = 99
A value is trying to be set on a copy of a slice from a DataFrame

See the caveats in the documentation:
In [7]:
Warning issued on line 4 of code block [6].                                                         

       1| # What about if I tried to use indirect chaining.  
       2| # There are two possibilities
       3| series = df.loc["b"]
     > 4| series["x"] = 99
In [8]:
I suspect that you used indirect chained indexing of a dataframe.     
First, you likely created a series using something like:              

series = df.loc[...]                                                  

This made a copy of the data contained in the dataframe. Next, you    
indexed that copy                                                     


This had no effect on the original dataframe. If your goal is to      
modify the value of the original dataframe, try something like the    
following instead:                                                    

df.loc[..., "x"]                                                      
In [9]:
# What if I do things in a different order
series_1 = df["x"]
series_1.loc["b"] = 99
A value is trying to be set on a copy of a slice from a DataFrame

See the caveats in the documentation:
In [10]:
Warning issued on line 3 of code block [9].                                                         

       1| # What if I do things in a different order
       2| series_1 = df["x"]
     > 3| series_1.loc["b"] = 99
In [11]:
I suspect that you used indirect chained indexing of a dataframe.     
First, you likely created a series using something like:              

series_1 = df[...]                                                    

This made a copy of the data contained in the dataframe. Next, you    
indexed that copy                                                     


This had no effect on the original dataframe. If your goal is to      
modify the value of the original dataframe, try something like the    
following instead:                                                    

df.loc[..., "b"]                                                      
In [12]:
# What if I had multiples data frames?
df2 = df.copy()
series = df.loc["b"]
series["x"] = 99
A value is trying to be set on a copy of a slice from a DataFrame

See the caveats in the documentation:
In [13]:
Warning issued on line 4 of code block [12].                                                        

       2| df2 = df.copy()
       3| series = df.loc["b"]
     > 4| series["x"] = 99
In [14]:
In your code, you have the following dataframes: {'df2', 'df'}. I do  
not know which one is causing the problem here; I will use the name   
df2 as an example.                                                    

I suspect that you used indirect chained indexing of a dataframe.     
First, you likely created a series using something like:              

series = df2.loc[...]                                                 

This made a copy of the data contained in the dataframe. Next, you    
indexed that copy                                                     


This had no effect on the original dataframe. If your goal is to      
modify the value of the original dataframe, try something like the    
following instead:                                                    

df2.loc[..., "x"]                                                     

Monday, September 19, 2022

New milestone: friendly/friendly-traceback version 0.6 (and why not 1.0)

Just a few minutes ago, @isidentical tweeted that PyPy 3.9 had implemented the new enhanced tracebacks that are going to be part of cPython 3.11.  Of course, I had to reply to show that friendly/friendly-traceback have been able to do the same with all cPython version starting with 3.6.1.

A few hours before this tweet, I had bumped the minor version number of both friendly and friendly-traceback from 0.5 to 0.6. I always keep them in sync; previously, friendly-traceback was at 0.5.63 and friendly was at 0.5.42.

friendly builds on friendly-traceback and is the package you want to install as an end-user. If you're just interested at retrieving the data produced by friendly-traceback and format it your own way, as do and, then you only need to install friendly-traceback.  Both these websites have been making use of friendly-traceback quite successfully for quite a while. From that point of view, friendly-traceback is really mature enough to be considered as being a 1.0 version.  However, other  than always including more cases being covered, I have some interesting new additions planned, which makes me postpone giving it a 1.0 version number.

Quite a bit has been done since version 0.5. In particular, Tamil and Russian have been added as supported language. The syntax highlighting of the traceback location has been improved in friendly. Support for a new project, friendly_idle has been added. I've previously described

Note that, when some new highlighted code is shown with friendly_idle, the highlighting done on previous line of codes is removed: this is by design, to help focus the attention on the latest area with problems. 

I could say more about friendly ... but, why don't you try it out by yourself and see what you think. Feedback is always appreciated!

Friday, June 17, 2022

friendly_idle is done!

friendly_idle is done!

I've found a better solution for the remaining issue I had mentioned in the previous blog post.

I also found a fix for an "annoyance" mentioned by Raymond Hettinger on Twitter!

I could have changed the version to 1.0 ... but decided to wait until I get more feedback from users.

Tuesday, June 14, 2022

Friendly IDLE

friendly_idle is now available.  This is just a quick announcement. Eventually I plan to write a longer blog post explaining how I use import hooks to patch IDLE and to provide seamless support for friend/friendly_traceback.  Before I incorporated "partial" support for IDLE within friendly, I had released a package named friendly_idle ... but this is really a much better version.

When you launch it from a terminal, the only clue you get that this is not your regular IDLE is from the window title.

Since Python 3.10 (and backported to Python 3.8.10 and 3.9.5), IDLE provide support for sys.excepthook() (see announcement).  Actually, in the announcement, it is not pointed out that this is only partial support: exceptions raised because of syntax errors cannot be captured by user-defined exception hooks.  However, fear not, friendly_idle is perfectly capable to help you when your code has some syntax errors.

And, of course, it can also do so for runtime errors.

The same is true for code run from a file as well:

If the code in a file contains some syntax error, friendly_idle is often much more helpful than IDLE. Here's an example from IDLE:
And the same example run using friendly_idle

Unfortunately, the tkinter errorbox does not use a monospace font (assumed by friendly/friendly_traceback for the formatting), and does not allow customization.  I might have to figure out how to create my own dialog, hopefully with support for monospace font and colour highlighting. If anyone has some experience doing this, feel free to contact me! ;-)

Saturday, June 11, 2022

Nicer arithmetic with Python

Beginning programmers are often surprised by floating point arithmetic inaccuracies. If they use Python, many will often write posts saying that Python is "broken" when the see results as follows:

>>> 0.1 + 0.2

This particular result is not limited to Python. In fact, it is so common that there exists a site with a name inspired by this example (, devoted to explaining the origin of this puzzling result, followed by examples from many programming languages.

Python provides some alternatives to standard floating point operations. For example, one can use the decimal module to perform fixed point arithmetic operations. Here's an example.

>>> from decimal import Decimal, getcontext
>>> getcontext().prec = 7
>>> Decimal(0.1) + Decimal(0.2)
>>> print(_)

While one can set the precision (number of decimals) with which operations are performed, printed values can carry extra zeros: 0.3000000 does not look as "nice" as 0.3.

Another alternative included with Python's standard library is the fractions module: it provides support for rational number arithmetic.

>>> from fractions import Fraction
>>> Fraction("0.1") + Fraction("0.2")
Fraction(3, 10)
>>> print(_)

However, the fractions module can yield some surprising results if one does not use string arguments to represent floats, as was mentioned by Will McGugan (of Rich and Textual fame) in a recent tweet.

>>> from fractions import Fraction as F
>>> F("0.1")
Fraction(1, 10)
>>> F(0.1)
Fraction(3602879701896397, 36028797018963968)

In the second case, 0.1 is a float which means that it carries some intrinsic inaccuracy. For the first case, some parsing is done by Python to determine the number of decimal places to use before converting the result into a rational number. A similar result can be achieved using the limit_denominator method of the Fraction class:

>>> F(0.1).limit_denominator(10)
Fraction(1, 10)

In fact, we do not have to be as restrictive in the limitation imposed on the denominator to achieve the same result

>>> F(0.1).limit_denominator(1_000_000_000)
Fraction(1, 10)

While we can achieve some "more intuitive" results for floating point arithmetic using special modules from Python, the notation that one has to use is not as simple as "0.1 + 0.2". As Raymond Hettinger often says: "There has to be a better way."

Using ideas

As readers of this blog already know, I created a Python package named ideas to facilitate the creation of import hooks and to enable easy experimentation with modified Python syntax. ideas comes with its own console that support modified Python syntax. It can also be used with IPython (and thus with Jupyter notebooks).

Using ideas, one can "instruct" python to perform rational arithmetic.  For example, suppose I have a Python file containing the following:


a = 0.2 + 0.1
b = 0.2 + 1/10
c = 2/10 + 1/10
print(a, b, c)

I can run this with Python, getting the expected "unintuitive" result:

> py
0.30000000000000004 0.30000000000000004 0.30000000000000004

Alternatively, using ideas, I can execute this file using rational arithmetic:

> ideas simple_math -a rational_math
3/10 3/10 3/10

Using a different import hook, I can have the result shown with floating point notation.

> ideas simple_math -a nicer_floats
0.3 0.3 0.3

Instead of executing a script, let's use the ideas console instead, starting with "nicer_float"

ideas> 0.1 + 0.2

ideas> 1/10 + 2/10
For "nicer_float", I've also adopted the Pyret's notation: floating-point number immediately preceded by "~" are treated as "approximate" floating points i.e. with the regular inaccuracy.
ideas> ~0.1 + 0.2

And, as mentioned before, I can use ideas with IPython. Here's a very brief example

IPython 8.0.0b1 -- An enhanced Interactive Python. Type '?' for help.

In [1]: from ideas.examples import rational_math

In [2]: hook = rational_math.add_hook()
   The following initializing code from ideas is included:

from fractions import Fraction

In [3]: 0.1 + 0.2
Out[3]: Fraction(3, 10)

Final thoughts

Given how confusing floating point arithmetic is to beginners, I think it would be nice if Python had an easy built-in way to switch modes and do calculations as done with ideas in the above examples. However, I doubt very much that this will ever happen. Fortunately, as demonstrated above, it is possible to use import hooks and modified interactive console to achieve this result.

Friday, May 13, 2022

Python 🐍 fun with emojis

At EuroSciPy in 2018, Marc Garcia gave a lightning talk which started by pointing out that scientific Python programmers like to alias everything, such as

import numpy as np
import pandas as pd

and suggested that they perhaps would prefer to use emojis, such as

import pandas as 🐼

However, Python does not support emojis as code, so the above line cannot be used.

A year prior, Thomas A Caswell had created a pull request for CPython that would have made this possible. This code would have allowed the use of emojis in all environments, including in a Python REPL and even in Jupyter notebooks. Unsurprisingly, this was rejected.

Undeterred, Geir Arne Hjelle created a project called pythonji (available on Pypi) which enabled the use of emojis in Python code, but in a much more restricted way. With pythonji, one can run modules ending with 🐍 instead of .py from a terminal. However, such modules cannot be imported, nor can emojis be used in a terminal.

When I learned about this attempt by Geir Arne Hjelle from a tweet by Mike Driscoll, I thought it would be a fun little project to implement with ideas.  Below, I use the same basic example included in the original pythonji project.

As you can see, it works in ideas' console, when importing module. It can also work when running the 🐍 file as source - but leaving the extension out.

And, it works in Jupyter notebooks too!

All of this without any need to modify CPython's source code!


Sunday, April 10, 2022

Natural syntax for units in Python

In the past week, there has been an interesting discussion on Python-ideas about Natural support for units in Python. As I have taught introductory courses in Physics for about 20 of the 30 years of my academic career, I am used to stressing the importance of using units correctly, but had never had the need to explore what kind of support for units was available in Python. I must admit to have been pleasantly surprised by many existing libraries.

In this blog post, I will give a very brief overview of parts of the discussion that took, and is still taking place, on Python-ideas about this topic. I will then give a very brief introduction to two existing libraries that provide support for units, before showing some actual code inspired by the Python-ideas discussion.

But first, putting my Physics teacher hat on, let me show you some partial Python code that I find extremely satisfying, and which contains a line that is almost guaranteed to horrify programmers everywhere, as it seemingly reuse the variable "m" with a completely different meaning.

>>> g = 9.8[m/s^2]
>>> m = 80[kg]
>>> weight = m * g
>>> weight
<Quantity(784.0, 'kilogram * meter / second ** 2')>
>>> tolerance = 1.e-12[N]
>>> abs(weight - 784[N]) < tolerance

Discussion on Python-ideas

The discussion on Python-ideas essentially started with the suggestion that "it would be nice if Python's syntax supported units".  That is, if you could basically do something like:

length = 1m + 3cm
# or even
length = 1m 3cm

and it just worked as "expected". Currently, identifiers in Python cannot start with a number, and writing "3cm" is a SyntaxError. So, in theory, one could add support for this type of construct without causing any backward incompatibility.

While I never thought of it before, as I use Python as a hobby, I consider the idea of supporting handling units correctly to be an absolute requirement for any scientific calculations. Much emphasis is being made on adding type information to ensure correctness: to my mind, adding *unit* information to ensure correctness is even more important than adding type information.

During the course of the discussion on Python-ideas, other possible suggestions were made, some of which are actually supported by at least a couple of existing Python libraries. These suggestions included constructs like the following:

length = 1*m + 3*cm
speed = 4*m / 1*s  # or speed = 4 * m / s

length = m(1) + cm(3)
speed = m_s(4)

length = 1_m + 3_cm
speed = 4_m_s

length = 1[m] + 3[cm]
speed = 4[m/s]

length = 1"m" + 3"m"
speed = 4"m/s"

density = 1.0[kg/m**3]
density = 1.0[kg/m3]
# No one suggested something like the following
density = 1.0[kg/m^3]

I will come back to looking at potential new syntax for units, as it currently my main interest in this topic. But first, I want to highlight one other main point of the discussion on Python-ideas, namely: Should the units be defined globally for an entire application, or locally according to the standard Python scopes?

My first thought was "of course, it should follow Python's normal scopes". 

Thinking of the opposite argument, what happen if one uses units other than S.I. units in different module, including those from external libraries?  Take for example "mile", and have a look at its Wikipedia entry. If one uses units with the same name but different values in different parts of an application, any pretense of using quantities with units to ensure accuracy goes out the window. Furthermore, many units libraries make it possible for users to define they own custom units. What happens if the same name is used for different custom units in different modules, with variables or functions using variables with units in one module are used in a second module?

Still, as long as libraries do not, or cannot change unit definitions globally, and if they provide clear and well-documented access to the units they use, then the normal Python scopes would likely be the best choice.

[For a detailed discussion of these two points of view, have a look at the thread on Python-ideas mentioned above. There doesn't seem to be a consensus as to what the correct approach should be.]

A brief look at two unit libraries

There are many unit libraries available on Pypi. After a brief look at many of them, I decided to focus on only two: astropy.units and pint. These seemed to be the most complete ones currently available, with source code and good supporting documentation available.

I will first look at an example that shows how equivalent description of units are easily handled in both of them. First, I use the units module from astropy:

>>> from astropy import units as u
>>> p1 = 1 * u.N / u.m**2
>>> p1
<Quantity 1. N / m2>
>>> p2 = 1 * u.Pa
>>> p1 == p2

Next, doing the same with pint.

>>> import pint
>>> u = pint.UnitRegistry()
>>> p1 = 1 * u.N / u.m**2
>>> p1
<Quantity(1.0, 'newton / meter ** 2')>
>>> p2 = 1 * u.Pa
>>> p1 == p2

In astropy, all the units are defined in a single module.  Instead of prefacing the units with the name of the module, one can import units directly

>>> from astropy.units import m, N, Pa
>>> p1 = 1 * N / m**2
>>> p2 = 1 * Pa
>>> p1 == p2

The same cannot be done with pint.

A custom syntax for units

As I was reading posts from the discussion on Python-ideas, I was thinking that it might be fun to come up with a way to "play" with some code written in a more user-friendly syntax for units. After reading the following, written by Matt del Valle, I decided that I should definitely do it.

My personal preference for adding units to python would be to make instances of all numeric classes subscriptable, with the implementation being roughly equivalent to:

def __getitem__(self, unit_cls: type[T]) -> T: return unit_cls(self)

We could then discuss the possibility of adding some implementation of units to the stdlib. For example:

from import km, m, N, Pa

3[km] + 4[m] == 3004[m] # True 5[N]/1[m**2] == 5[Pa] # True

My first thought was to create a custom package building from and depending on astropy.units, as I had looked at it before looking at pint and found it to have everything one might need.  However, as I read its rather unusual license, I decided that I should take another approach: I chose to simply add a new example to my ideas library, making it versatile enough so that it could be used with any unit library that uses the standard Python notation for multiplication, division and power of units, which both pint and astropy do. Note that my ideas library has been created to facilitate quick experiments and is not meant to be used in production code.

First, here's an example that mimics the example given by Matt del Valle above, with what I think is an even nicer (more compact) notation.

python -m ideas -t easy_units

Ideas Console version 0.0.29. [Python version: 3.9.10]

>>> from astropy.units import km, m, N, Pa
>>> 3[km] + 4[m] == 3004[m]
>>> 5[N/m^2] == 5[Pa]

In addition to allowing '**' for powers of units (not shown above), I chose to also recognize as equivalent the symbol '^' which is more often associated with exponentiation outside of the (Python) programming world.

Let's do essentially the same example using pint instead, and follow it with a few additional lines to illustrate further.

Ideas Console version 0.0.29. [Python version: 3.9.10]

>>> import pint
>>> unit = pint.UnitRegistry()
>>> 3[km] + 4[m] == 3004[m]
>>> 5[N/m^2] == 5[Pa]
>>> pressure = 5[N/m^2]
>>> pressure
<Quantity(5.0, 'newton / meter ** 2')>
>>> pressure = 5[N/m*m]
>>> pressure
<Quantity(5.0, 'newton / meter ** 2')>

In the last example, I made sure that "N/m*m" did not follow the regular left-to-right order of operation which might have resulted in unit cancellation as we first divide and then multiply by meters.

A look at some details

Using ideas with a "verbose" mode (-v or --verbose), one can see how the source is transformed prior to its execution.  Furthermore, in the case of easy_units, sometime a "prefix" is "extracted" from the code, ensuring that the correct names are used.  Here's a very quick look.

python -m ideas -t easy_units -v

Ideas Console version 0.0.29. [Python version: 3.9.10]

>>> import pint
>>> un = pint.UnitRegistry()
>>> pressure = 5[N/m^2]
pressure = 5 * un.N/(un.m**2)
>>> pressure
<Quantity(5.0, 'newton / meter ** 2')>


Prior to reading the discussion on Python-ideas, I was only vaguely aware of the existence of some units libraries available in Python, and had no idea about their potential usefulness. Many unit libraries are, in my opinion, much  less user-friendly than astropy and pint. Still, I do find the requirements to add explicit multiplication symbols to be more tedious and much less readable than the alternative that I have shown.  While introducing a syntax like the one I have shown would not cause any backward incompatibilities, I doubt very much that anything like it would be added to Python, as it would likely be considered to be too specific to niche applications. I find this unfortunate ... However, I know that I can use ideas in my own projects if I ever want to use units together with a friendlier syntax.

I wrote the easy_units module in just a few hours. It is likely to contain some bugs [1], and is most definitely written as a quick hack not following the best practice. If you do try it, and find some bugs, feel free to file an issue; don't bother looking at the code. ;-)

[1] Indeed, I found and fixed a couple while writing this post.

Tuesday, February 08, 2022

Friendly-traceback and IPython: update

In my previous post, I mentioned that, unlike IPython, friendly/friendly-traceback included values of relevant objects in a traceback.  As I wrote in the update, Alex Hall pointed out that one could get this information by using a verbose mode in IPython.  Here is the previous example when using the verbose mode.

In (1) I enabled the verbose mode. In (3), we see its effect.   (2) is a reminder of the highlighting when it spans many lines.  Regarding the highlighting, here's what I had in the previous blog post:

Alex Hall (yes, him again), the author of stack_data used by both IPython and friendly-traceback, suggested that perhaps a better way would be to have a common indentation. This is what I implemented next:

In my code, this is done in a rather convoluted way. Following a suggestion by Alex, I implemented a change in stack_data itself which yields the correct result, at least when using carets (^) as marker for the location.  If Alex can confirm that it works for stack_data in all cases, this new way of highlighting consecutive lines would likely be automatically incorporated into IPython.

The reason I go into all these datails is as follows: I'm really interested in getting feedback from users so as to make friendly/friendly-traceback even more useful.  So, don't be shy! :-)

Friday, February 04, 2022

Friendly-traceback: trying to stay ahead of IPython

UPDATE: Alex Hall pointed out that IPython can display the values of variables in the highlighted sections using %xmode verbose. He also suggested a different highlighting strategy when the problematic code spans multiple lines.  I go into more details about these two issues in a future blog post.


I'm writing this blog post in the hope that some people will be encouraged to test friendly/friendly-traceback with IPython/Jupyter and make suggestions as to how it could be even more useful.

However, before you read any further...

Important clarification: IPython is a professionally developed program which is thoroughly tested, and is an essential tool for thousands of Python programmers.  By contrast, friendly/friendly-traceback is mostly done by a hobbyist (myself) and is not nearly as widely used nor as reliable as IPython. Any comparison I make below should be taken in stride.  Still, I can't help but draw your attention to this recent tweet from Matthias Bussonnier, the IPython project leader:

I don't believe that friendly/friendly-traceback is mature and stable enough to become part of IPython's distribution. However, it is because of this endorsement that I decided to see what I could do to improve friendly/friendly-traceback's integration with IPython.

IPython news

The recent release of IPython included many traceback improvements. One of these changes, shown with the screen capture below:

is something that I am happy to have implemented many months ago as mentioned in this blog post. I have no reason to believe that my idea was the impetus for this change in IPython's formatting of tracebacks. Still, I think it validates my initial idea.

However, there have been other changes introduced in this latest IPython release, such as using colour instead of ^^ to highlight the location of the code causing a traceback is something that I had done only for IDLE but not for other environments such as IPython/Jupyter.  So, I felt that I had to catch up with what IPython has implemented and, if possible, do even better.  Of course, I must recognize that this work is greatly facilitated since I use Alex Hall's excellent stack_data (as well as some other of his packages on which stack_data depends) in friendly-traceback: stack_data is now used by IPython to generate these tracebacks. So, in principle, there is no reason why I shouldn't be able to implement similar features in friendly/friendly-traceback.

Again, I must note that the way I use stack_data is a bit hackish, and definitely not as elegant as it is used within IPython. 

Enough of a preamble, time to provide some actual examples.

First example

Consider the following module which will generate a traceback when imported in IPython:

Here is the result:

We can see not only the lines of code that caused the traceback, but actually the specific parts of each line that caused a problem.  Notice how the display jumps from line 6 to line 8: this is because line 7 is an empty line. Such empty lines are removed to reduce the vertical space require for the display.

I could replicate this example using the friendly console but, instead, I will use the specific IPython integration to see what else we could do at this point. 

We see a traceback that is somewhat similar to a standard CPython traceback, but with an additional hint at the end which gives us an additional clue as to what the cause of the error might be.  Friendly/friendly-traceback can give more information about what() a particular exception means in general, why() it might have happened in this particular case and where() it occurred:

By design, the information provided by where() only focus on the beginning and the end of the traceback, so as to not overwhelm beginners with often irrelevant steps in between. However, notice that in addition to the highlighted parts (new feature!), we also see the values of some objects from these highlighted regions.

Until recently, this was all the information that one could get. However, it is now possible to get more details, in a way similar to that provided by IPython, but with the addition of the values of various objects.  (Note that the syntax shown below to obtain this information is subject to change; it is just a proof of concept.)

Other than the different colour chosen for highlighting (both IPython and this example are done in a Windows Terminal), I also chose to ensure that one line per frame was highlighted, such as the line "import example1".  Do you think this is a good choice, or should I do like IPython?
Finally, I included line 7 which is an empty line, so that beginners (my original target audience) might better recognize their own code instead of seeing a more vertically compressed version. If more than one blank line would be included, they would be replaced by "(...)" indicating that some lines
were skipped.

If the highlighting is not adequate, it can be changed by using either named colours (converted to lowercase with spaces removed) or hexadecimal values; the name of the function and its arguments are subject to change:

Any such change is written to a settings file so that it is remembered for future sessions. Those that prefer traditional Python's notation with ^^ can do so by using None as an argument:

Finally, one can go back to the defaults by specifying no argument:

Example 2

In the previous example, all highlighting regions were part of a single line. However, sometimes the code at fault will spill over two lines. Here's how IPython does its highlighting:

Instead of highlighting each line from the beginning, I chose to not highlight the indentation; is this a better choice?

Jupyter notebook

When IPython is used in a Jupyter notebook (or lab), I chose yet again a different way to present the result. First, let's have a look at a simple example using the Jupyter default.

In this example, only two frames are highlighted.  Let's see the result, using friendly.

We get a basic error message with a button to click if we want to have more details.

We already have seen the output of what() and why() before; this time let's just click on where():

Since we only had two frames in the traceback, where() gives us all the relevant information.

What happens if we have more than two frames in the traceback?  First, let's give an example with the Jupyter default.

What happens if we use friendly in this case?  Below I show the result after clicking "more"

An additional button has appeared.  Note that this is something new that I just did earlier today (before writing this blog post). It is quite possible that there might be bugs if you try it.


These new features are simple proofs of concept that have not been thoroughly tested.  If you read this far, and hopefully tried it on your own, I would really appreciate getting your feedback regarding the choices I made and any improvement you might be able to suggest.