classification
Title: Offer suggestions on AttributeError
Type: enhancement Stage: patch review
Components: Interpreter Core Versions: Python 3.9
process
Status: open Resolution:
Dependencies: Superseder:
Assigned To: Nosy List: BTaskaya, james, pablogsal, serhiy.storchaka, vstinner, xtreak, yselivanov
Priority: normal Keywords: patch

Created on 2019-10-19 18:57 by pablogsal, last changed 2020-01-14 15:04 by vstinner.

Pull Requests
URL Status Linked Edit
PR 16850 closed pablogsal, 2019-10-19 18:58
PR 16856 open pablogsal, 2019-10-19 23:12
Messages (25)
msg354954 - (view) Author: Pablo Galindo Salgado (pablogsal) * (Python committer) Date: 2019-10-19 18:57
To improve the debugging experience in both interactive and non-interactive code, I propose to offer suggestions when attribute access fails. For example:

>>> class A: foo = None
... 
>>> A.fou
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: type object 'A' has no attribute 'fou'

Did you mean: foo?

This also applies to imports from modules and other situations:

>>> import collections
>>> collections.NamedTuple
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: module 'collections' has no attribute 'NamedTuple'

Did you mean: namedtuple?
msg354955 - (view) Author: Pablo Galindo Salgado (pablogsal) * (Python committer) Date: 2019-10-19 19:00
PR 16850 shows an initial prototype for the idea
msg354956 - (view) Author: Batuhan (BTaskaya) * Date: 2019-10-19 19:01
It already exists as a 3rd party module and it would be really cool to have this in core level.

https://github.com/dutc/didyoumean (by James Powell)
msg354957 - (view) Author: Pablo Galindo Salgado (pablogsal) * (Python committer) Date: 2019-10-19 19:22
I am not super convinced that this is a great idea because it has some performance cost (although somewhat controlled) but I want to open a discussion.
msg354958 - (view) Author: Karthikeyan Singaravelan (xtreak) * (Python committer) Date: 2019-10-19 19:23
Ruby has it integrated into the core : https://bugs.ruby-lang.org/issues/11252 . It was initially a gem that got merged into core.

> methosd
undefined local variable or method `methosd' for main:Object
Did you mean?  methods
               method
(repl):1:in `<main>'
msg354959 - (view) Author: Pablo Galindo Salgado (pablogsal) * (Python committer) Date: 2019-10-19 19:23
Idea: we could only do this in interactive mode if we consider that is expensive enough.
msg354960 - (view) Author: Pablo Galindo Salgado (pablogsal) * (Python committer) Date: 2019-10-19 19:25
I am running pyperformance to check the performance cost of this.
msg354964 - (view) Author: Pablo Galindo Salgado (pablogsal) * (Python committer) Date: 2019-10-19 20:45
Slower (27):
- pathlib: 25.2 ms +- 0.4 ms -> 105 ms +- 2 ms: 4.18x slower (+318%)
- sympy_str: 315 ms +- 3 ms -> 500 ms +- 3 ms: 1.59x slower (+59%)
- sympy_sum: 203 ms +- 2 ms -> 286 ms +- 2 ms: 1.41x slower (+41%)
- sqlalchemy_imperative: 36.5 ms +- 0.9 ms -> 41.8 ms +- 0.8 ms: 1.14x slower (+14%)
- python_startup: 12.4 ms +- 0.1 ms -> 13.9 ms +- 0.0 ms: 1.12x slower (+12%)
- python_startup_no_site: 9.19 ms +- 0.04 ms -> 10.3 ms +- 0.0 ms: 1.12x slower (+12%)
- sympy_integrate: 24.9 ms +- 0.1 ms -> 27.6 ms +- 0.3 ms: 1.11x slower (+11%)
- scimark_sparse_mat_mult: 5.21 ms +- 0.05 ms -> 5.49 ms +- 0.06 ms: 1.05x slower (+5%)
- unpickle_list: 5.78 us +- 0.07 us -> 6.08 us +- 0.07 us: 1.05x slower (+5%)
- 2to3: 392 ms +- 2 ms -> 411 ms +- 10 ms: 1.05x slower (+5%)
- nbody: 155 ms +- 2 ms -> 160 ms +- 2 ms: 1.03x slower (+3%)
- scimark_fft: 423 ms +- 2 ms -> 432 ms +- 3 ms: 1.02x slower (+2%)
- float: 142 ms +- 2 ms -> 145 ms +- 2 ms: 1.02x slower (+2%)
- unpack_sequence: 68.6 ns +- 2.5 ns -> 69.9 ns +- 2.1 ns: 1.02x slower (+2%)
- pickle_list: 5.65 us +- 0.05 us -> 5.76 us +- 0.07 us: 1.02x slower (+2%)
- fannkuch: 569 ms +- 3 ms -> 579 ms +- 6 ms: 1.02x slower (+2%)
- xml_etree_parse: 196 ms +- 2 ms -> 199 ms +- 2 ms: 1.02x slower (+2%)
- sqlalchemy_declarative: 210 ms +- 2 ms -> 213 ms +- 3 ms: 1.01x slower (+1%)
- unpickle: 16.9 us +- 0.1 us -> 17.2 us +- 0.7 us: 1.01x slower (+1%)
- regex_effbot: 3.93 ms +- 0.12 ms -> 3.97 ms +- 0.04 ms: 1.01x slower (+1%)
- scimark_monte_carlo: 128 ms +- 2 ms -> 129 ms +- 2 ms: 1.01x slower (+1%)
- scimark_lu: 186 ms +- 5 ms -> 188 ms +- 6 ms: 1.01x slower (+1%)
- regex_dna: 272 ms +- 1 ms -> 274 ms +- 1 ms: 1.01x slower (+1%)
- xml_etree_iterparse: 130 ms +- 2 ms -> 131 ms +- 3 ms: 1.01x slower (+1%)
- genshi_xml: 74.3 ms +- 0.8 ms -> 74.8 ms +- 0.8 ms: 1.01x slower (+1%)
- regex_v8: 29.7 ms +- 0.3 ms -> 29.9 ms +- 0.2 ms: 1.01x slower (+1%)
- mako: 19.3 ms +- 0.3 ms -> 19.4 ms +- 0.2 ms: 1.01x slower (+1%)


The current approach is too expensive, so I'm closing PR 16850.
msg354965 - (view) Author: Serhiy Storchaka (serhiy.storchaka) * (Python committer) Date: 2019-10-19 20:56
AFAIK there is existing issue for this idea.

I have doubts about performance. I added _PyObject_LookupAttr in particularly to avoid an overhead of raising and silencing an AttributeError. I believe most performance sensitive code in the core now uses it and will not be affected, but there is other code which silences it (_PyObject_LookupAttr itself silences an AttributeError raised in called functions), third-party code which uses PyObject_HasAttr or PyObject_GetAttr can be affected.

It might be simpler to implement it in sys.excepthook or sys.displayhook, but at that point we do not have attribute name and a reference to the object. There is an issues and a PEP about adding references to AttributeError. It could help to implement this feature.
msg354967 - (view) Author: Serhiy Storchaka (serhiy.storchaka) * (Python committer) Date: 2019-10-19 21:04
I am surprised that it was SO expensive.

Pathlib would largely benefit from cached_property if it be compatible with slots.
msg354969 - (view) Author: Pablo Galindo Salgado (pablogsal) * (Python committer) Date: 2019-10-19 21:10
Serhiy, do you think we could attach the object and the name to some private fields of the AttributeError and check that in sys.excepthook if they are present?
msg354970 - (view) Author: Pablo Galindo Salgado (pablogsal) * (Python committer) Date: 2019-10-19 21:15
I will also repeat the pyperformance results locally just in case something was off on the speed.python.org server.
msg354971 - (view) Author: Serhiy Storchaka (serhiy.storchaka) * (Python committer) Date: 2019-10-19 21:16
Why private? They should be public.

But the problem is that making a reference to the object we can prolong its lifetime and even create a reference loop. There was a proposition to create a weak reference, but not all types support weak reference.
msg354972 - (view) Author: Pablo Galindo Salgado (pablogsal) * (Python committer) Date: 2019-10-19 21:19
> Why private? They should be public.

I was suggesting orivate for now until the/a PEP to modify the exception is approved. In this way we could try to implement the feature that way.

------

On the other hand do you see any way to make the current approach not that slow? Maybe activating it only on interactive mode?
msg354973 - (view) Author: Serhiy Storchaka (serhiy.storchaka) * (Python committer) Date: 2019-10-19 21:21
See issue18156, issue22716 and PEP 473.
msg354975 - (view) Author: Pablo Galindo Salgado (pablogsal) * (Python committer) Date: 2019-10-19 21:28
> But the problem is that making a reference to the object we can prolong its lifetime and even create a reference loop.

If I'm not mistaken, as long as the traceback is alive, the object is alive beacuse the frames will contain it. The other case is if the exception is not propagated, but in that case it should just die unless explicitly captured. 

The cycle only happens if the object has a reference to the exception, and that should not happen in the general case.
msg354979 - (view) Author: Pablo Galindo Salgado (pablogsal) * (Python committer) Date: 2019-10-19 23:15
I have opened PR 16856 adding fields to the AttributeError and implementing the feature in PyErr_Display.
msg354981 - (view) Author: Pablo Galindo Salgado (pablogsal) * (Python committer) Date: 2019-10-20 03:22
With the new approach, there is no measurable different in performance:

venv ❯ pyperf compare_to json/2019-10-19_20-01-master-24dc2f8c5669.json.gz json/2019-10-20_01-32-suggestions2-21404456383b.json.gz -G

Slower (3):
- 2to3: 400 ms +- 2 ms -> 405 ms +- 3 ms: 1.01x slower (+1%)
- sqlite_synth: 3.46 us +- 0.08 us -> 3.50 us +- 0.08 us: 1.01x slower (+1%)
- nqueens: 121 ms +- 1 ms -> 121 ms +- 1 ms: 1.01x slower (+1%)

Faster(4):
- hexiom: 11.9 ms +- 0.1 ms -> 11.8 ms +- 0.1 ms: 1.01x faster (-1%)
- xml_etree_generate: 113 ms +- 2 ms -> 113 ms +- 2 ms: 1.01x faster (-1%)
- python_startup_no_site: 9.25 ms +- 0.06 ms -> 9.20 ms +- 0.01 ms: 1.01x faster (-1%)
- go: 317 ms +- 4 ms -> 315 ms +- 3 ms: 1.01x faster (-1%)
- sympy_integrate: 25.1 ms +- 0.2 ms -> 25.0 ms +- 0.1 ms: 1.01x faster (-1%)


Benchmark hidden because not significant (57): chameleon, chaos, crypto_pyaes, deltablue, dulwich_log, fannkuch, float, genshi_text, genshi_xml, json_dumps, json_loads, logging_format, logging_silent, logging_simple, mako, meteor_contest, nbody, pathlib, pickle, pickle_dict, pickle_list, pickle_pure_python, pidigits, python_startup, raytrace, regex_compile, regex_dna, regex_effbot, regex_v8, richards, scimark_fft, scimark_lu, scimark_monte_carlo, scimark_sor, scimark_sparse_mat_mult, spectral_norm, sqlalchemy_declarative, sqlalchemy_imperative, sympy_expand, sympy_sum, sympy_str, telco, unpack_sequence, unpickle, unpickle_list, unpickle_pure_python, xml_etree_parse, xml_etree_iterparse, xml_etree_process
msg355499 - (view) Author: Pablo Galindo Salgado (pablogsal) * (Python committer) Date: 2019-10-27 19:55
I think I am going to proceed to modify PR 16856 by adding the name and the object to the AttributeError exceptions.

This should not extend the lifetime of the object more than the current exception is doing as the exception keeps alive the whole frame stack in the __traceback__ attribute. Consider this code for example:

class Target:
    def __del__(self):
        print("The object is dead!")

def f():
    g()

def g():
    h()

def h():
    theobj = Target()
    theobj.thevalue

try:
    f()
except Exception as e:
    print(e.__traceback__.tb_next.tb_next.tb_next.tb_frame.f_locals["theobj"])
    print("End of except")
print("Begining of the next line")


This code prints:

<__main__.Target object at 0x7f064adbfe10>
End of except
The object is dead!
Beginning of the next line

We can notice two things:
   * The target objects are reachable from the exception.
   * When the exception dies, the target object dies.

Adding another reference to the target object to the exception won't change the current lifetime, neither will create reference cycles as the target object will not have (unless explicitly created) a reference to the exception. We can conclude that this change should be safe.

In the resolution email for PEP473 the council stated that "Discussions about adding more attributes to built-in exceptions can continue on the issue tracker on a per-exception basis". As I think this will be very beneficial for the feature discussed in the issue, I will proceed as indicated.

Notice that if we want to change all internal CPython code that raises AttributeError to use the new fields, this should be done in a different PR to keep this minimal. For this feature, we still need to intercept AttributeError raised by the user without these fields added.
msg355503 - (view) Author: Serhiy Storchaka (serhiy.storchaka) * (Python committer) Date: 2019-10-27 20:20
Interesting, why locals are not cleared when an exception leaves a frame?
msg355527 - (view) Author: STINNER Victor (vstinner) * (Python committer) Date: 2019-10-28 11:17
Helping the developer to suggest a fix introduces a minor but non-zero overhead, I would prefer to only enable it as an opt-in option. Maybe enable it using in the development mode (-X dev/PYTHONDEVMODE=1)?

> https://github.com/dutc/didyoumean (by James Powell)

This project hooks into PyObject_GetAttr() by modifying PyObject_GetAttr() machine code, which is definitely not a portable approach.

Maybe one approach would be to add a way to install a hook to customize AttributeError exceptions?

Can this issue be implemented using sys.excepthook?

--

This issue is specific to AttributeError, but I vaguely recall that Yury Selivanov told me that he wanted to something but for any exception. Detect the most common mistakes and propose a solution. I don't think that he ever sent anything in public sadly.

I add Yury in the nosy list.

--

Here is another project which also catch NameError, AttributeError, ImportError, TypeError, ValueError, SyntaxError, MemoryError, OverflowError, OSError, RuntimeError, etc. :
https://github.com/SylvainDe/DidYouMean-Python

It is implemented with sys.excepthook, but it is also compatible with IPython "custom exception handler" (call get_ipython().set_custom_exc()).

By the way, does IPython have a feature like this?

In short, https://github.com/SylvainDe/DidYouMean-Python seems to already implement this issue in the proper way, no?

--

Similar project for Ruby:
https://github.com/yuki24/did_you_mean
msg355530 - (view) Author: Pablo Galindo Salgado (pablogsal) * (Python committer) Date: 2019-10-28 11:24
> By the way, does IPython have a feature like this?

Not that I know of.

> In short, https://github.com/SylvainDe/DidYouMean-Python seems to already implement this issue in the proper way, no?

I briefly checked the project. My current approach exposes the object and the name directly on the exception, so the display hook can directly using them instead of fiddling with frames and other things.

Additionally, I think it would be very beneficial to have it in the core, as many people learning python for example cannot install packages.
msg355531 - (view) Author: Pablo Galindo Salgado (pablogsal) * (Python committer) Date: 2019-10-28 11:25
> Helping the developer to suggest a fix introduces a minor but non-zero overhead, I would prefer to only enable it as an opt-in option. Maybe enable it using in the development mode (-X dev/PYTHONDEVMODE=1)?

I think doing that would make it lose almost completely its value.
msg355533 - (view) Author: STINNER Victor (vstinner) * (Python committer) Date: 2019-10-28 11:28
> Ruby has it integrated into the core : https://bugs.ruby-lang.org/issues/11252 . It was initially a gem that got merged into core.

GCC also provides more and more hints. Example:

   int main() { int hello = 1; return helo; }

GCC:

   error: 'helo' undeclared (first use in this function); did you mean 'hello'?
msg359977 - (view) Author: STINNER Victor (vstinner) * (Python committer) Date: 2020-01-14 15:04
Related issue: PEP 534 -- Improved Errors for Missing Standard Library Modules
https://www.python.org/dev/peps/pep-0534/
History
Date User Action Args
2020-01-14 15:04:18vstinnersetmessages: + msg359977
2019-10-28 11:28:17vstinnersetmessages: + msg355533
2019-10-28 11:25:44pablogsalsetmessages: + msg355531
2019-10-28 11:24:44pablogsalsetmessages: + msg355530
2019-10-28 11:17:59vstinnersetnosy: + yselivanov
messages: + msg355527
2019-10-27 20:20:10serhiy.storchakasetmessages: + msg355503
2019-10-27 19:55:48pablogsalsetmessages: - msg355498
2019-10-27 19:55:41pablogsalsetmessages: + msg355499
2019-10-27 19:55:01pablogsalsetmessages: + msg355498
2019-10-20 03:22:55pablogsalsetmessages: + msg354981
2019-10-19 23:15:25pablogsalsetmessages: + msg354979
2019-10-19 23:12:53pablogsalsetpull_requests: + pull_request16403
2019-10-19 21:28:04pablogsalsetmessages: + msg354975
2019-10-19 21:24:49pablogsalsetmessages: - msg354974
2019-10-19 21:24:13pablogsalsetmessages: + msg354974
2019-10-19 21:21:50serhiy.storchakasetmessages: + msg354973
2019-10-19 21:19:39pablogsalsetmessages: + msg354972
2019-10-19 21:16:43serhiy.storchakasetmessages: + msg354971
2019-10-19 21:15:02pablogsalsetmessages: + msg354970
2019-10-19 21:10:19pablogsalsetmessages: + msg354969
2019-10-19 21:04:48serhiy.storchakasetmessages: + msg354967
2019-10-19 20:56:01serhiy.storchakasetmessages: + msg354965
2019-10-19 20:45:21pablogsalsetmessages: + msg354964
2019-10-19 19:25:09pablogsalsetmessages: + msg354960
2019-10-19 19:23:31pablogsalsetmessages: + msg354959
2019-10-19 19:23:19xtreaksetmessages: + msg354958
2019-10-19 19:22:24pablogsalsetmessages: + msg354957
2019-10-19 19:12:59xtreaksetnosy: + xtreak
2019-10-19 19:02:27BTaskayasetnosy: + james
2019-10-19 19:01:04BTaskayasetnosy: + BTaskaya
messages: + msg354956
2019-10-19 19:00:42pablogsalsetmessages: + msg354955
2019-10-19 19:00:16pablogsalsetnosy: + vstinner, serhiy.storchaka
2019-10-19 18:58:15pablogsalsetkeywords: + patch
stage: patch review
pull_requests: + pull_request16400
2019-10-19 18:57:31pablogsalcreate