Offer suggestions on AttributeError and NameError

[pablogsal]

BPO	38530
Nosy	@vstinner, @aroberge, @serhiy-storchaka, @1st1, @pablogsal, @tirkarthi, @isidentical, @sweeneyde
PRs	bpo-38530: Offer suggestions on AttributeError #16850 bpo-38530: Offer suggestions on AttributeError #16856 bpo-38530: Offer suggestions on NameError #25397 bpo-38530: Clean exceptions if dir() fails when making suggestions for AttributeError #25408 bpo-38530: Optimize the calculation of string sizes when offering suggestions #25412 bpo-38530: Match exactly AttributeError and NameError when offering suggestions #25443 bpo-38530: Properly extend UnboundLocalError from NameError #25444 bpo-38530: Include builtins in NameError suggestions #25460 bpo-38530: Cover more error paths in error suggestion functions #25462 bpo-38530: Surround suggestions by quotes #25473 bpo-38530: Require 50% similarity in NameError suggestions #25584 bpo-38530: Refactor AttributeError suggestions #25776

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2021-04-14.23:32:49.056>
created_at = <Date 2019-10-19.18:57:31.623>
labels = ['interpreter-core', 'type-feature', '3.9']
title = 'Offer suggestions on AttributeError and NameError'
updated_at = <Date 2021-05-03.15:47:35.739>
user = 'https://github.com/pablogsal'

bugs.python.org fields:

activity = <Date 2021-05-03.15:47:35.739>
actor = 'pablogsal'
assignee = 'none'
closed = True
closed_date = <Date 2021-04-14.23:32:49.056>
closer = 'pablogsal'
components = ['Interpreter Core']
creation = <Date 2019-10-19.18:57:31.623>
creator = 'pablogsal'
dependencies = []
files = []
hgrepos = []
issue_num = 38530
keywords = ['patch']
message_count = 40.0
messages = ['354954', '354955', '354956', '354957', '354958', '354959', '354960', '354964', '354965', '354967', '354969', '354970', '354971', '354972', '354973', '354975', '354979', '354981', '355499', '355503', '355527', '355530', '355531', '355533', '359977', '391026', '391080', '391090', '391108', '391123', '391219', '391224', '391312', '391316', '391834', '392007', '392513', '392543', '392581', '392817']
nosy_count = 9.0
nosy_names = ['vstinner', 'aroberge', 'serhiy.storchaka', 'yselivanov', 'james', 'pablogsal', 'xtreak', 'BTaskaya', 'Dennis Sweeney']
pr_nums = ['16850', '16856', '25397', '25408', '25412', '25443', '25444', '25460', '25462', '25473', '25584', '25776']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue38530'
versions = ['Python 3.9']

[pablogsal]

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?

[pablogsal]

PR 16850 shows an initial prototype for the idea

[isidentical]

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)

[pablogsal]

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.

[tirkarthi]

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

'

[pablogsal]

Idea: we could only do this in interactive mode if we consider that is expensive enough.

[pablogsal]

I am running pyperformance to check the performance cost of this.

[pablogsal]

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.

[serhiy-storchaka]

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.

[serhiy-storchaka]

I am surprised that it was SO expensive.

Pathlib would largely benefit from cached_property if it be compatible with slots.

[pablogsal]

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?

[pablogsal]

I will also repeat the pyperformance results locally just in case something was off on the speed.python.org server.