[issue29116] Make str and bytes error messages on concatenation conform with other sequences

[Serhiy Storchaka]

Change by Serhiy Storchaka :


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resolution:  -> fixed
stage: patch review -> resolved
status: open -> closed

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[issue29116] Make str and bytes error messages on concatenation conform with other sequences

[Berker Peksag]

Change by Berker Peksag :


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pull_requests:  -4084

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[issue29116] Make str and bytes error messages on concatenation conform with other sequences

[Raymond Hettinger]

Raymond Hettinger  added the comment:

In the future, I don't think this sort of thing should be backported.  It isn't 
a bug, rather, it is a "thing x doesn't exactly look like thing y".

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[issue29116] Make str and bytes error messages on concatenation conform with other sequences

[Antoine Pietri]

Change by Antoine Pietri :


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keywords: +patch
pull_requests: +4084
stage:  -> patch review

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[issue29116] Make str and bytes error messages on concatenation conform with other sequences

[Serhiy Storchaka]

Serhiy Storchaka added the comment:


New changeset 6b5a9ec4788770c652bac3bf5d5a0a3b710b82ae by Serhiy Storchaka in 
branch 'master':
bpo-29116: Fix error messages for concatenating bytes and bytearray with 
unsupported type. (#709)
https://github.com/python/cpython/commit/6b5a9ec4788770c652bac3bf5d5a0a3b710b82ae


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[issue29116] Make str and bytes error messages on concatenation conform with other sequences

[Serhiy Storchaka]

Serhiy Storchaka added the comment:


New changeset 004e03fb0c2febe2ec8afbd28ffcb3e980c63228 by Serhiy Storchaka in 
branch 'master':
bpo-29116: Improve error message for concatenating str with non-str. (#710)
https://github.com/python/cpython/commit/004e03fb0c2febe2ec8afbd28ffcb3e980c63228


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[issue29116] Make str and bytes error messages on concatenation conform with other sequences

[Serhiy Storchaka]

Serhiy Storchaka added the comment:


New changeset 3d258b1eb453bcbc412d6b252f5bdceae0303f07 by Serhiy Storchaka in 
branch '3.6':
bpo-29116: Fix error messages for concatenating bytes and bytearray with 
unsupported type. (#709) (#723)
https://github.com/python/cpython/commit/3d258b1eb453bcbc412d6b252f5bdceae0303f07


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[issue29116] Make str and bytes error messages on concatenation conform with other sequences

[Serhiy Storchaka]

Serhiy Storchaka added the comment:


New changeset 2c5b2c3832d4d2af7b60333a5a8f73dd51ef6245 by Serhiy Storchaka in 
branch '3.5':
bpo-29116: Fix error messages for concatenating bytes and bytearray with 
unsupported type. (#709) (#724)
https://github.com/python/cpython/commit/2c5b2c3832d4d2af7b60333a5a8f73dd51ef6245


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[issue29116] Make str and bytes error messages on concatenation conform with other sequences

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

I don't know why statuses was not updated automatically. Updated them manually.

Ivan in the comment on GitHub suggested to use "concatenate" instead of 
"concat" in "can't concat  to bytes". Maybe make it more similar to 
messages for list, tuple, deque, array? "can only concatenate objects 
supporting the buffer protocol (not "") to bytes"?

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[issue29116] Make str and bytes error messages on concatenation conform with other sequences

[Ivan Levkivskyi]

Ivan Levkivskyi added the comment:

Something is strange: PRs 709, 723, 724 are shown as open in the "Pull 
Requests" section on this page. However, all four PRs are already merged.

Are other see the same? Shouldn't status be automatically updated?

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[issue29116] Make str and bytes error messages on concatenation conform with other sequences

[Serhiy Storchaka]

Changes by Serhiy Storchaka :


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pull_requests: +639

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[issue29116] Make str and bytes error messages on concatenation conform with other sequences

[Serhiy Storchaka]

Changes by Serhiy Storchaka :


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pull_requests: +638

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[issue29116] Make str and bytes error messages on concatenation conform with other sequences

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

PR 709 fixes the order of types for concatenating bytes and and bytearray. 
b''+0 will raise "can't concat str to bytes" rather than "can't concat bytes to 
str".

PR 710 makes error message for str concatenation more informative and similar 
to error messages for list, tuple, deque, array. '' + b'' will raise "can only 
concatenate str (not "bytes") to str" rather than "must be str, not bytes".

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[issue29116] Make str and bytes error messages on concatenation conform with other sequences

[Serhiy Storchaka]

Changes by Serhiy Storchaka :


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pull_requests: +630

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[issue29116] Make str and bytes error messages on concatenation conform with other sequences

[Serhiy Storchaka]

Changes by Serhiy Storchaka :


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pull_requests: +629

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[issue29116] Make str and bytes error messages on concatenation conform with other sequences

[Ned Deily]

Ned Deily added the comment:

I'm removing myself as assignee as this doesn't seem to need a 3.6 RM decision 
at this point.  If necessary, we can discuss a 3.6 backport after the issue has 
been resolved for 3.7.

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assignee: ned.deily -> 

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[issue29116] Make str and bytes error messages on concatenation conform with other sequences

[Jim Fasarakis-Hilliard]

Jim Fasarakis-Hilliard added the comment:

As I currently see this: 

 - The error message for str can be changed to the one used for other sequences 
'can only concatenate str (not "type") to str'

 - The error message for arrays can be changed to use concatenate instead of 
append, too.

For bytes I see a conundrum, on one hand it can be changed to mention the 
buffer protocol which might confuse new users more than it helps them or, it 
can try and mention the objects that currently conform to it which might be 
exhaustive and long.

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[issue29116] Make str and bytes error messages on concatenation conform with other sequences

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

You can concatenate any object supporting the buffer protocol to bytes and 
bytearray.

Current error message for bytes looks awkward too, because it says "bytes to 
othertype" instead of "othertype to bytes".

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[issue29116] Make str and bytes error messages on concatenation conform with other sequences

[Terry J. Reedy]

Terry J. Reedy added the comment:

By default, error message wording changes are 'enhancements' that wait for the 
next x.y.0 release unless the current wording is positively wrong'.  This is 
different from doc changes because there are tests depending on error messages. 
'Inconsistent' or 'awkward' is not the same as 'wrong'.

I do agree that the clipped "must be str, not bytes' is awkward.  What (which) 
is it that must be str?  A fleshed out and positive "can only concatenate str 
(not bytes) to str." would be clearer and educate users better.

As Serhiy said, the exact equivalent cannot be said for bytes.

>>> b'a' + bytearray(b'b')
b'ab'
>>> b'a' + memoryview(b'b')
b'ab'

However "Can only concatenate bytes, bytearray, or memoryview (not x) to 
bytes." would be good if this is in fact complete. I need to be educated on 
this ;-)  I was not sure about memoryview until I tried it.

--
nosy: +terry.reedy
type: behavior -> enhancement
versions:  -Python 3.6

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[issue29116] Make str and bytes error messages on concatenation conform with other sequences

[Raymond Hettinger]

Changes by Raymond Hettinger :


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nosy: +rhettinger

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[issue29116] Make str and bytes error messages on concatenation conform with other sequences

[Ivan Levkivskyi]

Changes by Ivan Levkivskyi :


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nosy: +levkivskyi

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[issue29116] Make str and bytes error messages on concatenation conform with other sequences

[Xiang Zhang]

Xiang Zhang added the comment:

> As for `operator.concat`, any reason why the check is made beforehand to see 
> if the first argument has a `__getitem__` method?

Concatenation is an operation of sequence (+ could be used both for 
concatenation and addition), so you have to check __getitem__ to make sure it's 
a sequence.

> Couldn't that just be removed allowing the exception from `concat(1, '')` to 
> just propagate to the caller?

If removed, `concat(1, '')` could propagate exceptions but not `concat(1, 1)`.

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[issue29116] Make str and bytes error messages on concatenation conform with other sequences

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

Originally this issue was raised on StackOverflow:

http://stackoverflow.com/questions/41388606/python-3-6-vs-3-5-typeerror-message-on-string-concatenation

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[issue29116] Make str and bytes error messages on concatenation conform with other sequences

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

Different cases accept different types. Not always we can say that only one 
specific type is accepted.

"list +" accepts only lists, "list +=" is a syntax sugar for list.extend and 
accepts any iterables. "bytes +" and "bytearray +=" accept any objects that 
support the buffer protocol. "array +" and "array +=" accept only arrays.

Changing semantic is out of the scope of this issue. I think there are reasons 
for current behavior.

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[issue29116] Make str and bytes error messages on concatenation conform with other sequences

[Jim Fasarakis-Hilliard]

Jim Fasarakis-Hilliard added the comment:

Should that message be the one predominantly used for sequences, i.e:

   TypeError: can only concatenate class1 (not "class2") to class1

or should another one be used like "Unsupported operand type(s) for op: 
'class1' and 'class2'?

The first is problematic with cases like `+=` where an iterable is accepted, 
the second seems better to me at least.

As for `operator.concat`, any reason why the check is made beforehand to see if 
the first argument has a `__getitem__` method? Couldn't that just be removed 
allowing the exception from `concat(1, '')` to just propagate to the caller?

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[issue29116] Make str and bytes error messages on concatenation conform with other sequences

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

Old error message is misleading. It implied that there are objects that can be 
converted to str implicitly. This was true in Python 2, but is false in Python 
3.

New error message conforms with TypeError messages produced by PyArg_Parse*(). 
It is the same in a number of str methods. It doesn't conform with the error 
messages for other produced by concatenating other types. But different 
messages are generated for different types:

>>> b'' + 1
Traceback (most recent call last):
  File "", line 1, in 
TypeError: can't concat bytes to int
>>> a = bytearray(); a += 1
Traceback (most recent call last):
  File "", line 1, in 
TypeError: can't concat int to bytearray
>>> [] + 1
Traceback (most recent call last):
  File "", line 1, in 
TypeError: can only concatenate list (not "int") to list
>>> a = []; a += 1
Traceback (most recent call last):
  File "", line 1, in 
TypeError: 'int' object is not iterable
>>> import array; array.array('b') + []
Traceback (most recent call last):
  File "", line 1, in 
TypeError: can only append array (not "list") to array
>>> import array; a = array.array('b'); a += []
Traceback (most recent call last):
  File "", line 1, in 
TypeError: can only extend array with array (not "list")
>>> import operator; operator.concat(1, '')
Traceback (most recent call last):
  File "", line 1, in 
TypeError: 'int' object can't be concatenated

I think it would be better to unify them all.

I'm not sure that this change can be considered as a bug fix rather than an 
enhancement. Leave this on 3.6 release manager.

--
assignee:  -> ned.deily
nosy: +ned.deily, serhiy.storchaka

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[issue29116] Make str and bytes error messages on concatenation conform with other sequences

[Jim Fasarakis-Hilliard]

New submission from Jim Fasarakis-Hilliard:

Specifically, bytes (always, from what I could find) had this error message:

>>> b'' + ''
TypeError: can't concat bytes to str

while str, after a change in issue26057, was made to:

>>> '' + b''
TypeError: must be str, not bytes

from the previous form of "TypeError: Can't convert 'bytes' object to str 
implicitly".

I think these could be changed to conform with what the other sequences 
generate and fall in line with the error messages produced for other operations.

Specifically changing them to:

>>> b'' + ''
TypeError: can only concatenate bytes (not 'str') to bytes

and similarly for str.

If this idea makes sense, I can attach a patch that addresses it.

--
components: Interpreter Core
messages: 284340
nosy: Jim Fasarakis-Hilliard
priority: normal
severity: normal
status: open
title: Make str and bytes error messages on concatenation conform with other 
sequences
type: behavior
versions: Python 3.6, Python 3.7

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