[issue28385] Bytes objects should reject all formatting codes with an error message

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

The proposition of making default __format__ equivalent to str() will be 
addressed in separate issue. Python-Dev thread: 
https://mail.python.org/pipermail/python-dev/2016-October/146765.html.

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

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[issue28385] Bytes objects should reject all formatting codes with an error message

[Eric V. Smith]

Eric V. Smith added the comment:

I don't have a strong feeling one way or the other. I'd be surprised if anyone 
is catching these errors.

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[issue28385] Bytes objects should reject all formatting codes with an error message

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

But on other hand, the error depends on the value of format specifier:

>>> format('abc', '')
'abc'
>>> format('abc', 'j')
Traceback (most recent call last):
  File "", line 1, in 
ValueError: Unknown format code 'j' for object of type 'str'
>>> format([1, 2, 3], '')
'[1, 2, 3]'
>>> format([1, 2, 3], 'j')
Traceback (most recent call last):
  File "", line 1, in 
TypeError: unsupported format string passed to list.__format__

If keep TypeError I think it would be better to restore former error message 
"non-empty format string".

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[issue28385] Bytes objects should reject all formatting codes with an error message

[Eric V. Smith]

Eric V. Smith added the comment:

TypeError is documented as "Raised when an operation or function is applied to 
an object of inappropriate type". I think that fits this case: you're applying 
some format code to a type that doesn't support it.

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[issue28385] Bytes objects should reject all formatting codes with an error message

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

Shouldn't the type of error be changed from TypeError to ValueError?

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[issue28385] Bytes objects should reject all formatting codes with an error message

[Roundup Robot]

Roundup Robot added the comment:

New changeset f43078ec598c by Serhiy Storchaka in branch '3.6':
Update the f-string test broken in issue #28385.
https://hg.python.org/cpython/rev/f43078ec598c

New changeset 931410a04240 by Serhiy Storchaka in branch 'default':
Update the f-string test broken in issue #28385.
https://hg.python.org/cpython/rev/931410a04240

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[issue28385] Bytes objects should reject all formatting codes with an error message

[Eric V. Smith]

Eric V. Smith added the comment:

I would change the f-string tests. I realize it's a fragile test, but it's the 
only way to test the strings in the exception.

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[issue28385] Bytes objects should reject all formatting codes with an error message

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

What is better?

1. Replace "unsupported format string passed to" back to "non-empty format 
string passed to " in generated error message? Note that full error message is 
changed in any case, this is the purpose of this issue.

2. Change the pattern "non-empty" to "unsupported" in f-string tests. Or to 
other part of new error message. It may be changed again in future.

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[issue28385] Bytes objects should reject all formatting codes with an error message

[Ned Deily]

Ned Deily added the comment:

Buildbots are failing:

==
FAIL: test_errors (test.test_fstring.TestCase) (str="f'{(lambda: 0):x}'")
--
TypeError: unsupported format string passed to function.__format__

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File 
"/root/buildarea/3.6.angelico-debian-amd64/build/Lib/test/test_fstring.py", 
line 20, in assertAllRaise
eval(str)
AssertionError: "non-empty" does not match "unsupported format string passed to 
function.__format__"

==
FAIL: test_errors (test.test_fstring.TestCase) (str="f'{(0,):x}'")
--
TypeError: unsupported format string passed to tuple.__format__

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File 
"/root/buildarea/3.6.angelico-debian-amd64/build/Lib/test/test_fstring.py", 
line 20, in assertAllRaise
eval(str)
AssertionError: "non-empty" does not match "unsupported format string passed to 
tuple.__format__"

--
Ran 48 tests in 1.389s

--
nosy: +ned.deily

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[issue28385] Bytes objects should reject all formatting codes with an error message

[Roundup Robot]

Roundup Robot added the comment:

New changeset 92cae79fa5d9 by Serhiy Storchaka in branch '3.5':
Issue #28385: An error message when non-empty format spec is passed to
https://hg.python.org/cpython/rev/92cae79fa5d9

New changeset 0a985f7c6731 by Serhiy Storchaka in branch '3.6':
Issue #28385: An error message when non-empty format spec is passed to
https://hg.python.org/cpython/rev/0a985f7c6731

New changeset 6e8183abcc35 by Serhiy Storchaka in branch 'default':
Issue #28385: An error message when non-empty format spec is passed to
https://hg.python.org/cpython/rev/6e8183abcc35

--
nosy: +python-dev

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[issue28385] Bytes objects should reject all formatting codes with an error message

[Serhiy Storchaka]

Changes by Serhiy Storchaka :


--
assignee:  -> serhiy.storchaka

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[issue28385] Bytes objects should reject all formatting codes with an error message

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

>From msg214162:

> This is basically the definition of object.__format__:
>
> def __format__(self, specifier):
>   if len(specifier) == 0:
> return str(self)
>   raise TypeError('non-empty format string passed to object.__format__')

This simple algorithm is implemented in object-__format__.patch.

But current implementation uses ``format(str(self), '')`` instead of just 
``str(self)``. object-__format__-2.patch keeps this.

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[issue28385] Bytes objects should reject all formatting codes with an error message

[STINNER Victor]

Changes by STINNER Victor :


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nosy: +haypo

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[issue28385] Bytes objects should reject all formatting codes with an error message

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

Removed item 3 from updated patch.

--
Added file: http://bugs.python.org/file45127/object-__format__-2.patch

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[issue28385] Bytes objects should reject all formatting codes with an error message

[Eric V. Smith]

Eric V. Smith added the comment:

LGTM, although I'm not so sure about your #3. Maybe it should be a separate 
issue and raised on python-dev? But I don't feel strongly enough about it to 
argue the point.

Thanks!

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[issue28385] Bytes objects should reject all formatting codes with an error message

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

Sorry, but it looks to me that issue28385.diff solves wrong issue. The issue is 
not in bytes, but in default __format__. object.__format__ should raise an 
error message that contains the name of the actual type. There are other issues 
with object.__format__, following patch tries to solve them.

1. Error message now contains the name of the actual type.

2. Format spec is checked before converting the value to str. Converting bytes 
and bytearray to str can raise a BytesWarning, and it is better if the type of 
this error doesn't depend on the -b option.

3. Since format spec is always empty, avoid calling PyObject_Format(). In 
theory PyObject_Str() can raise a subclass of str with non-trivial __format__, 
but I think we can ignore this subtle behavior difference.

--
nosy: +serhiy.storchaka
Added file: http://bugs.python.org/file45015/object___format__.patch

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[issue28385] Bytes objects should reject all formatting codes with an error message

[Eric V. Smith]

Eric V. Smith added the comment:

I left a comment on Rietveld, but repeating it here because for me those 
messages often go to spam:

I think you want to make this more general, by modifying the message for the 
TypeError that follows. Any type that doesn't provide it's own __format__ 
should produce an error when using a non-empty format string.

For example:

>>> format({}, 'a')
Traceback (most recent call last):
  File "", line 1, in 
TypeError: non-empty format string passed to object.__format__

Would be better as:
TypeError: non-empty format string passed to __format__ for object of type 
"class <'dict'>"

(Or some prettier way to print out the type).

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[issue28385] Bytes objects should reject all formatting codes with an error message

[Berker Peksag]

Berker Peksag added the comment:

Here's a patch. Changes in test_builtin might be redundant, but I added them 
anyway.

--
keywords: +patch
nosy: +berker.peksag
stage: needs patch -> patch review
Added file: http://bugs.python.org/file45011/issue28385.diff

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[issue28385] Bytes objects should reject all formatting codes with an error message

[Марк Коренберг]

Марк Коренберг added the comment:

Yes, that message will be sufficient. Perfectly.

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[issue28385] Bytes objects should reject all formatting codes with an error message

[Eric V. Smith]

Eric V. Smith added the comment:

Yes, I think we should implement this for object.__format__. Marking as easy.

--
components: +Interpreter Core -Library (Lib)
keywords: +easy (C)

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[issue28385] Bytes objects should reject all formatting codes with an error message

[R. David Murray]

R. David Murray added the comment:

Bytes don't support formatting codes, so they are passed through to object, 
which doesn't support any formatting codes, and produces the error message you 
see.  Since all other built in types reject invalid codes with a message that 
mentions their type, I think bytes should too.  This would change the message 
to:

  ValueError: Unknown format code 's' for object of type 'bytes'

Would that have lessened your confusion sufficiently?

--
nosy: +eric.smith, r.david.murray
stage:  -> needs patch
title: Non-informative exception while formatting strings. -> Bytes objects 
should reject all formatting codes with an error message
type:  -> behavior

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