[issue13790] In str.format an incorrect error message for list, tuple, dict, set

[Eric V. Smith]

Change by Eric V. Smith :


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[issue13790] In str.format an incorrect error message for list, tuple, dict, set

[Irit Katriel]

Irit Katriel  added the comment:

This seems complete.

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[issue13790] In str.format an incorrect error message for list, tuple, dict, set

[miss-islington]

miss-islington  added the comment:


New changeset 445152e0d3ab6e4381aef8d1404c2c17a516070f by Miss Islington (bot) 
in branch '3.8':
bpo-13790: Change 'string' to 'specification' in format doc (GH-18690)
https://github.com/python/cpython/commit/445152e0d3ab6e4381aef8d1404c2c17a516070f


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[issue13790] In str.format an incorrect error message for list, tuple, dict, set

[miss-islington]

miss-islington  added the comment:


New changeset 5157506e043f75f49caecae1c6afee8517a7bbb0 by Miss Islington (bot) 
in branch '3.7':
bpo-13790: Change 'string' to 'specification' in format doc (GH-18690)
https://github.com/python/cpython/commit/5157506e043f75f49caecae1c6afee8517a7bbb0


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[issue13790] In str.format an incorrect error message for list, tuple, dict, set

[miss-islington]

Change by miss-islington :


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[issue13790] In str.format an incorrect error message for list, tuple, dict, set

[miss-islington]

Change by miss-islington :


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[issue13790] In str.format an incorrect error message for list, tuple, dict, set

[Terry J. Reedy]

Terry J. Reedy  added the comment:


New changeset 916895f93905f8b8dad677cceff501833f5a633a by Terry Jan Reedy in 
branch 'master':
bpo-13790: Change 'string' to 'specification' in format doc (GH-18690)
https://github.com/python/cpython/commit/916895f93905f8b8dad677cceff501833f5a633a


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[issue13790] In str.format an incorrect error message for list, tuple, dict, set

[Terry J. Reedy]

Terry J. Reedy  added the comment:

PR-18690 makes the approved change of 'string' to 'specification'.  After 
merging, I will re-review the other changes in i13790b.diff and possibly make 
another PR for review.

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[issue13790] In str.format an incorrect error message for list, tuple, dict, set

[Terry J. Reedy]

Change by Terry J. Reedy :


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[issue13790] In str.format an incorrect error message for list, tuple, dict, set

[Mark Lawrence]

Change by Mark Lawrence :


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[issue13790] In str.format an incorrect error message for list, tuple, dict, set

[Terry J. Reedy]

Terry J. Reedy  added the comment:

In msg151730, R. David Murry said "Terry's [first] patch with the ("{}") 
removed should be committed, though."
In msg151738, Eric V. Smith said "I agree with your comment about Terry's 
patch."

My second patch removed "{}" but also made more text changes, explained in 
msg151757.  Someone should re-review

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[issue13790] In str.format an incorrect error message for list, tuple, dict, set

[Cheryl Sabella]

Cheryl Sabella  added the comment:

>From the examples in msg220401, issue28385 changed it to print the object type 
>in the message. 

>>> format([], 'd')
Traceback (most recent call last):
  File "", line 1, in 
TypeError: unsupported format string passed to list.__format__
>>> format((), 'd')
Traceback (most recent call last):
  File "", line 1, in 
TypeError: unsupported format string passed to tuple.__format__


Would the change left on this issue be to create a PR for Terry's documetation 
patch?

Thanks!

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[issue13790] In str.format an incorrect error message for list, tuple, dict, set

[Terry J. Reedy]

Changes by Terry J. Reedy :


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[issue13790] In str.format an incorrect error message for list, tuple, dict, set

[Terry J. Reedy]

Terry J. Reedy added the comment:

Yes, the deprecation in 3.3 did not apply to 2.7.

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[issue13790] In str.format an incorrect error message for list, tuple, dict, set

[py.user]

py.user added the comment:

Python 2.7.7 is still printing.

>>> format([], 'd')
Traceback (most recent call last):
  File "", line 1, in 
ValueError: Unknown format code 'd' for object of type 'str'
>>>

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[issue13790] In str.format an incorrect error message for list, tuple, dict, set

[Eric V. Smith]

Eric V. Smith added the comment:

I believe that comment was referring to the subject of this bug:

$ ./python
Python 3.4.1+ (3.4:bec6f18dd636, Jun 12 2014, 20:23:30)
[GCC 4.8.1] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> format([], 'd')
Traceback (most recent call last):
  File "", line 1, in 
TypeError: non-empty format string passed to object.__format__
>>> format((), 'd')
Traceback (most recent call last):
  File "", line 1, in 
TypeError: non-empty format string passed to object.__format__
>>> format({}, 'd')
Traceback (most recent call last):
  File "", line 1, in 
TypeError: non-empty format string passed to object.__format__
>>> format(set(), 'd')
Traceback (most recent call last):
  File "", line 1, in 
TypeError: non-empty format string passed to object.__format__

With the possible exception of listing the type in this error message, I think 
these are all fine.


I'm not sure what you'd expect format('a', 'd') to produce other than the error 
you're seeing. 'd' is in fact an unknown "format code" for str.

>>> format('a', 'd')
Traceback (most recent call last):
  File "", line 1, in 
ValueError: Unknown format code 'd' for object of type 'str'

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[issue13790] In str.format an incorrect error message for list, tuple, dict, set

[Mark Lawrence]

Mark Lawrence added the comment:

Python 3.4.1 (v3.4.1:c0e311e010fc, May 18 2014, 10:38:22) [MSC v.1600 32 bit 
(Intel)] on win32
Type "copyright", "credits" or "license()" for more information.
>>> '{0:d}'.format('a')
Traceback (most recent call last):
  File "", line 1, in 
'{0:d}'.format('a')
ValueError: Unknown format code 'd' for object of type 'str'

Nothing appears to have changed despite "the issue will go away in 3.4" in 
msg164373.  What should have happened here?

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[issue13790] In str.format an incorrect error message for list, tuple, dict, set

[Serhiy Storchaka]

Changes by Serhiy Storchaka :


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[issue13790] In str.format an incorrect error message for list, tuple, dict, set

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

> Serhiy: I'm not sure what you're saying. At the point that str.format() is 
> producing its error message, it doesn't know as much as %-formatting does 
> about the original arguments, so it can't produce a similar message.

I'm surprised that the code of the classic and the modern formatting is
so different. Looking deeper, I saw that the issue will go away in 3.4.
I agree with you in msg151728.

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[issue13790] In str.format an incorrect error message for list, tuple, dict, set

[Eric V. Smith]

Eric V. Smith  added the comment:

Serhiy: I'm not sure what you're saying. At the point that str.format() is 
producing its error message, it doesn't know as much as %-formatting does about 
the original arguments, so it can't produce a similar message.

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[issue13790] In str.format an incorrect error message for list, tuple, dict, set

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

>>> '%d' % ([],)
Traceback (most recent call last):
  File "", line 1, in 
TypeError: %d format: a number is required, not list

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[issue13790] In str.format an incorrect error message for list, tuple, dict, set

[Ezio Melotti]

Changes by Ezio Melotti :


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[issue13790] In str.format an incorrect error message for list, tuple, dict, set

[Terry J. Reedy]

Terry J. Reedy  added the comment:

Looking further, I noticed that 'string' needed to be changed to 
'specification' in the following sentence also. Then I decided that the 
preceding sentence
 
"Most built-in types implement the following options for format specifications, 
although some of the formatting options are only supported by the numeric 
types."

should really follow the one about non-empty format specs. This positioning 
should make it more obvious that most of the options affect the string 
representation of the object after, not before, the string is produced, and are 
therefore applicable to all objects and not just string and number objects. I 
also propose to modify it so it is shorter and no longer contradictory, to read

"Most built-in types implement various options for such modifications, although 
some are only supported by the numeric types."

Further on, under "The available string presentation types are:"
I think "``'s'`` String format. This is the default type for strings and may be 
omitted." should have 'and other non-numeric types ' inserted after strings. 
New patch i13790b.diff attached

The point of these additional changes is to make it clearer that the default 
formatting of non-number, non-string objects is to call str() and then apply 
the options to the resulting string. That makes something like
>>> format(range(5), '-^20s') # same with object.__format__(), 3.3.0a0
'range(0, 5)-'
predictable and comprehensible.

I agree with not making a temporary change (but see below ;-).

But it seems that the 3.4 message should at least be
"numeric format string passed to object.__format__" or
"format string with number-only options passed to object.__format__" or
"object.__format__ cannot handle number-only options"
as string formats work fine and, I presume, are not deprecated (?).

However, if the new ValueError message did not specify object.__format__ (which 
could still be confusing, even if more accurate), the change could be make now. 
For instance
'Numeric option 'd' for non-number object'.
It would not really matter if it is later raised in object.__format__ instead 
of str.__format__. I believe *all* of the format codes 'unknown' to str (and by 
extension, by default, to all other non-number types) *are* number codes.

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assignee: docs@python -> terry.reedy
Added file: http://bugs.python.org/file24290/i13790b.diff

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[issue13790] In str.format an incorrect error message for list, tuple, dict, set

[Eric V. Smith]

Eric V. Smith  added the comment:

The error message will be: "non-empty format string passed to 
object.__format__".

I agree with your comment about Terry's patch.

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[issue13790] In str.format an incorrect error message for list, tuple, dict, set

[R. David Murray]

R. David Murray  added the comment:

So the error is going to be something about the source type not supporting 
'__format__'?

That change will also address the OP's concern about truncated reprs when a 
fixed string length is specified, so I agree that the title issue can be 
closed.  Terry's patch with the ("{}") removed should be committed, though.

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[issue13790] In str.format an incorrect error message for list, tuple, dict, set

[Eric V. Smith]

Eric V. Smith  added the comment:

While looking at object.__format__, I recall that we've already addressed this, 
sort of. For a different reason, this is already deprecated in 3.3 and will 
become an error in 3.4. See issues 9856 and 7994.

$ ./python -Wd
Python 3.3.0a0 (default:40e1be1e0707, Jan 15 2012, 00:58:51) 
[GCC 4.6.1] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> format([], 'd')
__main__:1: DeprecationWarning: object.__format__ with a non-empty format 
string is deprecated
Traceback (most recent call last):
  File "", line 1, in 
ValueError: Unknown format code 'd' for object of type 'str'
[67288 refs]
>>> 

We could still have object.__format__ catch and re-throw the ValueError with a 
better message. I'd have to think it through if we could catch all ValueErrors, 
or if it's possible for another ValueError to be thrown and we'd only catch and 
rethrow this specific ValueError.

But since this is deprecated, I'm not sure it's worth the hassle. I'd advocate 
closing this issue as "won't fix".

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[issue13790] In str.format an incorrect error message for list, tuple, dict, set

[Terry J. Reedy]

Terry J. Reedy  added the comment:

OK, the example of an empty format spec should be dropped. Let people figure it 
out ;-).

>>> format([], 'd')
Traceback (most recent call last):
  File "", line 1, in 
ValueError: Unknown format code 'd' for object of type 'str'

One possibility is to give (str of) the object instead of the type:

ValueError: Unknown format code 'd' for object '[]'

The downside is a long message for long strings. It would need to be limited 
(as is done in test error reports).

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[issue13790] In str.format an incorrect error message for list, tuple, dict, set

[R. David Murray]

R. David Murray  added the comment:

Oh, never mind that comment about recursion, I wasn't thinking it through.

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[issue13790] In str.format an incorrect error message for list, tuple, dict, set

[R. David Murray]

R. David Murray  added the comment:

Oh, I see.  Yes, that is a problem.

object.__format__ knows the type of the object it was called on, right?  
Couldn't it catch the error and re-raise it with the correct type?  (If the 
type isn't str, of course, we don't want to get into an infinite recursion.)

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[issue13790] In str.format an incorrect error message for list, tuple, dict, set

[Eric V. Smith]

Eric V. Smith  added the comment:

I don't think "{}" is the correct way to document this. These all have an empty 
format specifier:

"{}".format(foo)
"{:}".format(foo)
"{0}".format(foo)
"{0:}".format(foo)
"{name}".format(name=foo)
format(foo, "")
format(foo)

That is, they all call foo.__format__(""). If foo.__format__ (well, really 
type(foo).__format__) doesn't exist, then object.__format__(foo, "") gets 
called. It's object.__format__ that's checking for the empty format string, and 
if so it returns str(foo).

What would you suggest changing the ':d' error message to, for objects that 
don't support a format type of 'd'? This makes sense to me:

>>> format('', 'd')
Traceback (most recent call last):
  File "", line 1, in 
ValueError: Unknown format code 'd' for object of type 'str'

The problem, if there is one, is:
>>> format([], 'd')
Traceback (most recent call last):
  File "", line 1, in 
ValueError: Unknown format code 'd' for object of type 'str'

The problem is that the str that's producing this error doesn't know that it 
exists because object.__format__ returned str([]).

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[issue13790] In str.format an incorrect error message for list, tuple, dict, set

[Terry J. Reedy]

Terry J. Reedy  added the comment:

Doc patch attached to make sure correct. Should {} be quoted?

Eric, do you want to close off the idea of changing :d errors, or switch back 
after the doc fix?

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stage:  -> test needed
Added file: http://bugs.python.org/file24287/i13790.diff

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[issue13790] In str.format an incorrect error message for list, tuple, dict, set

[Eric V. Smith]

Eric V. Smith  added the comment:

Changing to a documentation issue.

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components: +Documentation -Interpreter Core
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nosy: +docs@python
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[issue13790] In str.format an incorrect error message for list, tuple, dict, set

[R. David Murray]

R. David Murray  added the comment:

Good point.  That should be fixed.  It should be "empty format specification".

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[issue13790] In str.format an incorrect error message for list, tuple, dict, set

[py.user]

py.user  added the comment:

R. David Murray wrote:
> Putting nothing between the {}'s is an empty format string.

this is an empty replacement field

here:
http://docs.python.org/py3k/library/string.html#format-string-syntax

the definition of format string:
"Format strings contain “replacement fields” surrounded by curly braces {}. 
Anything that is not contained in braces is considered literal text, which is 
copied unchanged to the output."

"The grammar for a replacement field is as follows:"
replacement_field ::=  "{" [field_name] ["!" conversion] [":" format_spec] "}"

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[issue13790] In str.format an incorrect error message for list, tuple, dict, set

[R. David Murray]

R. David Murray  added the comment:

"an empty format string" is exactly what I was talking about.  Putting nothing 
between the {}'s is an empty format string.  I can't think of any way to make 
that wording clearer.

The format docs should not contains examples of the repr of all possible python 
objects.  The examples of what tuples and lists and dicts &c look like are 
shown in the docs for those objects.

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[issue13790] In str.format an incorrect error message for list, tuple, dict, set

[py.user]

py.user  added the comment:

also here:
http://docs.python.org/py3k/library/string.html#format-examples

there is no example with list or tuple to know exactly how they are formatted

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[issue13790] In str.format an incorrect error message for list, tuple, dict, set

[py.user]

py.user  added the comment:

R. David Murray wrote:
> it is made clear in various places that every object has an str

here:
http://docs.python.org/py3k/library/string.html#format-specification-mini-language

3rd paragraph:
"A general convention is that an empty format string ("") produces the same 
result as if you had called str() on the value. A non-empty format string 
typically modifies the result."

"an empty format string ("")" what does it mean ?

"".format(value) or "{}".format(value) or "{0}".format(value) ?

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[issue13790] In str.format an incorrect error message for list, tuple, dict, set

[R. David Murray]

R. David Murray  added the comment:

Oh, and when you say there is nothing in the documentation about the 's' case 
for arbitrary objects, it is made clear in various places that every object has 
an str, which defaults to its repr if it has no specific __str__.  Combine that 
with the description of what happens when you use a fixed field length for 's', 
and you get the results you see.  There should be nothing surprising about this 
to anyone who has read the tutorial, I think.  (But specific suggestions for 
improving the docs are always welcome.)

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[issue13790] In str.format an incorrect error message for list, tuple, dict, set

[R. David Murray]

R. David Murray  added the comment:

No, it wouldn't.  I expect

  "{}".format(x)

to produce something for an arbitrary x.  Breaking that would break a 
fundamental Python contract.

Improving the error message for 'd' is more possible.  Perhaps "the format code 
'd' is not implemented by objects of type "?

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[issue13790] In str.format an incorrect error message for list, tuple, dict, set

[py.user]

py.user  added the comment:

also strange(unobvious) behavior:
>>> '{0:.3s}'.format((i for i in (1, 2, 3)))
'>> '{0:.3s}'.format(range(10))
'ran'
>>> '{0:.3s}'.format(None)
'Non'
>>>

it would be better to print an error:
ValueError: Unknown format code 's' for object of type 'generator'

like in this:
>>> '{0:d}'.format(4.5)
Traceback (most recent call last):
  File "", line 1, in 
ValueError: Unknown format code 'd' for object of type 'float'
>>>

in the documentation there is nothing about it

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[issue13790] In str.format an incorrect error message for list, tuple, dict, set

[Eric V. Smith]

Eric V. Smith  added the comment:

I agree it's not the best error message. What's happening is that these types 
(list, tuple, etc.) do not implement __format__, so object.__format__ is used. 
It returns str(self). Then the resulting string is formatted with the given 
format_spec. Since str does not support the 'd' format type, the error you see 
is raised.

I'm open to suggestions on how to improve this, but I don't see how it's 
possible given what str.__format__ knows when it generates the error.

--
assignee:  -> eric.smith
nosy: +eric.smith
versions: +Python 3.2, Python 3.3 -Python 3.1

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[issue13790] In str.format an incorrect error message for list, tuple, dict, set

[py.user]

New submission from py.user :

>>> '{0:d}'.format('a')
Traceback (most recent call last):
  File "", line 1, in 
ValueError: Unknown format code 'd' for object of type 'str'
>>> '{0:d}'.format(1+1j)
Traceback (most recent call last):
  File "", line 1, in 
ValueError: Unknown format code 'd' for object of type 'complex'
>>> '{0:d}'.format([])
Traceback (most recent call last):
  File "", line 1, in 
ValueError: Unknown format code 'd' for object of type 'str'
>>>

also strange behavior:
>>> '{0:s}'.format((1, 2, 3))
'(1, 2, 3)'
>>> '{0:10.5s}'.format([1, 2, 3])
'[1, 2 '
>>>

--
components: Interpreter Core
messages: 151277
nosy: py.user
priority: normal
severity: normal
status: open
title: In str.format an incorrect error message for list, tuple, dict, set
type: behavior
versions: Python 3.1

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