[issue7951] Should str.format allow negative indexes when used for __getitem__ access?

[Eric V. Smith]

Eric V. Smith  added the comment:

I'm closing this as "won't fix" for the negative indexing functionality. If 
someone wants to open an new documentation issue (and ideally provide a PR), 
that would be welcome.

--
resolution:  -> wont fix
stage: needs patch -> resolved
status: open -> closed

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[issue7951] Should str.format allow negative indexes when used for __getitem__ access?

[Raymond Hettinger]

Raymond Hettinger  added the comment:

I recommend not adding support for negative indexing to format() for accessing 
positional arguments.  There is almost no reason to do this because it almost 
always makes the format string less readable, because the number of arguments 
is always known in advance, and because the arguments are almost always used 
entirely rather than selectively.

Negative index support isn't a feature of the language.  Instead, it is a 
feature provided on a class by class basis, if it makes sense for that class 
and for its use cases.

We are not obliged to provide negative index support in places where it doesn't 
make sense or where it makes code less readable.  For example, the islice() 
function doesn't support negative indices because it doesn't make sense there.  
Likewise, the Sequence ABC doesn't require negative index support or slice 
support.

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[issue7951] Should str.format allow negative indexes when used for __getitem__ access?

[Irit Katriel]

Irit Katriel  added the comment:

Reproduced on 3.11, and the error message is a little weirder now:

>>> "{0[-1]}".format('fox')
Traceback (most recent call last):
  File "", line 1, in 
TypeError: string indices must be integers, not 'str'

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[issue7951] Should str.format allow negative indexes when used for __getitem__ access?

[Ilya Kamenshchikov]

Ilya Kamenshchikov  added the comment:

Py3.6+ f-strings support any indexing as they actually evaluate python 
expressions. 

>>> a = ['Java', 'Python']
>>> var = f"Hello {a[-1]}"
Hello Python

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[issue7951] Should str.format allow negative indexes when used for __getitem__ access?

[Eric V. Smith]

Eric V. Smith  added the comment:

I'm not sure where (or if) it's defined in the Python docs, but in PEP 3101 
it's in 
https://www.python.org/dev/peps/pep-3101/#simple-and-compound-field-names: "It 
should be noted that the use of 'getitem' within a format string is much more 
limited than its conventional usage. In the above example, the string 'name' 
really is the literal string 'name', not a variable named 'name'. The rules for 
parsing an item key are very simple. If it starts with a digit, then it is 
treated as a number, otherwise it is used as a string.".

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[issue7951] Should str.format allow negative indexes when used for __getitem__ access?

[Éric Araujo]

Éric Araujo  added the comment:

A side question: where is it defined that in `{thing[0]}`, 0 will be parsed as 
an integer?

The PEP shows `{thing[name]}` and mentions that this is not Python but a 
smaller mini-language, with `name` always a string, no quotes needed or 
permitted.

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[issue7951] Should str.format allow negative indexes when used for __getitem__ access?

[Mark Lawrence]

Changes by Mark Lawrence :


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[issue7951] Should str.format allow negative indexes when used for __getitem__ access?

[Marco Buttu]

Marco Buttu added the comment:

The error message is misleading:

>>> s = '{names[-1]} loves {0[1]}'
>>> s.format(('C', 'Python'), names=('Dennis', 'Guido'))
Traceback (most recent call last):
...
TypeError: tuple indices must be integers or slices, not str

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[issue7951] Should str.format allow negative indexes when used for __getitem__ access?

[Mark Lawrence]

Mark Lawrence added the comment:

msg216038 suggests three options for the doc patch, does anybody have any 
preference or a better alternative?

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[issue7951] Should str.format allow negative indexes when used for __getitem__ access?

[Terry J. Reedy]

Changes by Terry J. Reedy tjre...@udel.edu:


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[issue7951] Should str.format allow negative indexes when used for __getitem__ access?

[Terry J. Reedy]

Terry J. Reedy added the comment:

The doc bug is that the grammar block uses 'integer' (linked to 
https://docs.python.org/3/reference/lexical_analysis.html#grammar-token-integer)
 in
  arg_name  ::=  [identifier | integer]
  element_index ::=  integer | index_string
when it should use 'decimalinteger' or even more exactly 'digit+'. The int() 
builtin uses the same relaxed rule when no base is given.
 011
SyntaxError: invalid token
 int('011')
11
 '{[011]}'.format('abcdefghijlmn')
'm'

One possibity is to replace 'integer' in the grammar block with 'digit+' and 
perhaps leave the text alone. Another is to replace 'integer' with 
'index_number', to go with 'index_string, and add the production index_number 
::= digit+. My though for the latter is that 'index_number' would connect 
better with 'number' as used in the text. A further option would be to actually 
replace 'number' in the text with 'index_number'.


PS to Todd. As much as possible, doc content changes should be separated from 
re-formatting. I believe the first block of your patch is purely a re-format

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[issue7951] Should str.format allow negative indexes when used for __getitem__ access?

[Eric V. Smith]

Changes by Eric V. Smith e...@trueblade.com:


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[issue7951] Should str.format allow negative indexes when used for __getitem__ access?

[Terry J. Reedy]

Terry J. Reedy added the comment:

Either leading sign, '+' or '-', cause string interpretation, so I think 
'unsigned integer' should be the term in the doc.

 '{0[-1]}'.format({'-1': 'neg int key'})
'neg int key'
 '{0[+1]}'.format({'+1': 'neg int key'})
'neg int key'
 '{0[+1]}'.format([1,2,3])
Traceback (most recent call last):
  File pyshell#16, line 1, in module
'{0[+1]}'.format([1,2,3])
TypeError: list indices must be integers, not str

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[issue7951] Should str.format allow negative indexes when used for __getitem__ access?

[Mark Lawrence]

Changes by Mark Lawrence breamore...@yahoo.co.uk:


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[issue7951] Should str.format allow negative indexes when used for __getitem__ access?

[Mark Lawrence]

Mark Lawrence added the comment:

Todd's patch strikes me as fine.  If something more detailed is needed I think 
it would be better to raise a separate issue.

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[issue7951] Should str.format allow negative indexes when used for __getitem__ access?

[Todd Rovito]

Changes by Todd Rovito rovit...@gmail.com:


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[issue7951] Should str.format allow negative indexes when used for __getitem__ access?

[Todd Rovito]

Todd Rovito added the comment:

Here is a simple patch that simply explains negative indexes and negative 
slices are not supported for the string format documentation.  Perhaps more 
documentation needs to be created else where to help explain why all 
collections do not need to support negative indexes and negative slices? If so 
please let me know and I will create it.  But I think this patch at least 
clarifies for the use case of String format.

--
keywords: +patch
versions: +Python 3.4 -Python 3.2
Added file: 
http://bugs.python.org/file29948/7951NegativeIndexesForStringFormat3dot4.patch

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[issue7951] Should str.format allow negative indexes when used for __getitem__ access?

[Éric Araujo]

Éric Araujo mer...@netwok.org added the comment:

Thank you for the detailed argument, Raymond.  I’m +1 on turning this into a 
doc bug.

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[issue7951] Should str.format allow negative indexes when used for __getitem__ access?

[Raymond Hettinger]

Changes by Raymond Hettinger rhettin...@users.sourceforge.net:


--
components: +Documentation -Interpreter Core

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[issue7951] Should str.format allow negative indexes when used for __getitem__ access?

[Florent Xicluna]

Changes by Florent Xicluna florent.xicl...@gmail.com:


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[issue7951] Should str.format allow negative indexes when used for __getitem__ access?

[Raymond Hettinger]

Raymond Hettinger rhettin...@users.sourceforge.net added the comment:

After more thought, I'm -1 on this.  Consistency is a weak argument in favor 
of this.  We need to be more use case drivenm and it there is no evidence that 
this is needed.  Also, there is a reasonable concern that using negative 
indices in a format string would be a bad design pattern that should not be 
encouraged by the language.  And, there is a maintenance burden (just getting 
it right in the first place; having other implementations try to get it right; 
and a burden to custom formatters to have to support negative indices).

I do think we have a documentation issue.  This thread shows a number of 
experienced Python programmers who get surprised or perceive consistency 
issues perhaps because there isn't a clear mental picture  of Python's layer 
structure (separation of concerns) and where the responsibility lies for the 
supporting negative indices.

For the record, here are a few notes on where negative index handling fits into 
the hierarchy:

Negative index support is not guaranteed by the collections.Sequence ABC nor by 
the grammar (see the subscript rule in Grammar/Grammar).  It does not appear 
in opcode handling (see BINARY_SUBSCR in Python/ceval.c) nor in the top 
abstract layer (see PyObject_GetItem() in abstract.c).  Instead, the support 
for slicing and negative index handling appears at the concrete layer (see 
listindex() in Objects/listobject.c for example).

We do guarantee negative index handling for builtin sequences and their 
subclasses (as long as they don't override __getitem__), and we provide a fast 
path for their execution (via an intermediate abstract layer function, 
PySequence_GetItem() in Objects/abstract.c), but other sequence-like objects 
are free to make their own decisions about slices and negative indices at the 
concrete layer.

Knowing this, a person should not be surprised when one sequence has support 
for negative indices or slicing and another does not.  The choice belongs to 
the implementer of the concrete class, not to the caller of a[x].  There is 
no consistency issue here.

IOW, we're not required to implement negative slice handling and are free to 
decide whether it is a good idea or not for the use-case of string formatting.  
There is some question about whether it is a bad practice for people to use 
negative indices for string formatting.  If so, that would be a reason not to 
do it.  And if available, it would only work for builtin sequences, but not 
sequence like items in general.  There is also a concern about placing a burden 
on other implementations of Python (to match what we do in CPython) and on 
placing a burden on people writing their own custom formatters (to closely as 
possible mimic builtin formatters).  If so, those would be reasons not to do it.

my-two-cents,

Raymond

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[issue7951] Should str.format allow negative indexes when used for __getitem__ access?

[Raymond Hettinger]

Raymond Hettinger rhettin...@users.sourceforge.net added the comment:

Fixing-up str formatting idiosyncracies does not fall under the moratorium and 
is helpful in getting 3.x to be usable.

That being said, I'm not convinced that this is actually a helpful feature.  
Not all objects supporting __getitem__ offer support for negative indexing.  
Also, there's a case to be made that using negative indices in a formatting 
string is an anti-pattern, causing more harm than good.

--
keywords:  -after moratorium, patch
versions: +Python 3.2 -Python 3.3

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[issue7951] Should str.format allow negative indexes when used for __getitem__ access?

[Matthew Barnett]

Matthew Barnett pyt...@mrabarnett.plus.com added the comment:

I agree with Kamil and Germán. I would've expected negative indexes for 
sequences to work. Negative indexes for fields is a different matter.

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[issue7951] Should str.format allow negative indexes when used for __getitem__ access?

[Terry J. Reedy]

Terry J. Reedy tjre...@udel.edu added the comment:

I believe this is covered by the PEP3003 3.2 change moratorium.

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[issue7951] Should str.format allow negative indexes when used for __getitem__ access?

[Germán L . Osella Massa]

Germán L. Osella Massa gose...@gmail.com added the comment:

Well, using negative indexes for fields can be thought as a new feature with 
all the consequences mentioned before BUT negative indexes for accessing 
elements from a sequence, IMHO, is something that anyone would expected to 
work. That's why at first I thought it was a bug and I fill an issue about it.

The code that parses the fields and the indexes is the same, so when I change 
it to accept negative indexes, it worked for both cases. I'm attaching a patch 
that checks if a negative index is used in a field and reverts to the old 
behavior in that case, allowing only negative indexes for accessing sequences ( 
{-1} will raise KeyError because it will be threated as '-1').

Perhaps in this way this issue could be partially fixed.

--
keywords: +patch
Added file: 
http://bugs.python.org/file17768/format_no_fields_with_negative_indexes-2.7.diff

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[issue7951] Should str.format allow negative indexes when used for __getitem__ access?

[Kamil Kisiel]

Kamil Kisiel kisi...@users.sourceforge.net added the comment:

While I agree this functionality isn't strictly necessary I think it makes 
sense from a semantic point of view. I ran in to this issue today while writing 
some code and I simply expected the negative syntax to work, given that the 
format string syntax otherwise very closely resembles standard array and 
attribute access.

It would be nice to see this make it in eventually for consistency's sake.

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[issue7951] Should str.format allow negative indexes when used for __getitem__ access?

[Germán L . Osella Massa]

Germán L. Osella Massa gose...@gmail.com added the comment:

I finally managed to get the time to finish the patch that allows negative 
indexes inside square brackets so now they work with the same semantics as in a 
python expression:

 '{0[-1]}'.format(['abc', 'def'])
'def'
 '{0[-2]}'.format(['abc', 'def'])
'abc'
 '{0[-1][-1]}'.format(['abc', ['def']])
'def'

They work auto-numbered fields too:
 '{[-1]}'.format(['abc', 'def'])
'def'

Also, a positive sign is now accepted as part of a valid integer:

 '{0[+1]}'.format(['abc', 'def'])
'def'

As a bonus, negatives indexes are also allowed to refer to positional arguments:

 '{-1}'.format('abc', 'def')
'def'
 '{-2}'.format('abc', 'def')
'abc'

I'm attaching a patch against trunk. I added some tests for this functionality 
in test_str.py.

By the way, this code doesn't work anymore:

 {[-1]}.format({'-1': 'X'})
Traceback (most recent call last):
  File stdin, line 1, in module
KeyError: -1L

But now it behaves in the same way as:
 {[1]}.format({'1': 'X'})
Traceback (most recent call last):
  File stdin, line 1, in module
KeyError: 1L

I didn't attempt to ignore whitespaces when trying to parse the index as an 
integer (to allow that { 0 } can be treated as {0} and {0[1]} as { 0 [ 1 
] }) because I'm not sure if this behavior is desirable.

--
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Added file: http://bugs.python.org/file17712/format_negative_indexes-2.7.diff

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[issue7951] Should str.format allow negative indexes when used for __getitem__ access?

[Germán L . Osella Massa]

Changes by Germán L. Osella Massa gose...@gmail.com:


Added file: http://bugs.python.org/file17713/format_negative_indexes-3.2.diff

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[issue7951] Should str.format allow negative indexes when used for __getitem__ access?

[Germán L . Osella Massa]

Germán L. Osella Massa gose...@gmail.com added the comment:

I forgot to mention that I also made a patch against py3k (was the same code).

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[issue7951] Should str.format allow negative indexes when used for __getitem__ access?

[Raymond Hettinger]

Raymond Hettinger rhettin...@users.sourceforge.net added the comment:

Perhaps this ought to be discussed on python-ideas or python-dev for a bit.  It 
is not entirely clear that this is a GoodThingToDo(tm) nor is it clear that we 
want other Python implementations to have to invest the same effort.

The spirit of the language freeze suggests that we shouldn't add this unless we 
really need it.  The goal was to let other implementations catch up, not to add 
to their list of incompatabilites.

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[issue7951] Should str.format allow negative indexes when used for __getitem__ access?

[Eric Smith]

Eric Smith e...@trueblade.com added the comment:

I agree with Raymond. I'm not convinced it allows you to write any code that 
you can't currently write, and I'm fairly sure it violates the moratorium. 
Implementing this would clearly put a burden on other implementations.

Marking as after moratorium.

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[issue7951] Should str.format allow negative indexes when used for __getitem__ access?

[Mark Dickinson]

Mark Dickinson dicki...@gmail.com added the comment:

I (reluctantly) agree it's surprising that {0[-1]}.format(args) fails.  And I 
suppose that if it were allowed then it would also make sense to consider 
{-1}.format(*args) as well, in order to preserve the equivalence between 
{n}.format(*args) and {0[n]}.format(args).  And then:

 {-0}.format(*['calvin'], **{'-0': 'hobbes'})
'hobbes'

would presumably produce 'calvin' instead of 'hobbes'...

On '+': if {0[-1]} were allowed, I'm not sure whether the +1 in 
{0[+1]}.format(...) should also be interpreted as a list index.  I don't 
really see the value of doing so apart from syntactic consistency: there are 
very few other places in Python that I'm aware of that accept 
-one-or-more-digits but not +one-or-more-digits.

FWIW, my overall feeling is that the current rules are simple and adequate, and 
there's no great need to add this complication.

I do wonder, though:

How complicated would it be to make {0[1]}.format({'1':'foo'}) a bit magical? 
 That is, have the format method pass an integer to __getitem__ if the 
corresponding format argument is a sequence, and a string argument if it's a 
mapping (not sure what the criterion would be for distinguishing).  Is this too 
much magic?  Is it feasible implementation-wise?

I don't think it's do-able for simple rather than compound field names: e.g.,  
{0}.format(*args, **kwargs), since there we've got both a sequence *and* a 
dict, so it's not clear whether to look at args[0] or kwargs['0']. (Unless 
either args or kwargs is empty, perhaps.)  This is all getting a bit 
python-ideas'y, though.

BTW, I notice that PEP 3101's Simple field names are either names or numbers 
[...] if names, they must be valid Python identifiers isn't actually true:

 {in-valid #identifier}.format(**{'in-valid #identifier': 42})
'42'

Though I don't have a problem with this;  indeed, I think this is preferable to 
checking for a valid identifier.

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[issue7951] Should str.format allow negative indexes when used for __getitem__ access?

[Eric Smith]

Eric Smith e...@trueblade.com added the comment:

Addressing just the last part of Mark's message right now:

The PEP goes on to say:
Implementation note: The implementation of this proposal is
not required to enforce the rule about a simple or dotted name
being a valid Python identifier.  ...

I rely on getattr lookup failing for dotted names, but for simple names there's 
no check at all. I agree it's desirable to leave this behavior.

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[issue7951] Should str.format allow negative indexes when used for __getitem__ access?

[Matthew Barnett]

Matthew Barnett pyt...@mrabarnett.plus.com added the comment:

Re: msg107776.

If it looks like an integer (ie, can be converted to an integer by 'int') then 
it's positional, otherwise it's a key. An optimisation is to perform a quick 
check upfront to see whether it starts like an integer.

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[issue7951] Should str.format allow negative indexes when used for __getitem__ access?

[Mark Dickinson]

Mark Dickinson dicki...@gmail.com added the comment:

Matthew:

would that include allowing whitespace, then?

 int('\t\n+56')
56

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[issue7951] Should str.format allow negative indexes when used for __getitem__ access?

[Matthew Barnett]

Matthew Barnett pyt...@mrabarnett.plus.com added the comment:

That's a good question. :-)

Possibly just an optional sign followed by one or more digits.

Another possibility that occurs to me is for it to default to positional if it 
looks like an integer, but allow quoting to force it to be a key:

 {0}.format(foo, **{0: bar})
'foo'
 {'0'}.format(foo, **{0: bar})
'bar'

Or is that taking it too far?

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[issue7951] Should str.format allow negative indexes when used for __getitem__ access?

[Eric Smith]

Eric Smith e...@trueblade.com added the comment:

I can see the point of allowing negative indices for a consistency point, but 
is there really any practical problem that's currently causing people hardship 
that this would solve?

As for the rest of it, I think it's just not worth the additional burden on 
CPython and other implementations.

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[issue7951] Should str.format allow negative indexes when used for __getitem__ access?

[Matthew Barnett]

Matthew Barnett pyt...@mrabarnett.plus.com added the comment:

Your original:

{0[-1]}.format('fox')

is a worse gotcha than:

{-1}.format('fox')

because you're much less likely to want to do the latter.

It's one of those things that it would be nice to have fixed, or we could just 
add a warning to the documentation that it _might_ be fixed in the future, so 
people shouldn't rely on the current behaviour. :-)

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[issue7951] Should str.format allow negative indexes when used for __getitem__ access?

[Eric Smith]

Changes by Eric Smith e...@trueblade.com:


--
versions:  -Python 2.7

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[issue7951] Should str.format allow negative indexes when used for __getitem__ access?

[Eric Smith]

Eric Smith e...@trueblade.com added the comment:

Closed issue 8985 as a duplicate of this; merging nosy lists.

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nosy: +gosella, mark.dickinson, merwok

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[issue7951] Should str.format allow negative indexes when used for __getitem__ access?

[Eric Smith]

Changes by Eric Smith e...@trueblade.com:


--
stage:  - needs patch

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[issue7951] Should str.format allow negative indexes when used for __getitem__ access?

[Matthew Barnett]

Matthew Barnett pyt...@mrabarnett.plus.com added the comment:

On a related note, this doesn't work either:

 {-1}.format(x, y, z)
Traceback (most recent call last):
  File pyshell#3, line 1, in module
{-1}.format(x, y, z)
KeyError: '-1'

It could return z.

It also rejects a leading '+', but that would be optional anyway.

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nosy: +mrabarnett

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[issue7951] Should str.format allow negative indexes when used for __getitem__ access?

[Eric Smith]

New submission from Eric Smith e...@trueblade.com:

It surprised me that this doesn't work:
 {0[-1]}.format('fox')
Traceback (most recent call last):
  File stdin, line 1, in module
TypeError: string indices must be integers

I was expecting it to be equivalent to:

 {0[2]}.format('fox')
'x'

I don't think there's any particular reason this doesn't work. It would, 
however break the following code:

 {0[-1]}.format({'-1':'foo'})
'foo'

But note that this doesn't work currently:

 {0[1]}.format({'1':'foo'})
Traceback (most recent call last):
  File stdin, line 1, in module
KeyError: 1

--
assignee: eric.smith
components: Interpreter Core
keywords: easy
messages: 99482
nosy: eric.smith
priority: normal
severity: normal
status: open
title: Should str.format allow negative indexes when used for __getitem__ 
access?
type: feature request
versions: Python 2.7, Python 3.2

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