[issue29568] undefined parsing behavior with the old style string formatting

[Xiang Zhang]

Xiang Zhang added the comment:


New changeset 9f8ad3f39e0a92ed37d012b9dd237399524f0d51 by Xiang Zhang (Serhiy 
Storchaka) in branch 'master':
bpo-29568: Disable any characters between two percents for escaped percent "%%" 
 in the format string for classic string formatting. (GH-513)
https://github.com/python/cpython/commit/9f8ad3f39e0a92ed37d012b9dd237399524f0d51


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[issue29568] undefined parsing behavior with the old style string formatting

[Larry Hastings]

Changes by Larry Hastings :


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pull_requests:  -588

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[issue29568] undefined parsing behavior with the old style string formatting

[Larry Hastings]

Changes by Larry Hastings :


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pull_requests: +588

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[issue29568] undefined parsing behavior with the old style string formatting

[Xiang Zhang]

Changes by Xiang Zhang :


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

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[issue29568] undefined parsing behavior with the old style string formatting

[Serhiy Storchaka]

Changes by Serhiy Storchaka :


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pull_requests: +422

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[issue29568] undefined parsing behavior with the old style string formatting

[Jerry Dimitrov]

Jerry Dimitrov added the comment:

Thanks for the documentation reference. Can we at least add this link reference 
[1] as a note or something like that into those documentation sections: [2] 
https://docs.python.org/2/library/stdtypes.html#string-formatting and  [3] 
https://docs.python.org/3/library/string.html#string-formatting

I couldn't fine the printf specific documentation at first, so this is why I 
created the report.

Regards,
Jerry

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[issue29568] undefined parsing behavior with the old style string formatting

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

Here is a patch.

--
keywords: +patch
stage:  -> patch review
versions:  -Python 2.7, Python 3.5, Python 3.6
Added file: http://bugs.python.org/file46640/format-percent.patch

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[issue29568] undefined parsing behavior with the old style string formatting

[Xiang Zhang]

Xiang Zhang added the comment:

Or even not fix it in develop branch.

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[issue29568] undefined parsing behavior with the old style string formatting

[Xiang Zhang]

Xiang Zhang added the comment:

The documentation[1] explicitly states using % to do string format could be 
error-prone and recommends using str.format(). So +1 on no change in 
maintenance branches.

[1] 
https://docs.python.org/3/library/stdtypes.html#printf-style-string-formatting

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nosy: +xiang.zhang

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[issue29568] undefined parsing behavior with the old style string formatting

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

Yes, it is what I want. Current behavior is just an artefact of the 
implementation. I have doubts that any other implementation of printf-like 
formatting has such behavior.

Bytes formatting starves from the same issue.

>>> b'% %' % {}
b'%'

But there are differences:

>>> '%12%' % ()
'%'
>>> b'%12%' % ()
b'   %'

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[issue29568] undefined parsing behavior with the old style string formatting

[R. David Murray]

R. David Murray added the comment:

I don't think we should change the behavior in maintenance branches.  I take it 
what you want to do is make it so that if the parser ends up thinking it is 
seeing '%%' but there is stuff between the two %s, that's an error?

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nosy: +r.david.murray

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[issue29568] undefined parsing behavior with the old style string formatting

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

In this example characters between percents are parsed as for any conversion 
specifier but ignored and '%(a) %' is formatted to '%'. '(a)' specifies a 
mapping key, ' ' is a conversion flag, the second '%' is a conversion type.

>>> '%(a) d' % {'a': 123}
' 123'
>>> '%(a) %' % {'a': 123}
'%'

This behavior is explicable but looks weird and errorprone. I'm not sure about 
changing behavior in maintained branches (technically this may be not a bug), 
but I think it is worth to make this an error in the developed branch.

--
nosy: +benjamin.peterson, larry, ned.deily, rhettinger, serhiy.storchaka
versions:  -Python 3.3, Python 3.4

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[issue29568] undefined parsing behavior with the old style string formatting

[Jerry Dimitrov]

New submission from Jerry Dimitrov:

Hello everyone,
This is my first bug report to the python project, so please excuse me if the 
metadata for this particular issue is not 100% accurate.

Today I noticed (with the help from couple of people in IRC) a strange behavior 
in the python string formatting functionality.

Consider the following code snippets:

```
'%(a)s %(b)' % {'a': '1', 'b': '2'}

# result:
Traceback (most recent call last):
  File "", line 1, in 
ValueError: incomplete format
```


```
'%(a) %(b)s' % {'a': '1', 'b': '2'}

# result:
'%(b)s'

# expected result:
Traceback (most recent call last):
  File "", line 1, in 
ValueError: incomplete format
```

It seems that there is some kind of inconsistent (undefined) behavior, during 
the parsing of the type character for the formatted string (tested across all 
major python 2.x/3.x versions).

According to the documentation for string formatting and the relevant PEPs, 
there is no additional info about this particular case.

I want to say thank you to Yhg1s, JustASlacker, Jerub and lz1irq for 
discovering this 'bug/feature' and the additional information about it.

Please let me know if this is a bug, since I am not 100% sure if this is the 
case.
Thanks in advance for your time!

Best Regards,
Jerry

--
components: Interpreter Core
messages: 287857
nosy: Jerry Dimitrov
priority: normal
severity: normal
status: open
title: undefined parsing behavior with the old style string formatting
type: behavior
versions: Python 2.7, Python 3.3, Python 3.4, Python 3.5, Python 3.6, Python 3.7

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