[issue31907] Clarify error message when attempting to call function via str.format()

[mickey695]

mickey695  added the comment:

Well, it has been more than the 2 weeks I promised however now I finally have 
time to look into this.

I did some fuzzing to build a list quickly and the following ASCII characters 
can not be used as part of the argument name:
exclamation mark(!)
period(.)
colon(:)
opening square bracket([)
opening/closing curly brackets({})

also, the argument name may not start with digits from zero to nine(0-9)

Basically what needs to be changed is arg_name under the section "Format String 
Syntax" in \Doc\library\string.rst

Problem is, I really don't understand the syntax of the Python/sphinx formal 
language.

The following expression is what I came up with:
arg_name ::= [^`decinteger`] [ | `integer` ]+

If anyone from the documentation team could look into it I would be grateful.

Moreover, I think attribute_name should probably be changed as it does not have 
to be a valid identifier. In fact, it can be any text. Eric V. Smiths' example 
can be used with any text as the attribute name. A comment should be added to 
the aforementioned section explaining that normally only valid identifiers are 
accepted, however by implementing your own __getattr__ this can be changed.

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components: +Documentation

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[issue31907] Clarify error message when attempting to call function via str.format()

[mickey695]

mickey695  added the comment:

I think it should be documented properly.

In roughly two weeks I will have some time to look into it. 
So I could probably document the current behaviour by the start of 
December(unless someone beats me to it)

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[issue31907] Clarify error message when attempting to call function via str.format()

[Eric V. Smith]

Eric V. Smith  added the comment:

Guido's suggestion in 
https://mail.python.org/pipermail/python-dev/2017-October/150055.html is to not 
change anything.

Assuming we do that, it's still an open issue as to if/how we should document 
and test the current behavior. I'd be surprised if it's tested, but I haven't 
looked.

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[issue31907] Clarify error message when attempting to call function via str.format()

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

I have wrote a patch that change the parser to only accept valid identifiers. 
But now I have the same doubts and want first to discuss this [1]. This change 
can break the existing code as Eric's example shows.

[1] https://mail.python.org/pipermail/python-dev/2017-October/150052.html

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nosy: +serhiy.storchaka
status: pending -> open

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[issue31907] Clarify error message when attempting to call function via str.format()

[Eric V. Smith]

Eric V. Smith  added the comment:

How would you distinguish this from the case where an actually missing 
attribute was given?

>>> "{0.ctimex}".format(d)
Traceback (most recent call last):
  File "", line 1, in 
AttributeError: 'datetime.datetime' object has no attribute 'ctimex'

I guess it would be possible to change the parser to only look for valid chars 
in an identifier and give an error if there were "extra" chars found, but I'm 
not sure it's worth the hassle. The error message seems clear enough to me. 
Also, I wouldn't want to change this to not be an AttributeError (for backward 
compatibility), and I'm not sure an AttributeError with a different message 
would be good.

Of course, this works with f-strings:

>>> f"{d.ctime()}"
'Tue Oct 31 00:00:00 2017'

And I just realized that your code actually is valid, with a custom 
__getattr__. And while I wouldn't recommend doing this, I also wouldn't want to 
break it. For all I know, people have written custom evaluators in __getattr__:

>>> class C:
... def __getattr__(self, name):
... if name == 'ctime()':
... return 'okay'
... raise AttributeError('not found')
...

>>> '{0.x}'.format(C())
Traceback (most recent call last):
  File "", line 1, in 
  File "", line 5, in __getattr__
AttributeError: not found

>>> '{0.ctime()}'.format(C())
'okay'

I'm changing versions because this would be a new feature, since it breaks 
valid code.

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assignee:  -> eric.smith
nosy: +eric.smith
status: open -> pending
versions:  -Python 2.7, Python 3.4, Python 3.5, Python 3.6, Python 3.8

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[issue31907] Clarify error message when attempting to call function via str.format()

[mickey695]

New submission from mickey695 :

PEP 3101 states that format strings may only use the "."(getattr) or the "[]" 
(getitem) operators to address either attributes or items of parameters.
Should a programmer attempt to, for example, call a function of a parameter as 
follows: 

>>> d = datetime.datetime(2017, 10, 31)
>>> "{0.ctime()}".format(d)

they will receive an error message such as:

AttributeError: 'datetime.datetime' object has no attribute 'ctime()'

Proposal:
Raise an error stating that cannot embed arbitrary expressions in str.format() 
format strings

--
components: Interpreter Core
messages: 305263
nosy: mickey695
priority: normal
severity: normal
status: open
title: Clarify error message when attempting to call function via str.format()
type: enhancement
versions: Python 2.7, Python 3.4, Python 3.5, Python 3.6, Python 3.7, Python 3.8

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