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

[mickey695]

Change by mickey695 :


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versions: +Python 3.10

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

[mickey695]

mickey695  added the comment:

Happy passover Irit(and/or anyone else celebrating)!

While your suggestion is accurate, I can't say whether it's enough or not.
This is up to you and the other python language development forum to decide :)

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[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()

[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

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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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