[Python-Dev] Fwd: deep question re dict as formatting input

[Ethan Furman]

Eric Smith wrote:
> On 2/22/2011 6:28 PM, Steve Holden wrote:
>> On Feb 22, 2011, at 3:08 PM, Eric Smith wrote:
>>> Quoting PEP 3101:
>>>
>>> An example of the 'getitem' syntax:
>>>
>>>         "My name is {0[name]}".format(dict(name='Fred'))
>>>
>>> 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.
>>>
>> That's not strictly true:
>>
>>--> d = {"Steve":"Holden", "Guido":"van Rossum", 21.2:"float"}
>>--> d[21.1]
>> Traceback (most recent call last):
>>    File "<stdin>", line 1, in<module>
>> KeyError: 21.1
>>--> d[21.2]
>> 'float'
>>--> "{0[21.2]}".format(d)
>> Traceback (most recent call last):
>>    File "<stdin>", line 1, in<module>
>> KeyError: '21.2'
>>
> 
> You are correct, I didn't exactly implement the PEP on this point, 
> probably as a shortcut. I think there's an issue somewhere that 
> discusses this, but I can't find it. The CPython implementation is 
> really using "If every character is a digit, then it is treated as an 
> integer, otherwise it is used as a string".

Given the representation issues with floating point, I think the current 
behavior is desirable.  Also, leaving digits with periods as strings 
would, I think, be more useful (Dewey Decimal, anyone?).

~Ethan~