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

[Eric Smith]

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.


On 2/22/2011 6:01 PM, Steve Holden wrote:
> One of the students on an introductory Python 3 class asks a very good
> question about string formatting. This could be because the course
> materials are misleading, so I would like to understand. It would appear
> from tests that "{0[X]}".format(...) first tries to convert the string
> "X" to in integer. If it succeeds then __getitem__() is called with the
> integer as an argument, otherwise it is called with the string itself as
> an argument. Is this correct?
>
> The documentation at
> http://docs.python.org/library/string.html#formatspec is silent on
> whether strings were ever intended to be used as subscripts. Does this
> seem sensible? Was it considered during design? Should I alter the
> materials so that only integer subscripts are used?
>
> regards
> Steve
>
>
> Begin forwarded message:
>
>> *From: *kirby urner <kirby.urner at gmail.com <mailto:kirby.urner at gmail.com>>
>> *Date: *February 22, 2011 2:31:08 PM PST
>> *To: *Steve Holden <steve at holdenweb.com <mailto:steve at holdenweb.com>>
>> *Subject: **deep question re dict as formatting input*
>>
>>>>> d
>> {'Steve': 'Holden', 'Tim': 'Peters', 'Guido': 'van Rossum', '1':
>> 'string', 1: 'integer'}
>>>>> "{0[Guido]} is cool".format(d)
>> 'van Rossum is cool'
>>>>> "{0[1]} is cool".format(d)
>> 'integer is cool'
>>>>> "{0['1']} is cool".format(d)
>> Traceback (most recent call last):
>> File "<pyshell#19>", line 1, in <module>
>> "{0['1']} is cool".format(d)
>> KeyError: "'1'"
>>
>>
>> Student question:
>>
>> Good morning!
>>
>> Question on .format(), interactive session follows:
>>
>> --> d = {"Steve": "Holden",
>> ... "Guido": "van Rossum",
>> ... "Tim": "Peters",
>> ... "1": "string",
>> ... 1: "integer"}
>>
>> --> d
>> {'Steve': 'Holden', 'Tim': 'Peters', '1': 'string', 1: 'integer',
>> 'Guido': 'van Rossum'}
>>
>> --> d[1]
>> 'integer'
>>
>> --> d['1']
>> 'string'
>>
>> --> "{dct[1]}".format(dct=d)
>> 'integer'
>>
>> --> "{dct[Guido]}".format(dct=d)
>> 'van Rossum'
>>
>> --> "{dct['1']}".format(dct=d)
>> Traceback (most recent call last):
>> File "<console>", line 1, in <module>
>> KeyError: "'1'"
>>
>> Question: If {dct[Guido]} treats Guido as str, why doesn't {dct[1]}
>> treate 1 as str? Feels like an automatic conversion from str to int.
>> Furthermore, how does one access the key '1' in a format statement?
>>
>> ~Ethan~
>
>
>
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