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

[Steve Holden]

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>
> Date: February 22, 2011 2:31:08 PM PST
> To: Steve Holden <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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