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.ur...@gmail.com <mailto:kirby.ur...@gmail.com>>
*Date: *February 22, 2011 2:31:08 PM PST
*To: *Steve Holden <st...@holdenweb.com <mailto:st...@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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