On 25Apr2021 10:54, Cameron Simpson <[email protected]> wrote:
>On 24Apr2021 22:35, Stephen J. Turnbull <[email protected]>
>wrote:
>[...]
>> > My use case is presupplied strings, eg a command line supplied
>> > format string.
>>
>>In that case the format string is user input, and x is a variable in
>>the program that the user can have substituted into their string?
>>
>>Assuming that *exact* use case, wouldn't
>>
>> >>> class LowerableStr(str):
>> ... def __format__(self, fmt):
>> ... if fmt == 'lc':
>> ... return self.lower()
>> ... else:
>> ... return str.__format__(self, fmt)
>> ...
>> >>> "{x} is {x:lc} in lowercase".format_map({'x' : LowerableStr("This")})
>> 'This is this in lowercase'
>>
>>do?
>
>You're perfectly correct. ":lc" can be shoehorned into doing what I ask.
>But __format__ is in the wrong place for how I'd like to do this.
Just to follow up on this, I've been experimenting with Stephen's
suggestion of using ":foo" style format specifiers. With enough success
that I'm probably going to run with it is I can make the implementation
clean enough.
I'm currently making a subclass of the Formatter builtin class, which
supplies the string parser and lets one override various methods to
implemenent the "{...}" parts.
Currently I can do this:
I've got a little ontology, saying that a tag named "colour" is "a
colour, a hue" and expects to be a Python str.
It also has metadata for the particular colour "blue".
{ 'type.colour': TagSet:{'description': 'a colour, a hue', 'type': 'str'},
'meta.colour.blue': TagSet:{'url': 'https://en.wikipedia.org/wiki/Blue',
'wavelengths': '450nm-495nm'}
}
And I've got a tag set describing some object which is blue:
{'colour': 'blue', 'labels': ['a', 'b', 'c'], 'size': 9}
The TagSet class has a .format_as(format_string) method which hooks into
the Formatter subclass and formats a string according to the TagSet.
A little test programme to write information into some command line
supplied format strings:
[~/hg/css-tagsets(hg:tagsets)]fleet2*> py3 -m cs.tagset '{colour}'
'{colour:meta}' '{colour:meta.url}'
{colour}
=> 'blue'
{colour:meta}
=> 'url="https://en.wikipedia.org/wiki/Blue"; wavelengths="450nm-495nm"'
{colour:meta.url}
=> 'https://en.wikipedia.org/wiki/Blue'
Looking promising to me.
Cheers,
Cameron Simpson <[email protected]>
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