Issue 36159: Modify Formatter Class to handle arbitrary objects (original) (raw)

The only point in the string.Formatter class that really depends on string output is line 245: return ''.join(result), auto_arg_index.

I'm currently working on a problem that would be much easier if I could get access to the result list instead of having that join called.

I propose creating another overridable method in string.Formatter, say oformat that calls _vformat and is called by vformat. oformat would have the guts of vformat and just return the result list. vformat would then consist of the call ot oformat and the join. See Below. The recursive call to _vformat would also need some slight modification.

The work would not be difficult and I'm willing to do it, provided there is sufficient interest.

def vformat(self, format_string, args, kwargs): result = self.oformat(format_string, args, kwargs) return ''.join(result)

def oformat(self, format_string, args, kwargs): used_args = set() result, _ = self._vformat(format_string, args, kwargs, used_args, 2) self.check_unused_args(used_args, args, kwargs) return result

I'm currently writing a language translator between two domain specific computer languages. Because some expressions occur repeatedly, but in slightly different contexts, I make multiple passes. The first pass reduces everything it can and leaves place holder objects for things it can't reduce. Later passes replace the objects with their final expression in the new language. The final expression varies by context, so every time it's reevaluated, it could change. I would really like to handle things like a + b as

"{a} + {b}".format(a=a, b=b)

This works great when a and b are strings. But when they are place holder objects, I wasn't able to find a good solution. Although the Formatter class came so close that I thought I would suggest the change. What I ended up doing was replacing objects with unique strings so that I could use format and then using regular expressions on the output string to split it into an array and replace the string identifiers with the original objects. The change I've suggested to the Formatter class would have allowed me to skip the regular expressions.

Ross

On Mon, Mar 11, 2019 at 11:47 PM Lisa Roach <report@bugs.python.org> wrote:

Lisa Roach <lisaroach14@gmail.com> added the comment:

Can you give an example use case for this? F-strings are the newer method of string interpolation, I'm not sure it's worth putting effort into adding features to string.Formatter.

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nosy: +lisroach

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Python tracker <report@bugs.python.org> <https://bugs.python.org/issue36159>

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