Currently str.format() and f-strings support three converters.
Converting is specified by the "!" character followed by a letter which
denotes the converter.
s: str()
r: repr()
a: ascii()
In some cases it would be useful to apply other converters. One obvious
example is escaping special characters ("&", "<", ">", "'" and '"') for
XML and HTML. Other applications may need other converters, for example
escaping special characters (like \xff) for Telnet, or escaping non-BMP
characters for displaying on Tk widget, or escaping delimiters and
special characters for shell, or translating to other language.
I do not think that it is practical to provide additional standard
converters for all above cases, but what if create a special registry
similar to registries for encodings and error handlers and allow a user
to register custom converters? We could extend str.format() and
f-strings to accept arbitrary letters after "!", or maybe even allow
multi-character names of converters.
It is less important for f-strings because you can use arbitrary
expressions, but even in this case f"Hello, {name!x}!" or f"Hello,
{name!xml}!" looks better than f"Hello, {html.escape(name)}!" or
f"Hello, {x(name)}!".
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