reprlib.Repr truncation ignores the length of a custom fillvalue #161289

Bug report

Bug description:

Since Python 3.11, reprlib.Repr.fillvalue can be set to any string, but Repr.repr_str(), Repr.repr_int() and Repr.repr_instance() still compute how many characters to keep around it as if it were always three characters long (the length of the default '...'):

i = max(0, (self.maxstring-3)//2)
j = max(0, self.maxstring-3-i)
s = s[:i] + self.fillvalue + s[len(s)-j:]

As a result, when the fill value has a different length, a truncated representation no longer fits maxstring, maxlong or maxother:

>>> import reprlib
>>> r = reprlib.Repr(maxstring=10, maxlong=10, maxother=10, fillvalue='<snip>')
>>> r.repr(12345678901234567890)  # 13 characters, maxlong is 10
'123<snip>7890'
>>> r.repr('abcdefghijklmnopqrstuvwxyz')  # 13 characters, maxstring is 10
"'ab<snip>xyz'"
>>> r = reprlib.Repr(maxstring=10, fillvalue='…')
>>> r.repr('abcdefghijklmnopqrstuvwxyz')  # 8 characters, 2 left unused
"'ab…xyz'"

The documentation describes maxlong as the "Maximum number of characters in the representation for an integer". Using len(self.fillvalue) instead of 3 makes the result fit the limit, and leaves the output with the default '...' unchanged.

CPython versions tested on:

3.12, 3.15, CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-161289: Respect fillvalue length when truncating in reprlib.Repr #161290