!r is one of a class of conversion flags, others are !a which uses ascii(), !s which uses str().

“=” in the format-spec (the thing that comes after the “:”) is the numeric padding alignment specifier (other alignment specifiers are left “<”, right “>”, and center “^”.)

Oh, neat, so I can stack them:

  foo = 'foo'
  print(f"{foo=!r:>20}")
  print(f"{foo.upper()=!r:>20}")

The `=` you pointed out is a special case, however, and not part of the alignment specification. Sadly, this means that the above snippet is not as nicely formatted as I'd like. Still, it does seem useful for these cases:

  print(f"{x=:.2f} {dx=:.2f} {dot_product=:.4f}")

Yeah, there seem to be a number of special cases.

The rules for the trailing `=` case are particularly complicated:

    {x=} -> "x="+repr(x)
    {x=:.2f} -> "x="+format(x, ".2f")
    {x=:} -> "x="+format(x, "")
    {x=:!s:20} -> "x="+format(str(x), "20")
    {x=:!r:20} -> "x="+format(repr(x), "20")

https://bugs.python.org/msg341732

You really have to understand how it's translated into actual code, and of course it's Python, so there's some legacy syntax to take into account. A `:` just means call format(lhs, rhs), for example.

You can also actually combine those syntaxes - I often use f"{foo:%Y-%m-%d!r}" to get a quoted, properly escaped date in a specific format, for example.

What if your format string started with "r" or "=", or was simply just one of those characters?