Amend bc96a889

mpmath/libmp/libmpf.py

@@ -1728,7 +1728,7 @@ def format_digits(num, format_dict, prec, rnd, _pretty_repr_dps, unique):
                                            10, round_nearest)
             prev_digits = digits
             prev_exp = exp
-            while True:
+            while dps > 1:
                 dps -= 1
                 new_digits, new_exp = round_digits(num, digits, exp, dps,
                                                    10, round_down)
@@ -1743,6 +1743,9 @@ def format_digits(num, format_dict, prec, rnd, _pretty_repr_dps, unique):
                         break
                 prev_digits = new_digits
                 prev_exp = new_exp
+            else:
+                digits = new_digits
+                exp = new_exp
         else:
             num = mpf_pos(num, prec, rnd)  # workaround issue 1158
             # Here be dragons.

mpmath/tests/test_format.py

@@ -472,6 +472,8 @@ def test_mpf_fmt_cpython():
     assert f'{mp.pi}' == '3.14159265358979'
     mp.pretty_dps = 'repr'
     assert f'{mp.pi}' == '3.1415926535897931'
+    mp.shortest_str = True
+    assert f'{mp.mpf("1e100000")}' == '1e+100000'
 
 
 @given(fmt_str(types=list('fFeEgG%') + ['']),