thank you very much about the hints, i've improved the code as follows.

def rnd(x, n=0):
   a = x*10**(n + 1)
   b = int(a)
   if abs(a - b) >= 0.5: b += 1 if x >= 0 else -1
   a = b/10
   b = int(a)
   if abs(a - b) >= 0.5: b += 1 if x >= 0 else -1
   return b/10**n

def rndup(x, n=0):
   a = x*10**(n + 1)
   b = int(a)
   if abs(a - b) > 0: b += 1 if x >= 0 else -1
   a = b/10
   b = int(a)
   if abs(a - b) > 0: b += 1 if x >= 0 else -1
   return b/10**n

now it manage well your cases ...

print(rnd(2.8 - 1.3), 2.8 - 1.3)
print(rnd(1.8 - 1.3), 1.8 - 1.3)
print(rnd(-2.8 + 1.3), -2.8 + 1.3)
print(rnd(-1.8 + 1.3), -1.8 + 1.3)

results ...

2.0 1.4999999999999998
1.0 0.5
-2.0 -1.4999999999999998
-1.0 -0.5

we have to define limits caused by the floating point internal representation but for general use in my opinion can work fine. do you see other cases where it cracks?