All-in-one number base converter
A couple of simplifications are possible that will make this code more Pythonic.



Instead of declaring an empty number = [] and then appending elements,
use a list comprehension:

number = [int(i) for i in reversed(n)]


Notice that I used reversed(...) instead of number[::-1],
which seems a bit more intuitive to me.



Instead of this:


if n[0] == '-':
    n = n.replace('-', '')



I'd find it more clear:

if n.startswith('-'):
    n = n[1:]


At this point it's also clear that n is not a good name.
n is usually used as a number,
but here it's a string.
Maybe "string" or "numString" would be better.

number is also not a good name.
Since it's a list, making the name plural would be more natural.



Instead of this:


        number[i] = number[i] * -1
        number[i] = number[i] * (base ** i)



You could write in a shorter, more natural way:

        number[i] *= base ** i * -1




Similar to the previous point, this can be shortened:


        n = abs(n)
        n //= base
        n = -n



To this:

        n = abs(n) // base * -1





  Basically this program can convert any number from any base to another base including negative bases as long as the input is coherent.


For input arguments 255 10 16 it gives me:


You entered 255 in base 10.
Your number in base 16 is 165.



Normally it should be ff, or at least (15, 15) or something...