when does 3*0.1 make 0.30000000000000004 ?
YES it is the same program!

This is how binary floating-point arithmetic works.  See

http://docs.python.org/3/tutorial/floatingpoint.html

for some explanations.

can you add an approximation of the result in the command?
(ex: the biggest precision in the values is 0.1, so it won't show after 4.0)
meen while, thank you.

> can you add an approximation of the result in the command?

I don't really understand what you're asking here.

If you're asking for the behaviour of multiplication to change so that it becomes more do-what-I-mean-ish, that's not going to happen.  You could try writing your own DWIM-style multiplication if that's what you want, but the basic multiplication operator should stay as it is now: a simple wrapper around the C multiplication, which on most machines has a simple, easily-stated and well-defined behaviour: return me the floating-point number that's closest to the exact mathematical result of the multiplication.

@Liam, try using the "decimal" module instead.  That follows rules much like the ones people learn as kids.

>>> from decimal import Decimal as D
>>> D("0.1") * 3  # decimal results are computed exactly
Decimal('0.3')
>>> D("1.01") - D(".01")  # number of significant digits is preserved
Decimal('1.00')

Etc.