I have this code:

variable = "FFFF"
message = bytearray( variable.decode("hex") )

after this, I want to perform something like this:

message.len()

but it seems that bytearray does not have something like "len()" implemented, is it possible to know the lenght?

The idiom used for this in python is len(obj), not obj.len()

>>> v = 'ffff'
>>> msg = bytearray(v.decode('hex'))
>>> len(msg)
2

If you are creating your own class that should be able to report its length, implement the magic method __len__(self), which will be called when the built-in function len() is called and passed an instance of your class.

Why?

See this old post from Guido:

(a) For some operations, prefix notation just reads better thanpostfix — prefix (and infix!) operations have a long tradition inmathematics which likes notations where the visuals help themathematician thinking about a problem. Compare the easy with which werewrite a formula like x*(a+b) into xa + xb to the clumsiness ofdoing the same thing using a raw OO notation.

(b) When I read code that says len(x) I know that it is asking for thelength of something. This tells me two things: the result is aninteger, and the argument is some kind of container. To the contrary,when I read x.len(), I have to already know that x is some kind ofcontainer implementing an interface or inheriting from a class thathas a standard len().