What is None doing in the code object's co_consts attribute?

Question

The following function returns None:

In [5]: def f():
   ...:     pass

So I was not surprised by this output:

In [8]: dis.dis(f)
  2           0 LOAD_CONST               0 (None)
              3 RETURN_VALUE        

In [10]: f.__code__.co_consts
Out[10]: (None,)

Ok, this makes sense. But now, consider the following function:

In [11]: def g():
   ....:     return 1

In [12]: dis.dis(g)
  2           0 LOAD_CONST               1 (1)
              3 RETURN_VALUE        

In [13]: g.__code__.co_consts
Out[13]: (None, 1)

g does not use None, so why is it in co_consts?

Answer 1

The default return value for a function is None, so it is always inserted. Python doesn't want to have to analyse if you are always going to reach a return statement.

Consider for example:

def foo():
    if True == False:
        return 1

The above function will return None, but only because the if statement is never going to be True.

In your simple case, it is obvious to us humans that there is just the one RETURN_VALUE opcode, but to extend this to the general case for a computer to detect is not worth the effort. Better just store the None reference and be done with it, that one reference is extremely cheap.