Methods don't chain in Python set

Question

I have a Python set consisting of several values and I want to use method chaining like this:

>>> f = {1, 2, 3}
>>> g = f.copy().discard(3)
>>> g
>>>

but g becomes empty. However, it works without chaining:

>>> g = f.copy()
>>> g
{1, 2, 3}
>>> g.discard(3)
>>> g
{1, 2}

Can somebody explain me this behaviour?

Answer 1

discard() discards an element in the set and returns None.

so when you assign

g = f.copy().discard(3)

this is equivalent to

h = f.copy() # now h = f
g = h.discard(3) # g = None; h = {1,2}

which leaves you with g = None.

Answer 2

When you do g = f.copy().discard(3), you are storing in g the return value of the method "discard(3)". In this case, it returns nothing, but alters the object. This is why on the second scenario it works.