Why does [].append() not work in python?

Question

Why does this work -

a = []
a.append(4)
print a

But this does not -

print [].append(4)

The output in second case is None. Can you explain the output?

Answer 1

The append method has no return value. It changes the list in place, and since you do not assign the [] to any variable, it's simply "lost in space"

class FluentList(list):
    def append(self, value):
        super(FluentList,self).append(value)
        return self

    def extend(self, iterable):
        super(FluentList,self).extend(iterable)
        return self

    def remove(self, value):
        super(FluentList,self).remove(value)
        return self

    def insert(self, index, value):
        super(FluentList,self).insert(index, value)
        return self 

    def reverse(self):
        super(FluentList,self).reverse()
        return self

    def sort(self, cmp=None, key=None, reverse=False):
        super(FluentList,self).sort(cmp, key, reverse)
        return self

li = FluentList()
li.extend([1,4,6]).remove(4).append(7).insert(1,10).reverse().sort(key=lambda x:x%2)
print li

I didn't overload all methods in question, but the concept should be clear.

Answer 2

The method append returns no value, or in other words there will only be None

a is mutable and the value of it is changed, there is nothing to be returned there.