Cannot append to a returned list?

def f():
    lst = ['a', 'b', 'c']
    return lst[1:]

why is f().append('a') is None == True even though f().__class__ is <type 'list'>开发者_JAVA技巧 and f() == ['b', 'c']

Because append() returns None and not the list object. Use

l = f()
l.append('a')
...

Because append() modifies the list, but does not return it.

Try this:

f()+['a']

Hope this helps

In this context it's always good to be fully aware of the difference between expressions and commands. There are basically two ways to append a value x to a list l

1. Using a command: l.append(x). Usually a command doesn't return any value; it performs some kind of side-effect.
2. Using an expression, namely l+[x] which stands for a value and does nothing. I.e. you assign l=l+[x]