Question

When I execute this code in python 2.6

reduce(lambda x,y: x+[y], [1,2,3],[])

I get [1, 2, 3] as expected.
But when I execute this one (I think it is equivalent to previous)

reduce(lambda x,y: x.append(y), [1,2,3],[])

I get an error message

Traceback (most recent call last):
File "<stdin>", line 1, in <module>
  File "<stdin>", line 1, in <lambda>
AttributeError: 'NoneType' object has no attribute 'append'

Why these two lines of code do not give the same result?

Answer 1

x.append(y) is not equivalent to x+[y]; append modifies a list in place and returns nothing, while x+[y] is an expression that returns the result.