Question

I am learning the concept of filters in Python. I am running a simple code like this.

>>> def f(x): return x % 2 != 0 and x % 3 != 0
>>> filter(f, range(2, 25))

But instead of getting a list, I am getting some message like this.

<filter object at 0x00FDC550>

What does this mean? Does it means that my filtered object i.e list to come out is stored at that memory location? How do I get the list which I need?

Answer 1

It looks like you’re using python 3.x. In python3, filter, map, zip, etc return an object which is iterable, but not a list. In other words,

filter(func,data) #python 2.x

is equivalent to:

list(filter(func,data)) #python 3.x

I think it was changed because you (often) want to do the filtering in a lazy sense — You don’t need to consume all of the memory to create a list up front, as long as the iterator returns the same thing a list would during iteration.

If you’re familiar with list comprehensions and generator expressions, the above filter is now (almost) equivalent to the following in python3.x:

( x for x in data if func(x) ) 

As opposed to:

[ x for x in data if func(x) ]

in python 2.x