Couldn't close file in functional way in python3.1?

Question

I wrote a line of code using lambda to close a list of file objects in python2.6:

map(lambda f: f.close(), files)

It works, but doesn't in python3.1. Why?

Here is my test code:

import sys

files = [sys.stdin, sys.stderr]

for f in files: print(f.closed)   # False in 2.6 & 3.1

map(lambda o : o.close(), files)

for f in files: print(f.closed)   # True in 2.6 but False in 3.1

for f in files: f.close()        

for f in files: print(f.closed)   # True in 2.6 & 3.1

Answer 1

map returns a list in Python 2, but an iterator in Python 3. So the files will be closed only if you iterate over the result.

Never apply map or similar "functional" functions to functions with side effects. Python is not a functional language, and will never be. Use a for loop:

for o in files:
    o.close()

Answer 2

Because map in Python 3 is a lazy iterator. Quoting the docs:

Return an iterator that applies function to every item of iterable, yielding the results.

E.g. in Python 2, map(f, seq) equivalent to [f(i) for i in seq], but in Python 3, it's (f(i) for i in seq) - subtly different syntax, but very different semantics. To make the map variant work, you'd need to consume the iterator. Ergo, it's simpler (and more idiomatic: map, comprehension and generators shouldn't have side effects!) to use an explicit for-loop.