Update Python 2/3 porting HOWTO's suggestion for dealing with map()

[brettcannon]

BPO	21237
Nosy	@brettcannon, @MojoVampire

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/brettcannon'
closed_at = <Date 2014-05-09.18:35:11.558>
created_at = <Date 2014-04-15.15:45:50.531>
labels = ['easy', 'type-feature', 'docs']
title = "Update Python 2/3 porting HOWTO's suggestion for dealing with map()"
updated_at = <Date 2014-05-09.18:35:11.557>
user = 'https://github.com/brettcannon'

bugs.python.org fields:

activity = <Date 2014-05-09.18:35:11.557>
actor = 'brett.cannon'
assignee = 'brett.cannon'
closed = True
closed_date = <Date 2014-05-09.18:35:11.558>
closer = 'brett.cannon'
components = ['Documentation']
creation = <Date 2014-04-15.15:45:50.531>
creator = 'brett.cannon'
dependencies = []
files = []
hgrepos = []
issue_num = 21237
keywords = ['easy']
message_count = 3.0
messages = ['216315', '216458', '218191']
nosy_count = 2.0
nosy_names = ['brett.cannon', 'josh.r']
pr_nums = []
priority = 'normal'
resolution = 'wont fix'
stage = 'needs patch'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue21237'
versions = []

[brettcannon]

In Python 2.6 and newer you can import future_builtins and use everything from there. In Python 2.5 it should be itertools.imap(). This is much cleaner than the current suggestion at https://docs.python.org/2/howto/pyporting.html#update-map-for-imbalanced-input-sequences

[MojoVampire]

I think the suggestion is intended for "how do I keep Python 2 semantics in Python 3?", not "how can I write my Python 2 code so it will run equivalently in Python 3?"

It wouldn't be a bad idea to point out that you can adopt Py3 semantics initially so as to avoid surprises later on; sadly, unlike a __future__ import, if you want cross compatible code you have to do stupid stuff like:

try:
from future_builtins import *
except ImportError:
# Py3 is already the future
pass

[brettcannon]

I never bothered to mention the iter* methods in the HOWTO, and since it becomes very obvious very fast to tweak them I'm not going to worry about it and add more complexity to the doc.