"Don't rebind built-in names*" - it confuses readers

[Chris Angelico]

On Thu, Jun 13, 2013 at 9:07 AM, Mark Janssen <dreamingforward at gmail.com> wrote:
>>> You're right.  I was being sloppy.
> Okay, now I'm a bit confused.  "print" is both a <keyword> and a
> member of the builtins.  What happens then?

Ah! I see where we are getting confused. When you said keyword, did
you mean keyword, a person who has lost his parents... oops, that's
Pirates of Penzance. Ahem.

In Python 2.x, 'print' is actually a keyword. It has its own special
syntax (eg printing to something other than stdout), and absolutely
cannot be overridden:

Python 2.7.4 (default, Apr  6 2013, 19:54:46) [MSC v.1500 32 bit
(Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> print=1
  File "<stdin>", line 1
    print=1
         ^
SyntaxError: invalid syntax

But in Python 3, it's a builtin function:

Python 3.3.0 (v3.3.0:bd8afb90ebf2, Sep 29 2012, 10:55:48) [MSC v.1600 32 bit (In
tel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> foo=print
>>> print=1
>>> foo("Hello, world!")
Hello, world!

> And abs(), max(), hex()  and such seemed like keywords to my
> scientific self (due to never having to "include"/import them), but
> clearly their not.

The builtins don't need to be imported, but they're identifiers like
anything else. They're a namespace that gets searched after
module-globals.

int = 1
def foo():
   int = 2
   print(int)

This has double shadowing :) It'll print 2, because locals get
searched first, but if you remove that assignment then it'll print 1,
and if you remove _that_ one, then it'll print "<class 'int'>".
There's nothing magical about the name int, but before raising
NameError, the interpreter will look for it in builtins. You can even
add more "builtins", though I would *strongly* advise against this
unless you have a really REALLY good reason:

>>> __builtins__.helloworld=123
>>> helloworld
123
>>> helloworld=234
>>> helloworld
234
>>> del helloworld
>>> helloworld
123

> And int, list, tuple, dict and such always seemed
> like keywords to my CS self because they were included in Python's
> type system (like "int" would be in C).

Yep, but in Python, types/classes are themselves objects, so you can
pass them around like anything else. This also downgrades them from
"language keyword" to "always-available name", which in effect
upgrades your _own_ classes to the same level.

> They are all one-step removed from keywords.   And yet, since they are
> not in a separate namespace, they should not be used as variable
> names.  Perhaps since they are very different from one another, they
> should be put in separate namespaces off of a global, root
> namespace...  (math, string, etc.)

There's no point forcing them to be looked up in a two-step process.
If you want that, you can simply reference them as
__builtins__.whatever, but you can instead just reference them as the
unadorned name whatever. They contribute heavily to the simplicity and
readability of Python code - imagine if every call to len() had to be
qualified.

ChrisA