[Python-ideas] Python hook just before NameError

Chris Angelico rosuav at gmail.com
Mon Dec 29 09:00:31 CET 2014

 On 28/12/2014 16:42, Chris Angelico wrote:
> At the point where a global/builtin name lookup is about to raise
> NameError, first try calling a Python function, along the same lines
> as __getattr__. If that function hasn't been defined, raise NameError
> as normal; but if it has, let it either raise NameError or return some
> object, which is then used as if the name had been bound to it.

On Mon, Dec 29, 2014 at 6:46 PM, Mark Lawrence <breamoreboy at yahoo.co.uk> wrote:
> +1 from me as I'm always forgetting the "obvious" imports such as sys and os
> when trying things interactively.

On Mon, Dec 29, 2014 at 11:37 AM, Steven D'Aprano <steve at pearwood.info> wrote:
> An interesting idea, but I don't actually think much of it for
> interactive use. Having modules magically import themselves without an
> import is a bad habit for beginners to learn, and less useful for
> experienced users who know to import things.

This is why the feature is not "auto-import anything", but "give
Python code the chance to deal with NameError". Open to bikeshedding
about whether it's better per-module or global or what, but the point
is that you, as the programmer, get the flexibility. Maybe you want to
white-list potential imports, which would make this like pre-importing
those names but lazily. Maybe you want to allow a half-dozen common
"from" imports, but not just anything. Maybe you'd like to have some
completely different magic - like REXX mode:

>>> import sys
>>> sys.__getglobal__ = lambda name: name.upper()
>>> foo = "Ham "
>>> foo + spam
'Ham SPAM'

Or something more suitable for floating point work:

>>> sys.__getglobal__ = lambda name: float("nan")
>>> x = 4
>>> x + y

The power is in your hands. And by default, nothing is any different:

$ python3
Python 3.5.0a0 (default:1c51f1650c42+, Dec 29 2014, 02:29:06)
[GCC 4.7.2] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> foo
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'foo' is not defined

Don't like auto-importing? Don't use it. That simple. :)

Incidentally, I fully expect that the dunder name __getglobal__ will
be rejected. But the proposal works equally well with any name.


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