Namespace puzzle, list comprehension fails within class definition

Mon Jan 12 23:49:34 EST 2015

On 2015年1月12日 12:40:13 -0800, Ethan Furman wrote:
> On 01/12/2015 12:25 PM, John Ladasky wrote:
>> d = {0:"a", 1:"b", 2:"c", 3:"d"}
>> e = [d[x] for x in (0,2)]
>>>> class Foo:
>> f = {0:"a", 1:"b", 2:"c", 3:"d"}
>> print(f)
>> g = [f[x] for x in (0,2)]
>> In Foo 'f' is part of an unnamed namespace; the list comp 'g' has its
> own namespace, effectively making be a nonlocal; class name lookup skips
> nonlocal namespaces.

Actually, no it doesn't.
py> def factory():
... x = 23
... class Inner(object):
... print('x is', x)
... return Inner
... 
py> o = factory()
x is 23
The "problem" is that *functions* lookup don't include the class body in 
their scope. This is by design, and goes back to Python 1.5 or older:
[steve at ando ~]$ python1.5
Python 1.5.2 (#1, Aug 27 2012, 09:09:18) [GCC 4.1.2 20080704 (Red Hat 
4.1.2-52)] on linux2
Copyright 1991-1995 Stichting Mathematisch Centrum, Amsterdam
>>> class Outer:
... 	x = 23
... 	f = lambda: x+1
... 	y = f()
... 
Traceback (innermost last):
 File "<stdin>", line 1, in ?
 File "<stdin>", line 4, in Outer
 File "<stdin>", line 3, in <lambda>
NameError: x
> Workaround: use an actual for loop.

Sad but true.
-- 
Steven