[issue3692] improper scope in list comprehension, when used in class declaration

[Mark Dickinson]

Change by Mark Dickinson :


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nosy: +mark.dickinson

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[issue3692] improper scope in list comprehension, when used in class declaration

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

Indeed, this issue is more complex than it looked to me.

Technically, LOAD_FAST, which is used for reading the loop
control variables in comprehensions, uses the array f->f_localsplus, while 
LOAD_NAME uses f->f_locals and f->f_globals. When executing class bodies, 
f->f_localsplus is NULL, and if executing functions f->f_locals usually is a 
NULL or a cached copy of a dict created from f->f_localsplus. But we can create 
a hybrid frame for comprehensions in class body, which will contain independent 
f->f_localsplus and f->f_locals. References to the loop
control variables and to the class variables will work as expected, just the 
meaning of locals() will be changed.

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[issue3692] improper scope in list comprehension, when used in class declaration

[Guido van Rossum]

Guido van Rossum  added the comment:

> I don't think this will need drastic changes.

IIRC LOAD_NAME loads from the *local* scope, which will be the synthetic
function created for the comprehension (whose scope contains the loop
control variables). We may need a new opcode similar to LOAD_GLOBAL that
looks in two non-local directories rather than just one before falling back
to builtins; and the function object would need to have a link to both the
class dict and the global dict -- currently function objects gave a
__globals__ link but there's no chaining. Or perhaps __globals__ could be
set to a chainmap referencing the class dict and the globals?

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[issue3692] improper scope in list comprehension, when used in class declaration

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

I don't think this will need drastic changes. Just setting some flags here or 
there for making comprehensions using LOAD_NAME instead of LOAD_GLOBAL. The fix 
should lie near the fix for issue33346.

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[issue3692] improper scope in list comprehension, when used in class declaration

[Ivan Levkivskyi]

Ivan Levkivskyi  added the comment:

> Bug or not this is not going to change without a PEP, so I'm not sure it's a 
> good idea to keep this issue open.

I agree this requires a PEP. I would like to keep this open as a place for 
pre-PEP discussions among those interested (and as a remainder to maybe write 
such PEP).

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[issue3692] improper scope in list comprehension, when used in class declaration

[Guido van Rossum]

Guido van Rossum  added the comment:

There's a proposal for a fix currently in PEP 572. I'm linking to the commit 
here because we're likely to remove it from PEP 572, but it may be proposed as 
a separate PEP: 
https://github.com/python/peps/commit/71fea57f19231b9917ce6a558c0b763288385398

Bug or not this is not going to change without a PEP, so I'm not sure it's a 
good idea to keep this issue open.

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[issue3692] improper scope in list comprehension, when used in class declaration

[Ivan Levkivskyi]

Ivan Levkivskyi  added the comment:

See https://bugs.python.org/issue33346 for yet another example where implicit 
function scope complicates life.

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[issue3692] improper scope in list comprehension, when used in class declaration

[Ivan Levkivskyi]

Ivan Levkivskyi  added the comment:

Mariatta,

> While I understand why it behaves the way it is now, but it seems wrong 
> behavior. Perhaps we should look into finding a solution.

I am all in favour of dropping implicit function scope in comprehensions 
(mostly because of weirdness like described here, but it will also fix some 
`yield` issues and simplify some `await` aspects). But I was not able to 
convince others it worth the effort. Maybe we can make another attempt at 
discussing this?

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nosy: +gvanrossum, levkivskyi, serhiy.storchaka

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[issue3692] improper scope in list comprehension, when used in class declaration

[Mariatta Wijaya]

Mariatta Wijaya  added the comment:

I'm re-opening this, since the behavior sounds like a bug to me.

While I understand why it behaves the way it is now, but it seems wrong 
behavior. Perhaps we should look into finding a solution.

--
nosy: +Mariatta
resolution: wont fix -> 
status: closed -> open
versions: +Python 3.8 -Python 3.0

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[issue3692] improper scope in list comprehension, when used in class declaration

[Simon Charette]

Simon Charette  added the comment:

I stumble upon this bug when porting a Python 2 codebase to 3 and suddenly got 
a NameError for the following code.


class Foo:
a = [1,2,3]
b = [4,5,6]
c = [x * y for x in a for y in b]

NameError: name 'b' is not defined

Not sure what could be done at this point but I thought I'd leave some feedback 
given I was surprised this would break given it works just fine if not defined 
at the class level.

a = [1,2,3]
b = [4,5,6]
c = [x * y for x in a for y in b]

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nosy: +charettes

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[issue3692] improper scope in list comprehension, when used in class declaration

[John McDonald]

John McDonald added the comment:

Could we possibly revisit this? This feels like an implementation detail, not 
something specified. Consider the different cases:

Type help, copyright, credits or license for more information.
 class A:
...   b = 5
...   print(locals())
...   x = [i*i for i in range(b)]
...
{'__qualname__': 'A', '__locals__': {...}, '__module__': '__main__', 'b': 5}

This case works, and shows that 'b' is clearly in locals(). 

 class A:
...   b = 5
...   c = b * b
...   print(locals())
...   d = []
...   for i in range(b):
... d.append(i*i)
...   print(locals())
...   e = [i*i for i in range(b) if i*i  c]
...
{'__locals__': {...}, '__qualname__': 'A', 'b': 5, '__module__': '__main__', 
'c': 25}
{'__qualname__': 'A', 'b': 5, '__module__': '__main__', 'c': 25, 'i': 4, 'd': 
[0, 1, 4, 9, 16], '__locals__': {...}}
Traceback (most recent call last):
  File stdin, line 1, in module
  File stdin, line 9, in A
  File stdin, line 9, in listcomp
NameError: global name 'c' is not defined

Again, it feels really arbitrary that the variable can be used in certain 
places in the list comprehension, but not others.

And of course, all of this works properly if you place the definitions either 
at global scope or within a function.

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nosy: +John.McDonald

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[issue3692] improper scope in list comprehension, when used in class declaration

[Georg Brandl]

Georg Brandl [EMAIL PROTECTED] added the comment:

This won't change -- in 3.0, list comprehensions and generator
expressions are both implemented using a function, so it's like the
following:

class Foo:
attribute1 = 1
def f():
return attribute1
attribute2 = f()

--
nosy: +georg.brandl
resolution:  - wont fix
status: open - closed

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[issue3692] improper scope in list comprehension, when used in class declaration

[kai zhu]

New submission from kai zhu [EMAIL PROTECTED]:

in 3rd line, list comprehension tries to access class_attribute1 as a
global variable (code is valid in python 2.5)

 class Foo(object):
...   class_attribute1 = 1
...   class_attribute2 = [class_attribute1 for x in range(8)]
...
Traceback (most recent call last):
  File stdin, line 1, in module
  File stdin, line 3, in Foo
  File stdin, line 3, in listcomp
NameError: global name 'class_attribute1' is not defined

--
components: Interpreter Core
messages: 72002
nosy: kaizhu
severity: normal
status: open
title: improper scope in list comprehension, when used in class declaration
type: behavior
versions: Python 3.0

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[issue3692] improper scope in list comprehension, when used in class declaration

[Daniel Diniz]

Daniel Diniz [EMAIL PROTECTED] added the comment:

I believe the problem is that list comprehensions in 3.0 have scope like
that of genexprs in 2.5, but the change was deliberate (as it also
avoids leaking of temp variables).

Compare to 2.5:
 class Foo(object):
...class_attribute1 = 1
...class_attribute2 = (class_attribute1 for x in range(8))
...
 Foo.class_attribute2.next()
Traceback (most recent call last):
  File stdin, line 1, in module
  File stdin, line 3, in genexpr
NameError: global name 'class_attribute1' is not defined

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nosy: +ajaksu2

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