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Oddity in 2.4 with eval('None')

In Python 2.4, although None can't be directly assigned to,
globals()['None'] can still be; however, that won't change the value of
the expression "None" in ordinary statements. Except with the eval
function, it seems:

Python 2.4 (#2, Dec 3 2004, 17:59:05)
[GCC 3.3.5 (Debian 1:3.3.5-2)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
print None None print eval('None') None globals()['None'] = "spam"
print None None print eval('None')

spam

I don't really mind this weird behavior; I'm just curious about it. Does
anyone know what might be going on in Python's internals to cause the
difference between "print None" and "print eval('None')"?
Jul 18 '05 #1
6 1453
Leif K-Brooks wrote:
In Python 2.4, although None can't be directly assigned to,
globals()['None'] can still be; however, that won't change the value of
the expression "None" in ordinary statements. Except with the eval
function, it seems:

Python 2.4 (#2, Dec 3 2004, 17:59:05)
[GCC 3.3.5 (Debian 1:3.3.5-2)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> print None None >>> print eval('None') None >>> globals()['None'] = "spam"
>>> print None None >>> print eval('None') spam

I don't really mind this weird behavior; I'm just curious about it. Does
anyone know what might be going on in Python's internals to cause the
difference between "print None" and "print eval('None')"?


Yes. "print eval('None')" is printing the value of None as defined in
your module's global namespace:

Python 2.4 (#1, Dec 4 2004, 20:10:33)
[GCC 3.3.3 (cygwin special)] on cygwin
Type "help", "copyright", "credits" or "license" for more information.
import sys
globals()['None'] = "FooBar"
print sys.modules["__main__"].None FooBar print __builtins__.None None print eval("__builtins__.None")

None >>>

regards
Steve
--
Steve Holden http://www.holdenweb.com/
Python Web Programming http://pydish.holdenweb.com/
Holden Web LLC +1 703 861 4237 +1 800 494 3119
Jul 18 '05 #2
Python 2.2.2 (#37, Nov 26 2002, 10:24:37) [MSC 32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
dir() ['__builtins__', '__doc__', '__name__', 'shell'] globals()['None'] Traceback (most recent call last):
File "<input>", line 1, in ?
KeyError: None print None None print eval('None') None globals()['None']='wassabi'
print None wassabi print eval('None') wassabi dir() ['None', '__builtins__', '__doc__', '__name__', 'shell']


I think you are seeing two things and I admit I was confused a bit too.
None is becoming a constant and this is the first step. That is why you
see diffrent results than mine. Also notyice that None didn't exist in
your lnamespace till you defined it then you see all your trouble
start.Shadowing builtins and semi-constants are a bad thing
M.E.Farmer

Jul 18 '05 #3
Steve Holden wrote:
Yes. "print eval('None')" is printing the value of None as defined in
your module's global namespace:


Right, but why? The expression "None" doesn't worry about the global
namespace when used in normal code; why does it when used in eval()ed code?
Jul 18 '05 #4
Leif K-Brooks wrote:
Steve Holden wrote:
Yes. "print eval('None')" is printing the value of None as defined in
your module's global namespace:

Right, but why? The expression "None" doesn't worry about the global
namespace when used in normal code; why does it when used in eval()ed code?


I have no idea why. Given that
eval("globals()['__builtins__'].globals().keys()")

['None', '__builtins__', '__file__', 'sys', '__name__', '__doc__']

it's beginning to smell a bit like a buglet.

regards
Steve
--
Steve Holden http://www.holdenweb.com/
Python Web Programming http://pydish.holdenweb.com/
Holden Web LLC +1 703 861 4237 +1 800 494 3119
Jul 18 '05 #5
Leif K-Brooks wrote
Yes. "print eval('None')" is printing the value of None as defined in your module's global
namespace:


Right, but why? The expression "None" doesn't worry about the global namespace when used in normal
code; why does it when used in eval()ed code?


from what I can tell, the mapping of the None symbol to a constant is done
in the peephole optimizer, which doesn't seem to be used when compiling
expressions.

in 2.4:
dis.dis(compile("None", "", "exec")) 1 0 LOAD_CONST 0 (None)
3 POP_TOP
... dis.dis(compile("None", "", "eval")) 0 0 LOAD_NAME 0 (None)
3 RETURN_VALUE

in 2.3:
dis.dis(compile("None", "", "exec")) 1 0 LOAD_NAME 0 (None)
3 POP_TOP
... dis.dis(compile("None", "", "eval"))

0 0 LOAD_NAME 0 (None)
3 RETURN_VALUE

</F>

Jul 18 '05 #6
"Leif K-Brooks" <eu*****@ecritters.biz>
In Python 2.4, although None can't be directly assigned to,
globals()['None'] can still be; however, that won't change the value of
the expression "None" in ordinary statements. Except with the eval
function, it seems:

Python 2.4 (#2, Dec 3 2004, 17:59:05)
[GCC 3.3.5 (Debian 1:3.3.5-2)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> print None None >>> print eval('None') None >>> globals()['None'] = "spam"
>>> print None None >>> print eval('None')

spam

I don't really mind this weird behavior; I'm just curious about it. Does
anyone know what might be going on in Python's internals to cause the
difference between "print None" and "print eval('None')"?


It is a nuance of how None is being made constant.

For backwards compatability, None still has to be in the globals dictionary.
Like all entries in the globals dictionary, you can change it if you try hard
enough (which you did).

For Py2.4, whenever bytecode is generated for a code object, references to
"None" in the globals dictionary are bypassed and replaced with a constant
reference to Py_None, the one, true, singleton instance of None. That will
occur even if you've mucked with None entry in the globals dictionary.

Bytecode for eval() doesn't go through the bytecode optimizer so its dictionary
lookup is retained (producing the effect in your example).

To have made None a literal constant would have been a much more radical step.
Py2.4 simulates this and gives you the real benefit being sought, faster
function execution, without having incurred the costs of having a new literal.

It's possible to fool the simulation, but who cares.
Raymond Hettinger
Jul 18 '05 #7

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