[Python-Dev] classes and cell variables question (original) (raw)

Guido van Rossum guido at python.org
Tue Dec 19 23:24:52 CET 2006

On 12/19/06, tomer filiba <tomerfiliba at gmail.com> wrote:

to my understanding of the object model, the code of snippet 1 and snippet 2 should be equivalent. a class is just a "special function" that returns its locals automatically and passes them to the metaclass constructor:

--- snippet 1 --- class foo(object): x = 5 def f(self): print "f", x --- snippet 2 --- def bar(): x = 5 def g(self): print "g", x return locals() barcls = type("barcls", (object,), bar()) but there's one big difference. classes don't create cell variables to hold bound external variables. the "bar" version works, because "x" is a bound cell variable, but the "foo" version fails, as it attempts to access "x" as a global. .>>> barcls().g() g 5 .>>> foo().f() f Traceback (most recent call last): File "", line 1, in File "", line 4, in f NameError: global name 'x' is not defined for reference, i attached the code of all four functions below. my question is, how come classes don't create cell variables, like normal functions? was this done on purpose? does it have to do with inheritance? if so, what's wrong with my "bar" version?

I did this very much on purpose, although I have to think a bit to remember the reason. I think I didn't want "accidental Java code" where one would define a class variable and reference it from a method without prefixing it with self or the class name and it would accidentally work, but it would be very unpythonic. So I rigged it so that it wouldn't work.

[1] # code of class foo ############################################################ # 2 0 LOADNAME 0 (name) # 3 STORENAME 1 (module) # 3 6 LOADCONST 0 (5) # 9 STORENAME 2 (x) # # 4 12 LOADCONST 1 (<code object f at_ _009E5608, file "", line 4>) # 15 MAKEFUNCTION 0 # 18 STORENAME 3 (f) # 21 LOADLOCALS # 22 RETURNVALUE

[2] # code of foo.f: ############################################################ # 5 0 LOADCONST 1 ('f') # 3 PRINTITEM # 4 LOADGLOBAL 0 (x) # 7 PRINTITEM # 8 PRINTNEWLINE # 9 LOADCONST 0 (None) # 12 RETURNVALUE [3] # code of bar: ############################################################ # 2 0 LOADCONST 1 (5) # 3 STOREDEREF 0 (x) # # 3 6 LOADCLOSURE 0 (x) # 9 BUILDTUPLE 1 # 12 LOADCONST 2 (<code object g at_ _009F6698, file "", line 3>) # 15 MAKECLOSURE 0 # 18 STOREFAST 0 (g) # # 5 21 LOADGLOBAL 0 (locals) # 24 CALLFUNCTION 0 # 27 RETURNVALUE [4] # code of bar.g: ############################################################ # 4 0 LOADCONST 1 ('g') # 3 PRINTITEM # 4 LOADDEREF 0 (x) # 7 PRINTITEM # 8 PRINTNEWLINE # 9 LOADCONST 0 (None) # 12 RETURNVALUE

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-- --Guido van Rossum (home page: http://www.python.org/~guido/)

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