class closure question

[Steven W. Orr]

I want to indirectly change the value of a variable.

#! /usr/bin/python
foo = [44]
bar = foo
bar[0] = 55
print 'bar = ', bar
print 'foo = ', foo

This works fine.

bar = [55]
foo = [55]

But I want to do the same with a class value.

#! /usr/bin/python
S = None
dd = { 'class': [S] }
class C1(object):
def __init__(self):
print 'Hello from C1'

def mkclass(base):
class zzz(base):
pass
return zzz

dd['class'][0] = mkclass( C1 )
print "dd['class'][0].__bases__ = ", dd['class'][0].__bases__
print 'S = ', S

The answer is not what I want:

dd['class'][0].__bases__ = (<class '__main__.C1'>,)
S = None

The goal is for S to be set to the returned class from mkclass.

Can someone help?

--
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[Bruno Desthuilliers]

Steven W. Orr a écrit :

I want to indirectly change the value of a variable.

Are you sure this is the correct formulation of your problem ?

#! /usr/bin/python
foo = [44]
bar = foo
bar[0] = 55
print 'bar = ', bar
print 'foo = ', foo

This works fine.

bar = [55]
foo = [55]

But I want to do the same with a class value.

#! /usr/bin/python
S = None
dd = { 'class': [S] }
class C1(object):
def __init__(self):
print 'Hello from C1'

def mkclass(base):
class zzz(base):
pass
return zzz

dd['class'][0] = mkclass( C1 )

Hem... If your goal is to rebind S, then you got it all wrong. What
you're doing here is to rebind dd['class'][0] from S to the return value
of mkclass. This won't of course rebind S itself.

print "dd['class'][0].__bases__ = ", dd['class'][0].__bases__
print 'S = ', S

The answer is not what I want:

dd['class'][0].__bases__ = (<class '__main__.C1'>,)
S = None

The goal is for S to be set to the returned class from mkclass.

Seems like you still don't get how Python's assignment work.

FWIW, the type of object you want to bind to S is totally irrelevant, so
you could get rid of this mkclass stuff for the moment. Also, you don't
have to make dd['class'] point to a list containing S here - dicts are
themselves mutables, so your example would work the same with

dd = {'class':S}

and
dd['class'] = any_other_object;

Can someone help?

Not me, at least unless you explain your real use case - I mean, the
problem you're trying to solve by "indirectly chang(ing) the value of a
variable".

[Peter Otten]

Steven W. Orr wrote:

I want to indirectly change the value of a variable.

#! /usr/bin/python
foo = [44]
bar = foo
bar[0] = 55
print 'bar = ', bar
print 'foo = ', foo

This works fine.

bar = [55]
foo = [55]

But I want to do the same with a class value.

#! /usr/bin/python
S = None
dd = { 'class': [S] }
class C1(object):
def __init__(self):
print 'Hello from C1'

def mkclass(base):
class zzz(base):
pass
return zzz

dd['class'][0] = mkclass( C1 )
print "dd['class'][0].__bases__ = ", dd['class'][0].__bases__
print 'S = ', S

The answer is not what I want:

dd['class'][0].__bases__ = (<class '__main__.C1'>,)
S = None

The goal is for S to be set to the returned class from mkclass.

Can someone help?

What you want is not possible in Python. You can modify some objects
(called "mutable") but rebinding a name has to be explicit.

Peter