Use of __slots__

[Don Taylor]

Hi:

I am puzzled about the following piece of code which attempts to create
a class that can be used as record or struct with a limited set of
allowed attributes that can be set into an instance of the class.

class RecordClass(obj ect):
__slots__ = ["foo"]
def __init__(self, args):
print "Initial slots = ", RecordClass.__s lots__
RecordClass.__s lots__ = list(args)
print "Final slots = ", RecordClass.__s lots__
pass

def new_record(slot list):
return RecordClass(slo tlist)

if __name__ == "__main__":
record1 = new_record(["age", "name", "job"])
record1.age = 27
record1.name = 'Fred'
record1.job = 'Plumber'
record1.salary = 50000

When executed I get:

Initial slots = ['foo']
Final slots = ['age', 'name', 'job']
Traceback (most recent call last):
File "D:\Programming Projects\JustFo rTesting\record classes.py", line
37, in ?
record1.age = 27
AttributeError: 'RecordClass' object has no attribute 'age'

I don't understand why I cannot set an attribute 'age' into record1.

Thanks,

Don.

[Steven D'Aprano]

On Sun, 26 Feb 2006 17:01:45 -0500, Don Taylor wrote:

Hi:

I am puzzled about the following piece of code which attempts to create
a class that can be used as record or struct with a limited set of
allowed attributes that can be set into an instance of the class.

class RecordClass(obj ect):
__slots__ = ["foo"]
You define a RecordClass with a single slot, "foo".

[snip]
I don't understand why I cannot set an attribute 'age' into record1.

Because you defined the class with only a slot for "foo", not "age".

Re-defining the __slots__ attribute at runtime does not add or subtract
slots from the class. It just breaks your class.

If you want to add and delete arbitrary attributes at runtime, don't use
slots.
--
Steven.

[Crutcher]

Steven is right, however, there is a way:

def new_record(slot list):
class R(object):
__slots__ = slotlist
return R()

record1 = new_record(["age", "name", "job"])
record1.age = 27
record1.name = 'Fred'
record1.job = 'Plumber'
record1.salary = 50000