classes: need for an explanation

[0k-]

hello!

i started to write a game in python in which i want to implement
"dynamically attachable" attributes.
my intention is to generalise this and moreover these attributes can be
complex, so i'm creating classes to represent these attributes. but
somehow i cannot instantiate these attribute classes well...

here is the simple outline of my code:

class Attr(object):
name = ""
value = ""
def __init__(self, name, value):
self.name = name
self.value = value

# this is just an example, in the real game attrs. are more complex!
class TxtAttr(Attr):
name = "text"
def __init__(self, value):
self.value = value

class Thing(object):
props = {}
def __init__(self):
self.props["text"] = TxtAttr("something important")

t1 = Thing()
t2 = Thing()

t2.props["text"].value = "another string"

print "t1: %s\nt2: %s" % (t1.props["text"].value,
t2.props["text"].value)

the above code outputs:

t1: another string
t2: another string

so the problem i cannot get through is that both t1 and t2 have the
same attr class instance.
could somebody please explain me why?

 

[0k-]

oops, i forgot to mention i'm using interpreter version 2.4.3

 

[Peter Otten]

class Thing(object):
props = {}
def __init__(self):
self.props["text"] = TxtAttr("something important")

t1 = Thing()
t2 = Thing()

t2.props["text"].value = "another string"

print "t1: %s\nt2: %s" % (t1.props["text"].value,
t2.props["text"].value)

the above code outputs:

t1: another string
t2: another string

so the problem i cannot get through is that both t1 and t2 have the
same attr class instance.
could somebody please explain me why?

Putting an assignment into the class body makes the name a /class/ attribute
which is shared by all instances of the class:
.... props = {} # this is a class attribute
....True

The solution is to move the attribute creation into the initializer:
.... def __init__(self):
.... self.props = {} # an instance attribute
....False

Peter

 

[0k-]

thanks for the quick reply, Peter!

 

[Steven D'Aprano]

hello!

i started to write a game in python in which i want to implement
"dynamically attachable" attributes.

All attributes in Python are dynamically attachable.
.... pass
....'beautiful red'


[snip]

class Thing(object):
props = {}
def __init__(self):
self.props["text"] = TxtAttr("something important")

t1 = Thing()
t2 = Thing()

t2.props["text"].value = "another string"

Have you considered a simpler class like this?

class Thing(object):
def __init__(self, value="something important"):
self.text = TxtAttr(value)

t2 = Thing("another string")

Then, instead of writing t2.props["text"].value (four lookups) you would
just write t2.text (two lookups).