staticmethod behaviour

[Samu]

Hi,

I run today into some problems with my code and I realized that there
is something in the behaviours of the @staticmethod that I don't
really understand. I don't know if it is an error or not, actually,
only that it was, definitely, unexpected.

I wrote a small demo of what happens.

The code:
http://dpaste.com/hold/233795/

The answer I get:
User created with static: id, rights, rights2
1 ['read', 'write'] ['write2'] ['write3']
User created with User()
None [] ['write2'] ['write3']

I was expecting either all arrays from the second to be [] or to be a
copy of the first one.

If someone can provide an explanation, I would be thankful

Regards,
Samu

 

[Samu]

Hi,

I run today into some problems with my code and I realized that there
is something in the behaviours of the @staticmethod that I don't
really understand. I don't know if it is an error or not, actually,
only that it was, definitely, unexpected.

I wrote a small demo of what happens.

The code:http://dpaste.com/hold/233795/

The answer I get:
User created with static: id, rights, rights2
1 ['read', 'write'] ['write2'] ['write3']
User created with User()
None [] ['write2'] ['write3']

I was expecting either all arrays from the second to be [] or to be a
copy of the first one.

If someone can provide an explanation, I would be thankful

Regards,
Samu

In addition, if I don't define the function as static, but either as a
method of the object or a function outside of the class, something
like this:
def cr_user():
user = User(1, ['read'], rights3=[])
user.rights.append('write')
user.rights2.append('write2')
user.rights3.append('write3')
return user

I get instead:
User created with static: id, rights, rights2
1 ['read', 'write'] ['write2'] ['write3']
User created with User()
None [] ['write2'] []

There is some (maybe deep) concept that I don't get it seems, because
that output puzzles me...

 

[Peter Otten]

Hi,

I run today into some problems with my code and I realized that there
is something in the behaviours of the @staticmethod that I don't
really understand. I don't know if it is an error or not, actually,
only that it was, definitely, unexpected.

I wrote a small demo of what happens.

The code:

class User:
def __init__(self, id=None, rights=[], rights2=[], rights3=[]):
self.id = id
self.rights = rights
self.rights2 = rights2
self.rights3 = rights3
@staticmethod
def cr_user():
user = User(1, ['read'], rights3=[])
user.rights.append('write')
user.rights2.append('write2')
user.rights3.append('write3')
return user

print "User created with static: id, rights, rights2"
a = User.cr_user()
print a.id, a.rights, a.rights2, a.rights3
print "User created with User()"
b = User()
print b.id, b.rights, b.rights2, a.rights3

The answer I get:
User created with static: id, rights, rights2
1 ['read', 'write'] ['write2'] ['write3']
User created with User()
None [] ['write2'] ['write3']

I was expecting either all arrays from the second to be [] or to be a
copy of the first one.

If someone can provide an explanation, I would be thankful

The problem is not the staticmethod, it's the mutable default values for
__init__(). See

http://effbot.org/zone/default-values.htm

Peter

 

[Samu]

Hi,

I run today into some problems with my code and I realized that there
is something in the behaviours of the @staticmethod that I don't
really understand. I don't know if it is an error or not, actually,
only that it was, definitely, unexpected.

I wrote a small demo of what happens.
The code:
class User:
 def __init__(self, id=None, rights=[], rights2=[], rights3=[]):
  self.id = id
  self.rights = rights
  self.rights2 = rights2
  self.rights3 = rights3
 @staticmethod
 def cr_user():
  user = User(1, ['read'], rights3=[])
  user.rights.append('write')
  user.rights2.append('write2')
  user.rights3.append('write3')
  return user

print "User created with static: id, rights, rights2"
a = User.cr_user()
print a.id, a.rights, a.rights2, a.rights3
print "User created with User()"
b = User()
print b.id, b.rights, b.rights2, a.rights3
The answer I get:
User created with static: id, rights, rights2
1 ['read', 'write'] ['write2'] ['write3']
User created with User()
None [] ['write2'] ['write3']

I was expecting either all arrays from the second to be [] or to be a
copy of the first one.

If someone can provide an explanation, I would be thankful

The problem is not the staticmethod, it's the mutable default values for
__init__(). See

http://effbot.org/zone/default-values.htm

Peter

Ahh, thank you very much for the link. Now I understand. I remember
having read that before, but it is not until you face the problem that
the concept sticks. But why does it have a different behaviour the
staticmethod with the "rights3" case then?

 

[Peter Otten]

the concept sticks. But why does it have a different behaviour the
staticmethod with the "rights3" case then?

Moving from staticmethod to standalone function doesn't affect the output.
You have inadvertently changed something else.

Peter

 

[Samu]

Moving from staticmethod to standalone function doesn't affect the output..
You have inadvertently changed something else.

Peter

Absolutely right. Thank you very much for your time and answers,
Peter It helped me a lot!