What am I doing wrong?

[keithlackey]

I'm relatively new to python and I've run into this problem.


DECLARING CLASS

class structure:
def __init__(self, folders = []):
self.folders = folders

def add_folder(self , folder):
self.folders.ap pend(tuple(fold er))



Now I try to make an instance of this class

structure1 = structure()
structure1.add_ folder([('foo'),])
print structure1.fold ers

This returns: [('foo',)]

This works fine. But when I try to make another instance of that class...

structure2 = structure()
print structure2.fold ers

This now also returns: [('foo',)]
Even though I haven't added any folders to this new instance

What am I doing wrong?

[Larry Bates]

Re: What am I doing wrong?

You have been bitten by a well known "feature". You used
a mutable as default value in your argument list for __init__.

See:

http://www.nexedi.org/sections/education/python/tips_and_tricks/python_and_mutable_n/view

It would be better to write:

class structure:
def __init__(self, folders = None):
self.folders=fo lders or []

-Larry Bates


keithlackey wrote:[color=blue]
> I'm relatively new to python and I've run into this problem.
>
>
> DECLARING CLASS
>
> class structure:
> def __init__(self, folders = []):
> self.folders = folders
>
> def add_folder(self , folder):
> self.folders.ap pend(tuple(fold er))
>
>
>
> Now I try to make an instance of this class
>
> structure1 = structure()
> structure1.add_ folder([('foo'),])
> print structure1.fold ers
>
> This returns: [('foo',)]
>
> This works fine. But when I try to make another instance of that class...
>
> structure2 = structure()
> print structure2.fold ers
>
> This now also returns: [('foo',)]
> Even though I haven't added any folders to this new instance
>
> What am I doing wrong?
>[/color]

[Scott David Daniels]

Re: What am I doing wrong?

keithlackey wrote:[color=blue]
> I'm relatively new to python and I've run into this problem.[/color]
This has two very standard mistakes:
First, as noted by Sybren, messages should just use spaces in order to
be readable.

After correcting that one:[color=blue]
> class structure:
> def __init__(self, folders = []):
> self.folders = folders
> ...[/color]

Here is the second one. Default args are not rebuilt, but shared.
The correct way to do this is:
class structure:
def __init__(self, folders=None):
if folders is None:
self.folders = []
else:
self.folders = folders
...

--Scott David Daniels
Scott.Daniels@A cm.Org