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.append(tuple(folder))



Now I try to make an instance of this class

structure1 = structure()
structure1.add_folder([('foo'),])
print structure1.folders

This returns: [('foo',)]

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

structure2 = structure()
print structure2.folders

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

What am I doing wrong?

 

[Larry Bates]

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=folders or []

-Larry Bates

 

[Scott David Daniels]

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

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:

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

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
(e-mail address removed)