Brian Brinegar wrote:
Hmmm,
There seems to be a bug in the way parameters are referenced in python scripts. I have a simple script that has a list as a default parameter. The script then appends something to this list and returns the list. Each time I call the script the list gets to be one element longer. Here is an example script:
## Script (Python) "pyFix" ##bind container=container ##bind context=context ##bind namespace= ##bind script=script ##bind subpath=traverse_subpath ##parameters=a=['a'] ##title= ## a.append('b') return a
The output from the script is as follows: 1st call: ['a','b'] 2nd call: ['a','b','b'] 3rd call: ['a','b','b','b'] 4th call: ['a','b','b','b','b'] etc.
It seems that I'm getting a reference to the default parameter which I then modify for future calls. Currently I'm getting around this by having list parameters default to none and then setting the defaults within the script.
I have some security concerns since I can change the default parameters for other users on the server. Or so it seems.
Suggestions?
The behavior you are observing is not unique to PythonScripts; mutable default values are a notorious bug magnet for Python applications in general. Try the following in your Python interpreter:
def memoize(value, seen=[]): ... seen.append(value) ... return seen ... memoize('a') ['a'] memoize('b') ['a', 'b'] memoize('b') ['a', 'b', 'b']
The classic advice here is, "Don't do that." Instead, use a non-mutable default value (e.g. None or the empty tuple), and special case it. Even better, avoid writing to the default, e.g.,:: ## Script (Python) "pyFix" ##parameters=a=('a',) ## result.extend(a) result.append('b') return result Tres. -- =============================================================== Tres Seaver tseaver@zope.com Zope Corporation "Zope Dealers" http://www.zope.com