Question

So I know that in Python variables declared in a method are not visible outside of it. But what is their life cycle? I would expect that a variable declared inside a method is created each time that method is called.

I’m asking because of the following situation. I have a method as follows(this is used with cherrypy, probably not relevant but still I’ll add it for the off chance this influences it in any way).

First the custom Filter class I’m using:

class Filter():
"""
Class used to filter tables displayed in UI, based on few criteria.
Initialize filter with a list of fields, and correspondinf values.
"""
def __init__(self, display_name= "", fields= [], values= [], operations= []):
    self.display_name = display_name
    self.fields = fields
    self.values = values
    self.selected = False 
    self.operations = operations

Now the method I’m talking about

@cherrypy.expose
@logged() 
def getfiltereddatatypes(self, name, filters, datatype):
    ...some mumbo jumbo...

    default_filter = self._get_default_filters(inputTree, name)

    #default_filter here can be a object of type Filter if defined or None otherwise

    print "RECIEVED " + str(filters)
    if default_filter is not None:
        print "CREATING NEW"
        new_filter = copy.deepcopy(default_filter)
    else:
        print "CREATING NEW"
        new_filter = Filter()
    [new_filter.fields.append(value) for value in filters['fields']]
    [new_filter.operations.append(value) for value in filters['operations']]
    [new_filter.values.append(value) for value in filters['values']]
    print "LENGTH =" + str(len(new_filter.fields))

    ...Some other mumbo jumbo....

So basically I want to add the filter values recieved as parameter in the filters variable to the default_ones. But i want this to be done each time the method is called, or to better explain it, I made the prints to show my problem. So at first method call:

    RECIEVED {'operations': ['!='], 'fields': ['model.DataType.subject'], 'values': ['w']}
    CREATING NEW
    LENGTH =1

So as expected, a new Filter object is created, and the operations, fields, and values from the recieved parameter are added, length(fields) = 1. But at my second pass:

    RECIEVED {'operations': ['!='], 'fields': ['model.DataType.subject'], 'values': ['']}
    CREATING NEW
    LENGTH =2

Now this I can’t explain this behaviour. As you can see, the CREATING NEW brach of the if is called again, the filters variable holds one item lists but the length of the result is 2. If I run it again and again it will always increment. It’s like the new_filter = Filter() call returns the new_filter from the previous method call. What causes this kind of behaviour? I’m thiking maybe if I do a new_filter = copy.deepcopy(Filter()) it might solve it but why should I be forced to do that?

Regards,
Bogdan

Answer 1

def __init__(self, display_name= "", fields= [], values= [], operations= [])

The lists are created only once. So if you modify them the next call will have the modified lists. If you do not want this behaviour, simply use None as the default value and add something like this:

if fields is None:
    fields = []
self.fields = fields

Another solution would be creating a new list no matter what was passed:

self.fields = list(fields)

There’s also a detailed article about this behaviour at http://effbot.org/zone/default-values.htm

---

By the way, your abuse of list comprehensions is not nice.

[new_filter.fields.append(value) for value in filters['fields']]

should be

new_filter.fields += filters['fields']