Accessing instance variables using index

Question

I have a class which keeps track of errors encountered during a search operation

class SearchError(object): 
    def __init__(self,severity=0,message=''): 
        self.severity = severity
        self.message = message

My idea is to make the instance variables indexable.

So if I have

a=SearchError(1,"Fatal Error")

I get

>>> a[0]
1
>>> a[1]
'Fatal Error'
>>> a.severity
1
>>> a.message
'Fatal Error'

To do this I add a __getitem__ method to the class. The class now becomes

class SearchError(object): 
    def __init__(self,severity=0,message=''): 
        self.severity = severity
        self.message = message

    def __getitem__(self,val): 
        if isinstance(val,slice): 
            return [self.__getitem__(i) for i in xrange(val.start,val.stop,val.step)]
        elif val==0: 
            return self.severity
        elif val==1: 
            return self.message
        else: 
            raise IndexError

This does what I want but fails in cases such as

>>> a[:2]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 23, in __getitem__
TypeError: an integer is required

Or even

>>> a[-1]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 29, in __getitem__
IndexError

I understand my implementation of __getitem__ is limited. What I need to know is -

Is this the way to make instance variables indexable (Without using a list as variable container)?
How do I make the object behave 'sanely' as far as indexing goes?

jamylak · Accepted Answer · 2013-04-15 12:37:03Z

1

This does everything:

>>> from collections import namedtuple
>>> _SearchError = namedtuple("SearchError", "severity message")
>>> def SearchError(severity=0, message=''):
        return _SearchError(severity, message)

answered Apr 15, 2013 at 12:37

jamylak

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2 Comments

mgilson Over a year ago

Note that here your objects are immutable as a consequence of them being a tuple. That may be fine for OP's application, but it might not so I think it is worth mentioning.

RedBaron Over a year ago

What if the class SearchError also has some methods?

ecatmur · Accepted Answer · 2013-04-15 12:47:04Z

1

xrange requires all its arguments to be integers, but slice objects have None for unspecified attributes.

The best way to implement what you're after is to use namedtuple:

from collections import namedtuple
class SearchError(namedtuple('SearchError', 'severity message')):
    def __new__(cls, severity=0, message=''):
        return super(SearchError, cls).__new__(cls, severity, message)

edited Apr 15, 2013 at 12:47

answered Apr 15, 2013 at 12:41

ecatmur

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Comments

mgilson · Accepted Answer · 2013-04-15 15:00:47Z

1

The problem here is that slice objects default to having None values as attributes. So, a[:2] passes in slice(None,2,None). When you break this apart and try to pass it to xrange, you'll get a TypeError:

>>> xrange(None,2,None)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: an integer is required

Try a[0:2:1] and your code will work. Ultimately, you could do something like:

val.start = 0 if val.start is None else val.start
val.stop = 2 if val.stop is None else val.stop
val.stop = 2-val.stop if val.stop < 0 else val.stop
val.step = 1 if val.step is None else val.step

to unravel your slices into useable indices (In the general case, it'd be better to use len(self) instead of 2, but I don't know if your object has defined __len__.

Or, even better:

start,stop,step = val.indices(len(self))

Similarly, in the case where you do a[-1], you're not passing in a slice, a 0 or a 1, so you hit the else clause where you to raise an IndexError.

edited Apr 15, 2013 at 15:00

answered Apr 15, 2013 at 12:38

mgilson

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2 Comments

RedBaron Over a year ago

Thanks for clearing the slice part. I got why a[-1] raises an indexerror but was just curious how can I make my objects behave like other indexable objects (lists,tuples). I also read somewhere that slice has a method (probably indices() or something) which chops off non-relevant indices and produces sane defaults. But then I would need to define the number of variables (length of the indexable)

mgilson Over a year ago

@RedBaron -- Thanks for the .indices method. I hadn't read about that one (although I was always certain that something like that must exist). I'm glad to know what it is now -- That's a gem.

RedBaron · Accepted Answer · 2013-04-15 16:50:45Z

I mucked around the code and found the following solution.

It uses lists but to only store the names of the variables - Not the actual values. Additionally it also provides the method add to add a new variable with a given name and value. The new variable will also be indexable. (The add function is not needed by my class, but is nice to have around)

Thanks to @mgilson for nudging me in this direction

class SearchError(object): 
    def __init__(self,severity=0,message=''): 
        self.severity = severity
        self.message = message
        self._length = 2
        self._vars_list = ['severity','message',]

    def __getitem__(self,val): 
        if isinstance(val,slice): 
            steps = val.indices(self._length)
            return [self.__getitem__(i) for i in xrange(*steps)]
        elif val < 0: 
            i = self._length + val
            if i < 0: 
                raise IndexError,"Index Out of range for SearchError object"
            else: 
                return self.__getitem__(i)    
        else: 
            try: 
                return getattr(self,self._vars_list[val])
            except IndexError: 
                raise IndexError,"Index Out of range for SearchError object"

    def add(self,var_name,value): 
        self._vars_list.append(var_name)
        self._length += 1
        setattr(self,var_name,value)

The results

>>> a=SearchError(1,"Fatal Error")
>>> a[-1]
'Fatal Error'
>>> a[-2]
1
>>> a[-3]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 14, in __getitem__
IndexError: Index Out of range for SearchError object
>>> a[2]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 21, in __getitem__
IndexError: Index Out of range for SearchError object
>>> a[1]
'Fatal Error'
>>> a[0]
1
>>> a[:]
[1, 'Fatal Error']
>>> a.add('new_severity',8)
>>> a[:]
[1, 'Fatal Error', 8]
>>> a[3]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 21, in __getitem__
IndexError: Index Out of range for SearchError object
>>> a[2]
8
>>> a.new_severity
8
>>> a[:3]
[1, 'Fatal Error', 8]
>>> a[:4]
[1, 'Fatal Error', 8]
>>> a[:2]
[1, 'Fatal Error']

As far as I can see, you need lists (to either store the actual variables or their names). If someone has a better alternative please do post

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