Python global name not defined

Question

I'm trying to test the following simple object:

class WebCorpus(object):
    def __init__(self):
        _index = {}
        _graph = {}
        _ranks = {}
        _corpusChanged = False

    def lookup(self, keyword):
        if keyword in _index:
            return _index[keyword]
        return None
# (some irrelevant code)

With:

from WebCorpus import WebCorpus

def test_engine():
    print "Testing..."
    content = """This is a sample <a href="http://www.example.com">webpage</a> with 
    <a href="http://www.go.to">two links</a> that lead nowhere special."""
    outlinks = ["http://www.example.com", "http://www.go.to"]

    corpus = WebCorpus()
    assert corpus.lookup("anything") == None
#(some more code)
test_engine()

But it gives me an error: NameError: global name '_index' is not defined. I don't understand this, _index is clearly defined in the __init__ !? What is my mistake here? Help appreciated.

alecxe · Accepted Answer · 2013-07-13 21:56:18Z

4

In order to set class variables in the class method, you should use self:

class WebCorpus(object):
    def __init__(self):
        self._index = {}
        self._graph = {}
        self._ranks = {}
        self._corpusChanged = False

    def lookup(self, keyword):
        if keyword in self._index:
            return self._index[keyword]
        return None

Or, you can simplify the code and set variables like this (I've also simplified lookup method):

class WebCorpus(object):
    _index = {}
    _graph = {}
    _ranks = {}
    _corpusChanged = False

    def lookup(self, keyword):
        return self._index.get(keyword)

Note, the second example is not equivalent to the first one, because class-level variables are used, see comments below.

edited Jul 13, 2013 at 21:56

answered Jul 13, 2013 at 21:04

alecxe

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5 Comments

Sylvain Defresne Over a year ago

The two example are not equivalent, in the second case, all instance of WebCorpus will share the _index, _graph, and _ranks attributes as they are class attributes and not instance attributes.

alecxe Over a year ago

Agreed, but, keeping in mind that __init__.py doesn't accept any arguments, I think, this is what @ŞükrüHasdemir really wanted. Thank you.

Şükrü Hasdemir Over a year ago

Does that mean that once _index, for example, is set using one instance of WebCorpus, it will be the same for all instances @Sylvain?

Sylvain Defresne Over a year ago

@ŞükrüHasdemir: No, if you do corpus = WebCorpus() using the second implementation, then corpus._index = {"key": "value"} will override the dictionary for that specific instance. However, if you do c1, c2 = WebCorpus(), WebCorpus(), then c1._index["key"] = "value" will also change the content of c2._index (since they refer to the same object).

alecxe Over a year ago

Yes, _index will be shared between instances.

mattr555 · Accepted Answer · 2013-07-13 21:06:50Z

2

What's happening here is that it's defining _index but then losing it after the __init__ is run. You should append self to everything, so it's self._index, etc. This goes for the entire class, not just in the __init__.

answered Jul 13, 2013 at 21:06

mattr555

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