In Python, what is the underscore in front of the instance variable?

Question

What convention is it?

class IndexedText(object):
    def __init__(self, stemmer, text):
        self._text = text
        self._stemmer = stemmer
        self._index = nltk.Index((self._stem(word), i) for (i, word) in enumerate(text))

Answer 1

The _ signals that these are private members. It's not enforced by the language in any way, since Python programmers are all "consenting adults".

Answer 2

According to PEP 8:

In addition, the following special forms using leading or trailing underscores are recognized (these can generally be combined with any case convention):

• _single_leading_underscore: weak "internal use" indicator. E.g. from M import * does not import objects whose name starts with an underscore.

It doesn't actually refer to the use of a single underscore in a member of a class, but these tend to be used to imply "internal use".

For a stronger version of the same thing, use two leading underscores (e.g. self.__foo). Python will make a stronger attempt to prevent subclasses from accidentally overwriting the member, but determined code can of course still do so.

• __double_leading_underscore: when naming a class attribute, invokes name mangling (inside class FooBar, __boo becomes _FooBar__boo; see below).