How to imitate Python 3’s raise … from in Python 2?

There’s a raise_from in python-future; simply install it

pip install future

and import to use

from future.utils import raise_from
# or: from six import reraise as raise_from

class FileDatabase:
    def __init__(self, filename):
        try:
            self.file = open(filename)
        except IOError as exc:
            raise_from(DatabaseError('failed to open'), exc)

UPDATE

The compatibility package six also supports raise_from, from version 1.9 (released in 2015). It is used in the same manner as above.