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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"raise_from does not behave exactly like raise from","articleBody":"Here's a bit of sample code from [https://www.python.org/dev/peps/pep-3134/](PEP 3134):\n\n``` python\nclass DatabaseError(Exception):\n    pass\n\nclass FileDatabase(object):\n    def __init__(self, filename):\n        try:\n            self.file = open(filename)\n        except IOError as exc:\n            raise DatabaseError('failed to open') from exc\n\nfd = FileDatabase('dne')\n```\n\nThis, of course, does not work on Python 2. Using Python 3 I get the following output:\n\n```\nTraceback (most recent call last):\n  File \"pepex2.py\", line 7, in __init__\n    self.file = open(filename)\nFileNotFoundError: [Errno 2] No such file or directory: 'dne'\n\nThe above exception was the direct cause of the following exception:\n\nTraceback (most recent call last):\n  File \"pepex2.py\", line 11, in \u003cmodule\u003e\n    fd = FileDatabase('dne')\n  File \"pepex2.py\", line 9, in __init__\n    raise DatabaseError('failed to open') from exc\n__main__.DatabaseError: failed to open\n```\n\nWhen using `raise_from` for Python2 compatibility\n\n``` python\nfrom future.utils import raise_from\n# [...]\n            raise_from(DatabaseError('failed to open'), exc)\n```\n\nthis output (still with Python3) changes to:\n\n```\nFileNotFoundError: [Errno 2] No such file or directory\n\nThe above exception was the direct cause of the following exception:\n\nTraceback (most recent call last):\n  File \"pepex2.py\", line 13, in \u003cmodule\u003e\n    fd = FileDatabase('dne')\n  File \"pepex2.py\", line 11, in __init__\n    raise_from(DatabaseError('failed to open'), exc)\n  File \"/home/marc/.virtualenvs/scratch_future3/lib/python3.3/site-packages/future/utils/__init__.py\", line 382, in raise_from\n    exec(execstr, myglobals, mylocals)\n  File \"\u003cstring\u003e\", line 1, in \u003cmodule\u003e\n__main__.DatabaseError: failed to open\n```\n\nNote that the traceback and exception information are missing from the `FileNotFoundError`.\n","author":{"url":"https://github.com/mbr","@type":"Person","name":"mbr"},"datePublished":"2015-04-25T12:11:31.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":7},"url":"https://github.com/141/python-future/issues/141"}