[Tutor] How to parse large files

[Danny Yoo]

> AttributeError                            Traceback (most recent call last)
> <ipython-input-3-f1c2a78eeb9a> in <module>()
> ----> 1 with shelve.open("diz5") as db:
>       2     with open("tmp1.txt") as instream:
>       3         for line in instream:
>       4             assert line.count("\t") == 1
>       5             key, _tab, value = line.rstrip("\n").partition("\t")
>
> AttributeError: DbfilenameShelf instance has no attribute '__exit__'


The error that you're seeing is on this line:

    with shelve.open("diz5") as db:

so we should focus our efforts to know why this line is failing.


The with statement in Python has a requirement, that the resource
supports context management.  Context managers have to have a few
methods, according to:

https://docs.python.org/3/reference/compound_stmts.html#the-with-statement

https://docs.python.org/3/reference/datamodel.html#context-managers


However, the error message reports that it can't find a method that
it's looking for, "__exit__".  it looks like shelves don't have the
methods "__enter__" or "__exit__", which context managers must have.

It looks like a deficiency in 'shelve', but one that we can work
around without too much difficulty.  We can use the
contextlib.closing() function to adapt a thing that knows how to
close(), so that it works as a context manager.
https://docs.python.org/3/library/contextlib.html#contextlib.closing

It should be a matter of saying:

    import contextlib
    ...
    with contextlib.closing(shelve.open("diz5")) as db: ...


If you have questions, please feel free to ask.