[issue23174] shelve.open fails with error "anydbm.error: db type could not be determined"

[Raymond Hettinger]

Raymond Hettinger added the comment:

[Serhiy]
> Current error message looks good to me and I don't see how
> it can be enhanced.

I concur with Serhiy and David.

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resolution:  -> not a bug
stage:  -> resolved
status: open -> closed

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[issue23174] shelve.open fails with error "anydbm.error: db type could not be determined"

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

Proposed error message isn't correct. The file extension is not the only method 
for determining db type.

Current error message looks good to me and I don't see how it can be enhanced.

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[issue23174] shelve.open fails with error "anydbm.error: db type could not be determined"

[Anand Reddy Pandikunta]

Anand Reddy Pandikunta added the comment:

dbm.whichdb mentions that it returns empty string ('') if the file’s format 
can’t be guessed. Enhancing exception message should suffice.

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[issue23174] shelve.open fails with error "anydbm.error: db type could not be determined"

[Roundup Robot]

Changes by Roundup Robot :


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[issue23174] shelve.open fails with error anydbm.error: db type could not be determined

[Karl Richter]

Karl Richter added the comment:

For example, it should be clear why `shelve.open(tempfile.mkstemp()[1])` fails 
with the mentioned exception and `shelve.open(/tmp/bla)` fails. I still 
haven't figured out the constraints to create a working `shelve.Shelve` at all. 
It should be clear why `shelve.open(/tmp/tmphTTQLd)` fails and 
`shelve.open(/tmp/tmphTTQLda)` succeeds. There has to be something unrelated 
to extensions.

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[issue23174] shelve.open fails with error anydbm.error: db type could not be determined

[Karl Richter]

Karl Richter added the comment:

After checking the code, I think that it'd make more sense to document 
`whichdb.py`. It needs to be enhanced with references to criteria for the 
determination of the database type. Currently there're only function comments 
and the fact that some variables are named magic speaks for itself, i.e. it's 
ok for the implementation of the module to be a black box, but the requirements 
for the input, i.e. the potential database file, which is per se not part of 
the black box, needs to be specified. Then just link that in the `shelve` docs 
stating that `shelve.open` is basically a wrapper around `whichdb`.

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[issue23174] shelve.open fails with error anydbm.error: db type could not be determined

[Karl Richter]

Karl Richter added the comment:

That's nice, thanks. Considering your last comment, some points

  * If the issue can't go into the error message, than the essence of the 
discussion here should go into the docs (in 0.5 to 1.5 sentences).
  * It'd be nice if it was clear from the error message that shelve or 
underlying modules check the extension in filename, i.e. do `anydbm.error: db 
type could not be determined by file extension '%s' in filename '%s'` rather 
than `anydbm.error: db type could not be determined`.

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[issue23174] shelve.open fails with error anydbm.error: db type could not be determined

[R. David Murray]

R. David Murray added the comment:

The problem is that this is not the only situation in which that error can be 
generated.  If someone passed in an old (and possibly non-empty) file with no 
extension, the same code path would get executed.

The shelve/anydbm API is database agnostic, so there is no way to know a-priori 
what the valid extensions are.  The whichdbm module (which anydbm uses) could 
provide this information in theory, but it doesn't currently, and if it did 
that would be a new API.

Your problem is that the shelve API is designed so that you either pass it the 
base name of an existing database, *or* you pass it the base name of a *non* 
existent database, which it then creates all the right files for (there may be 
more than one file).  The fact that you are passing it a new existing file is 
the problem, and I'm not sure the error message can be effectively improved to 
handle that particular mistaken use of the API without breaking the message for 
the places it is more meaningful.

The best I can think of is to add the filename to the error message and list 
all the extensions and (if there are any) the 'magic' type guesses that 
whichdbm tried, but as I said that would be a new feature and thus could only 
go into 3.5.

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[issue23174] shelve.open fails with error anydbm.error: db type could not be determined

[R. David Murray]

R. David Murray added the comment:

What is it you want the docs to say?

For your second point, yes, that is what I was saying would be an enhancement 
(the extension is a list of possible extensions that varies per-platform and 
per how python is built/installed).

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[issue23174] shelve.open fails with error anydbm.error: db type could not be determined

[Karl Richter]

Karl Richter added the comment:

Then, let the error message say You are opening a just-created empty file.  
The db type of the file cannot, therefore, be determined. which is much 
clearer than anydbm.error: db type could not be determined which sounds like 
a generic fallback error message in an error occured-style.

It seems to be necessary that the filename passed to `shelve.open` has a suffix 
which is not very intuitive for Linux users. It'd be great to have validation 
of the suffix and raise a `ValueError` with an error message like `filename 
'%s' doesn't contain a suffix % (filename,)` when it isn't supplied. If 
there's another issue regarding the fact that the file is just-created and 
empty, this is independent of the issue above.

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[issue23174] shelve.open fails with error anydbm.error: db type could not be determined

[Karl Richter]

New submission from Karl Richter:

`shelve.open(tempfile.mkstemp()[1])` fails with error anydbm.error: db type 
could not be determined which is not explainable with the docs. Traceback is

Traceback (most recent call last):
  File ./cudaminer_param_checker.py, line 720, in module
plac.call(cudaminer_param_checker)
  File /usr/local/lib/python2.7/dist-packages/plac_core.py, line 309, in 
call
cmd, result = parser_from(obj).consume(arglist)
  File /usr/local/lib/python2.7/dist-packages/plac_core.py, line 195, in 
consume
return cmd, self.func(*(args + varargs + extraopts), **kwargs)
  File ./cudaminer_param_checker.py, line 715, in cudaminer_param_checker
visualize_cudaminer_param_checker_results_wxpython_gui()
  File ./cudaminer_param_checker.py, line 365, in 
visualize_cudaminer_param_checker_results_wxpython_gui
frame = CudaminerParamChecker(None, )
  File ./cudaminer_param_checker.py, line 378, in __init__
self.generator = CudaminerParamCheckerGenerator()
  File ./cudaminer_param_checker.py, line 160, in __init__
self.result_dict= shelve.open(storage_file_path) 
  File /usr/lib/python2.7/shelve.py, line 239, in open
return DbfilenameShelf(filename, flag, protocol, writeback)
  File /usr/lib/python2.7/shelve.py, line 223, in __init__
Shelf.__init__(self, anydbm.open(filename, flag), protocol, writeback)
  File /usr/lib/python2.7/anydbm.py, line 82, in open
raise error, db type could not be determined
anydbm.error: db type could not be determined

--
assignee: docs@python
components: Documentation
messages: 233488
nosy: docs@python, krichter
priority: normal
severity: normal
status: open
title: shelve.open fails with error anydbm.error: db type could not be 
determined
type: behavior
versions: Python 2.7

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[issue23174] shelve.open fails with error anydbm.error: db type could not be determined

[R. David Murray]

R. David Murray added the comment:

You are opening a just-created empty file.  The db type of the file cannot, 
therefore, be determined.  Which is what the error message says. 

How would you suggest the error message be improved?

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[issue23174] shelve.open fails with error anydbm.error: db type could not be determined

[Karl Richter]

Karl Richter added the comment:

EDIT 1: other examples, e.g.

import os
import shelve

curdir = os.path.dirname(__file__)
passwords = shelve.open(os.path.join(curdir, 'password_db'))

work, so there's need for usable error messages.

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[issue23174] shelve.open fails with error anydbm.error: db type could not be determined

[R. David Murray]

R. David Murray added the comment:

Sorry, I mean an empty file with no recognized extension.  I doubt supplying 
a suffix is what you want, though, since you probably want shelve to pick the 
persistent backend on db creation in order to be portable.

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