shelve.open of temporary file fails with error "anydbm.error: db type could not be determined"

[StephenLarroque]

BPO	31125
Nosy	@bitdancer

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2017-08-06.13:01:52.062>
created_at = <Date 2017-08-05.16:26:59.156>
labels = ['3.7', 'type-bug', 'library']
title = 'shelve.open of temporary file fails with error "anydbm.error: db type could not be determined"'
updated_at = <Date 2017-08-06.13:02:58.588>
user = 'https://bugs.python.org/StephenLarroque'

bugs.python.org fields:

activity = <Date 2017-08-06.13:02:58.588>
actor = 'r.david.murray'
assignee = 'none'
closed = True
closed_date = <Date 2017-08-06.13:01:52.062>
closer = 'r.david.murray'
components = ['Library (Lib)']
creation = <Date 2017-08-05.16:26:59.156>
creator = 'Stephen Larroque'
dependencies = []
files = []
hgrepos = []
issue_num = 31125
keywords = []
message_count = 3.0
messages = ['299790', '299797', '299798']
nosy_count = 2.0
nosy_names = ['r.david.murray', 'Stephen Larroque']
pr_nums = []
priority = 'normal'
resolution = 'rejected'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue31125'
versions = ['Python 2.7', 'Python 3.3', 'Python 3.4', 'Python 3.5', 'Python 3.6', 'Python 3.7']

[StephenLarroque]

This is a followup of bpo-23174.

When using shelve.open(tempfile.mkstemp()[1]), the shelve (or anydbm or dumbdbm) fail with "anydbm.error: db type could not be determined".

Instead, the module could detect the file is empty and use it as if it were not existing before ('c' or 'n' flag). For the moment, there is no way around except creating the temporary file and then delete it.

[bitdancer]

No, it should not. A DBM is not necessarily a single file. What you should be doing is creating a temporary *directory*, and opening your DB inside that.

[bitdancer]

By the way, if you want to open a doc issue with a suggestion of how to clarify this in the docs, that would be welcome.