Add Maildir.get_flags() to access message flags without opening the file

[gildea]

BPO	46734
Nosy	@gildea
PRs	gh-90890: Add method mailbox.Maildir.get_flags #31302

^{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 = None
created_at = <Date 2022-02-12.22:21:50.052>
labels = ['type-feature', 'library', '3.11']
title = 'Add Maildir.get_flags() to access message flags without opening the file'
updated_at = <Date 2022-02-12.22:49:37.459>
user = 'https://github.com/gildea'

bugs.python.org fields:

activity = <Date 2022-02-12.22:49:37.459>
actor = 'gildea'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2022-02-12.22:21:50.052>
creator = 'gildea'
dependencies = []
files = []
hgrepos = []
issue_num = 46734
keywords = ['patch']
message_count = 1.0
messages = ['413145']
nosy_count = 1.0
nosy_names = ['gildea']
pr_nums = ['31302']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue46734'
versions = ['Python 3.11']

Linked PRs

• gh-90890: New methods to access mailbox.Maildir message info and flags #103905
• gh-90890: Reorder mailbox.Maildir method documentation #113071

[gildea]

A message's flags are stored in its filename by Maildir, so the flags
are available without reading the message file itself. The structured
message file name makes it efficient to scan a large mailbox to select
only messages that are, for example, not Trashed.

The mailbox.Maildir interface does not expose these flags, however. The
only way to access the flags through the mailbox library is to create a
mailbox.MaildirMessage object, which has a get_flags() method. But
creating a MaildirMessage requires opening the message file, which is slow.

I propose adding a parallel get_flags(key) method to mailbox.Maildir,
so that the flags are available without having to create a
MaildirMessage object.

In iterating through a mailbox with thousands of messages, I find that
this proposed Maildir.get_flags() method is 50 times faster than
MaildirMessage.get_flags().

[gildea]

Ping. This issue has a patch that needs reviewing

[gildea]

I have updated the patch based on initial review. Please take another look.