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imaplib - bug/regression on login command with 3.14.7 #155775

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imaplib - bug/regression on login command with 3.14.7#155775
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3.14bugs and security fixesstdlibStandard Library Python modules in the Lib/ directorytype-bugAn unexpected behavior, bug, or error

Description

@QHivert
QHivert
opened on Aug 14, 2026

Bug report

Bug description:

Hello!

I notice a change of behavior with imaplib login command starting with 3.14.7. It was working fine with 3.14.6 and previous version.

However, I'm not sure if what happens with 3.14.7 is the right way and a correction or not.

Problem

In imap, after a successful login command like this, the response also has the capabilities

$ telnet 127.0.0.1 143

Trying 127.0.0.1...
Connected to 127.0.0.1.
Escape character is '^]'.
* OK [CAPABILITY IMAP4rev1 SASL-IR LOGIN-REFERRALS ID ENABLE IDLE LITERAL+ STARTTLS AUTH=PLAIN AUTH=LOGIN AUTH=XOAUTH2] Dovecot ready.
1 LOGIN username password
1 OK [CAPABILITY IMAP4rev1 SASL-IR LOGIN-REFERRALS ID ENABLE IDLE SORT SORT=DISPLAY THREAD=REFERENCES THREAD=REFS THREAD=ORDEREDSUBJECT MULTIAPPEND URL-PARTIAL CATENATE UNSELECT CHILDREN NAMESPACE UIDPLUS LIST-EXTENDED I18NLEVEL=1 CONDSTORE QRESYNC ESEARCH ESORT SEARCHRES WITHIN CONTEXT=SEARCH LIST-STATUS BINARY MOVE SNIPPET=FUZZY PREVIEW=FUZZY PREVIEW STATUS=SIZE SAVEDATE LITERAL+ NOTIFY QUOTA ACL RIGHTS=texk] Logged in.

The way to get those capabilities with imaplib has changed in 3.14.7

Prior 3.14.7, (works in 3.14.6 and prior)

To get those capabilities, I have to do that:

# Assume conn is a proper imaplib.IMAP4 object
conn.login(user, password)
cap_ret = conn.response('CAPABILITY')
# cap_ret = ('CAPABILITY', [b'bla blou bli']) with bla blou bli the capabilities

With imaplib.Debug = 4, notice the answer in two parts

20:46.30 > b'NDMB1 LOGIN [username](mailto:username) "password"'
20:46.32 < b'* CAPABILITY IMAP4rev1 SASL-IR LOGIN-REFERRALS ID ENABLE IDLE SORT SORT=DISPLAY THREAD=REFERENCES THREAD=REFS THREAD=ORDEREDSUBJECT MULTIAPPEND URL-PARTIAL CATENATE UNSELECT CHILDREN NAMESPACE UIDPLUS LIST-EXTENDED I18NLEVEL=1 CONDSTORE QRESYNC ESEARCH ESORT SEARCHRES WITHIN CONTEXT=SEARCH LIST-STATUS BINARY MOVE SNIPPET=FUZZY PREVIEW=FUZZY PREVIEW STATUS=SIZE SAVEDATE LITERAL+ NOTIFY QUOTA ACL RIGHTS=texk'
20:46.32 < b'NDMB1 OK Logged in'

With 3.14.7

With imaplib.Debug = 4, notice that the answer is is one part

22:31.17 > b'EHKG0 LOGIN LOGIN [username](mailto:username) "password"'
22:31.19 < b'EHKG0 OK [CAPABILITY IMAP4rev1 SASL-IR LOGIN-REFERRALS ID ENABLE IDLE SORT SORT=DISPLAY THREAD=REFERENCES THREAD=REFS THREAD=ORDEREDSUBJECT MULTIAPPEND URL-PARTIAL CATENATE UNSELECT CHILDREN NAMESPACE UIDPLUS LIST-EXTENDED I18NLEVEL=1 CONDSTORE QRESYNC ESEARCH ESORT SEARCHRES WITHIN CONTEXT=SEARCH LIST-STATUS BINARY MOVE SNIPPET=FUZZY PREVIEW=FUZZY PREVIEW STATUS=SIZE SAVEDATE LITERAL+ NOTIFY QUOTA ACL RIGHTS=texk] Logged in'

And in python:

# Assume conn is a proper imaplib.IMAP4 object
conn.login(user, password)
cap_ret = conn.response('CAPABILITY')
# cap_ret is now = ('CAPABILITY', [None]) because there is no response "* CAPABILITY"

Now I have to parse the response of conn.login() to get the capabilities.

Conclusion

I'm not sure which of 3.14.7 or previous version is the expected way for that. However, I would have to implement the two methods if I want my app to work whatever the 3.14.x.

CPython versions tested on:

3.14

Operating systems tested on:

Linux

Linked PRs

Activity

  1. QHivert
    added
    type-bugAn unexpected behavior, bug, or error
    on Aug 14, 2026

  2. picnixz commented on Aug 14, 2026

    @picnixz
    picnixz
    on Aug 14, 2026
    Member

    Likely #152752. cc @serhiy-storchaka

  3. aisk
    added
    stdlibStandard Library Python modules in the Lib/ directory
    3.14bugs and security fixes
    on Aug 14, 2026

  4. serhiy-storchaka commented on Aug 14, 2026

    @serhiy-storchaka
    serhiy-storchaka
    on Aug 14, 2026
    Member

    Thank you for the report. This is a regression from gh-63121 (#152752), which refreshes the capability list after a successful LOGIN or AUTHENTICATE.

    Refreshing was intentional; consuming the response was not — _refresh_capabilities() does self.untagged_responses.pop('CAPABILITY'), so response('CAPABILITY') right after login() now returns [None].

    Use the capabilities attribute instead, which now reflects the post-login capabilities.

    #155789 restores the old behaviour.

  5. QHivert commented on Aug 14, 2026

    @QHivert
    QHivert
    on Aug 14, 2026
    Author

    Thanks for the quick response and fix !

  6. serhiy-storchaka
    added a commit that references this issue on Aug 29, 2026

    gh-155775: Do not consume the CAPABILITY response in imaplib (GH-155789)

    9111820
  7. serhiy-storchaka
    closed this as completedon Aug 29, 2026
  8. serhiy-storchaka
    added 2 commits that reference this issue on Aug 29, 2026

    [3.14] gh-155775: Do not consume the CAPABILITY response in imaplib (G…

    807e46c

    [3.13] gh-155775: Do not consume the CAPABILITY response in imaplib (G…

    afe72b4
  9. clin1234
    added a commit that references this issue on Sep 12, 2026

    pythongh-155775: Do not consume the CAPABILITY response in imaplib (p…

    ca24302
  10. serhiy-storchaka
    added a commit that references this issue on Oct 10, 2026

    [3.15] gh-155775: Do not consume the CAPABILITY response in imaplib (G…

    3c49483
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