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PEP 836 Authors Withdraw Their CPython JIT Proposal, and Brett Cannon Says the Withdrawal Means the JIT Will Come Out of main

The authors of PEP 836 withdrew it on October 19; Brett Cannon wrote that the JIT work stops in CPython's repo, after the Steering Council's June six-month condition for keeping it in main.

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Overview

PEP 836, the proposal that was meant to justify keeping CPython’s experimental JIT compiler in the main branch, has been withdrawn by its authors. Savannah Ostrowski wrote on October 19 in the PEP’s discussion thread: “After further discussion, we have decided to withdraw PEP 836.” The PEP page now lists its status as Withdrawn with a resolution date of 19-Oct-2026. The Steering Council had said in June that the JIT code must come out of the main branch if no such PEP was accepted within six months, as previously reported.

What We Know

  • The Council’s condition. In its June 5 announcement, the Steering Council wrote: “We are setting a window of six months for a PEP to be submitted and resolved. If no such PEP is accepted within that window, the JIT code must be removed from the main branch and development must be continued outside the main Python repository.” In the same thread, Pablo Galindo Salgado wrote that the deadline was “a first goal to anchor, not a bureaucratic deadline we will hold.”
  • What the PEP proposed. According to the PEP, the JIT has been in the main branch since Python 3.13, and the PEP cites “about 4-12% geometric mean performance improvement across measured Tier 1 platforms” in Python 3.15. It set a target of at least 20% geometric mean improvement on pyperformance for the JIT plus free-threaded build by the first beta of Python 3.17, described as “a minimum bar for continued in-tree development of the JIT.”
  • Council feedback in September. On September 8, Barry Warsaw wrote in the discussion thread that “the Steering Council has not been formally asked to pronounce on PEP 836.” On September 21, Thomas Wouters posted feedback “for the Steering Council, minus Savannah as PEP co-author”, saying “We’re not evaluating, at this point, whether the JIT should be declared a supported feature.” The post asked for more detail on points including memory impact and security, and said a planned compatibility review in the last half year of the timeline was “way, way too late.” It concluded: “Bottom line, the PEP mostly answers the questions we asked, and there are no significant missing pieces… but we do hope for a bit more discussion on a few topics so we can gauge community support and consensus before we’re asked to pronounce on this PEP.” Ken Jin replied on September 25 that the authors would “work on it.”
  • The withdrawal. Ostrowski’s October 19 post said: “The Steering Council rightly sets a high bar for a proposal of this scope, and we don’t feel we have the personal bandwidth to carry the current version forward.” She added that the authors “continue to believe in the technical direction outlined in the PEP, including free-threaded support and a method-based frontend”, and said: “We’ll make the reference implementation available and link it from the withdrawn PEP so others who are interested can explore and build on that.”
  • What follows, per one participant. Asked by Barry Scott whether the JIT work is stopped, Brett Cannon replied in the thread: “In CPython’s repo, yes. Whether others in the community organize to keep it going is up to the community.” He added: “The point of the PEP was to justify the JIT work staying in main, so the withdrawal of the PEP means it will come out and live somewhere else”. When another participant asked whether an alternative PEP could still be submitted, Cannon answered that this was “Not really as all the people actively driving the JIT were either authors on that PEP or contributors to it,” and said the decision “was made at the core dev sprints where most of the people working on the JIT currently are.”

What We Don’t Know

  • Council statement. The discussion thread, as of its last post on October 21, contained no new post from the Steering Council responding to the withdrawal. Whether the Council will formally act, or when the JIT code would be removed from main, is not stated in the sources reviewed.
  • Reasons for the decision. On October 21, one participant asked for “insight into what were the deciding factors for the decision to abandon this effort (at least in its current form).” The thread contained no reply to that request. Ostrowski’s post cites the Council’s bar and the authors’ bandwidth.
  • Future of the code. The authors say the reference implementation will be linked from the withdrawn PEP. Whether a community effort outside the CPython repository will continue the work is, in Cannon’s words, “up to the community.”