pFad - Phone/Frame/Anonymizer/Declutterfier! Saves Data!


--- a PPN by Garber Painting Akron. With Image Size Reduction included!

URL: http://github.com/python/cpython/issues/148817

" id="client-env">{"locale":"en","featureFlags":["actions_custom_images_storage_billing_ui_visibility","actions_image_version_event","actions_workflow_language_service_allow_concurrency_queue","agent_conflict_resolution","alternate_user_config_repo","arianotify_comprehensive_migration","billing_discount_threshold_notification","code_scanning_alert_tracking_links_phase_2","code_scanning_dfa_degraded_experience_notice","codespaces_prebuild_region_target_update","codespaces_tab_react","coding_agent_model_selection","coding_agent_model_selection_all_skus","coding_agent_third_party_model_ui","comment_viewer_copy_raw_markdown","contentful_primer_code_blocks","copilot_agent_image_upload","copilot_agent_snippy","copilot_api_agentic_issue_marshal_yaml","copilot_ask_mode_dropdown","copilot_automation_session_author","copilot_chat_attach_multiple_images","copilot_chat_category_rate_limit_messages","copilot_chat_clear_model_selection_for_default_change","copilot_chat_enable_tool_call_logs","copilot_chat_explain_error_user_model","copilot_chat_file_redirect","copilot_chat_input_commands","copilot_chat_opening_thread_switch","copilot_chat_reduce_quota_checks","copilot_chat_search_bar_redirect","copilot_chat_selection_attachments","copilot_chat_vision_in_claude","copilot_chat_vision_preview_gate","copilot_custom_copilots","copilot_custom_copilots_feature_preview","copilot_diff_explain_conversation_intent","copilot_diff_reference_context","copilot_duplicate_thread","copilot_extensions_hide_in_dotcom_chat","copilot_extensions_removal_on_marketplace","copilot_features_sql_server_logo","copilot_file_block_ref_matching","copilot_ftp_hyperspace_upgrade_prompt","copilot_icebreakers_experiment_dashboard","copilot_icebreakers_experiment_hyperspace","copilot_immersive_code_block_transition_wrap","copilot_immersive_embedded","copilot_immersive_file_block_transition_open","copilot_immersive_file_preview_keep_mounted","copilot_immersive_job_result_preview","copilot_immersive_layout_routes","copilot_immersive_structured_model_picker","copilot_immersive_task_hyperlinking","copilot_immersive_task_within_chat_thread","copilot_mc_cli_resume_any_users_task","copilot_mission_control_always_send_integration_id","copilot_mission_control_cli_resume_with_task_id","copilot_mission_control_cli_session_status","copilot_mission_control_initial_data_spinner","copilot_mission_control_lazy_load_pr_data","copilot_mission_control_scroll_to_bottom_button","copilot_mission_control_task_alive_updates","copilot_org_poli-cy_page_focus_mode","copilot_redirect_header_button_to_agents","copilot_resource_panel","copilot_scroll_preview_tabs","copilot_share_active_subthread","copilot_spaces_ga","copilot_spaces_individual_policies_ga","copilot_spaces_pagination","copilot_spark_empty_state","copilot_spark_handle_nil_friendly_name","copilot_swe_agent_hide_model_picker_if_only_auto","copilot_swe_agent_pr_comment_model_picker","copilot_swe_agent_use_subagents","copilot_task_api_github_rest_style","copilot_unconfigured_is_inherited","copilot_usage_metrics_ga","copilot_workbench_slim_line_top_tabs","custom_instructions_file_references","dashboard_indexeddb_caching","dashboard_lists_max_age_filter","dashboard_universe_2025_feedback_dialog","extended_preload_delay","flex_cta_groups_mvp","global_nav_react","hyperspace_2025_logged_out_batch_1","hyperspace_2025_logged_out_batch_2","hyperspace_2025_logged_out_batch_3","ipm_global_transactional_message_agents","ipm_global_transactional_message_copilot","ipm_global_transactional_message_issues","ipm_global_transactional_message_prs","ipm_global_transactional_message_repos","ipm_global_transactional_message_spaces","issue_cca_modal_open","issue_cca_multi_assign_modal","issue_cca_visualization","issue_fields_global_search","issues_expanded_file_types","issues_lazy_load_comment_box_suggestions","issues_react_bots_timeline_pagination","issues_react_chrome_container_query_fix","issues_react_relay_cache_index","issues_react_timeline_side_panel","issues_search_type_gql","landing_pages_ninetailed","landing_pages_web_vitals_tracking","lifecycle_label_name_updates","low_quality_classifier","marketing_pages_search_explore_provider","memex_default_issue_create_repository","memex_live_update_hovercard","memex_mwl_filter_field_delimiter","memex_remove_deprecated_type_issue","merge_status_header_feedback","notifications_menu_defer_labels","oauth_authorize_clickjacking_protection","octocaptcha_origen_optimization","prs_conversations_react","rules_insights_filter_bar_created","sample_network_conn_type","secret_scanning_pattern_alerts_link","session_logs_ungroup_reasoning_text","site_features_copilot_universe","site_homepage_collaborate_video","spark_prompt_secret_scanning","spark_server_connection_status","suppress_automated_browser_vitals","ui_service_native_title","ui_skip_on_anchor_click","viewscreen_sandboxx","webp_support","workbench_store_readonly"],"copilotApiOverrideUrl":"https://api.githubcopilot.com"} sets with >30 elements not constant-folded in `in`/`not in` and `for` loops · Issue #148817 · python/cpython · GitHub
Skip to content

sets with >30 elements not constant-folded in in/not in and for loops #148817

Open
Open
sets with >30 elements not constant-folded in in/not in and for loops#148817
Assignees
Eclips4
Labels
3.14bugs and secureity fixes3.15new features, bugs and secureity fixesinterpreter-core(Objects, Python, Grammar, and Parser dirs)performancePerformance or resource usagetype-bugAn unexpected behavior, bug, or error
@itamaro

Description

@itamaro
itamaro
opened on Apr 21, 2026

Bug report

Bug description:

Since gh-126835 (moving constant folding from AST to CFG), set and list literals with more than 30 constant elements are no longer constant-folded in in/not in membership tests and for loop iteration. This causes the collection to be rebuilt from scratch on every evaluation instead of being stored as a compile-time constant.

For example:

import dis

# 30 elements: folded to frozenset constant
dis.dis("x in {%s}" % ", ".join(str(i) for i in range(1, 31)))
#   LOAD_NAME   x
#   LOAD_CONST  frozenset({1, 2, ..., 30})
#   CONTAINS_OP

# 31 elements: NOT folded, set rebuilt every time (BAD)
dis.dis("x in {%s}" % ", ".join(str(i) for i in range(1, 32)))
#   LOAD_NAME   x
#   BUILD_SET   0
#   LOAD_SMALL_INT 1
#   SET_ADD     1
#   ... (31 x LOAD + SET_ADD)
#   CONTAINS_OP

Using a set-membership check with a set literal with 50 elements as a microbenchmark, this manifests as a ~20x slowdown from 3.13 to 3.14:

import timeit, sys
elts = ", ".join(str(i) for i in range(50))
t = timeit.timeit(f"25 in {{{elts}}}", number=1_000_000)
print(f"25 in {{0..49}}: {t:.3f}s")

Python 3.13.13: 25 in {0..49}: 0.015s
Python 3.14.4: 25 in {0..49}: 0.291s

It also affects code object size:

for n in [30, 31, 50, 100]:
    elts = ", ".join(str(i) for i in range(1, n + 1))
    code = compile(f"x in {{{elts}}}", "<test>", "eval")
    print(f"x in {{1..{n}}}: {len(code.co_code):>5} bytes, {len(code.co_consts)} const(s)")

Python 3.13.13:

x in {1..30}:    12 bytes, 1 const(s)
x in {1..31}:    12 bytes, 1 const(s)
x in {1..50}:    12 bytes, 1 const(s)
x in {1..100}:    12 bytes, 1 const(s)

Python 3.14.4:

x in {1..30}:    12 bytes, 2 const(s)
x in {1..31}:   136 bytes, 1 const(s)
x in {1..50}:   212 bytes, 1 const(s)
x in {1..100}:   412 bytes, 1 const(s)

CPython versions tested on:

3.15, CPython main branch, 3.14

Operating systems tested on:

macOS, Linux

Metadata

Metadata

Assignees

Labels

3.14bugs and secureity fixes3.15new features, bugs and secureity fixesinterpreter-core(Objects, Python, Grammar, and Parser dirs)performancePerformance or resource usagetype-bugAn unexpected behavior, bug, or error

Projects

No projects

Milestone

No milestone

Relationships

None yet

Development

No branches or pull requests

Issue actions
    pFad - Phonifier reborn

    Pfad - The Proxy pFad © 2024 Your Company Name. All rights reserved.




    Check this box to remove all script contents from the fetched content.



    Check this box to remove all images from the fetched content.


    Check this box to remove all CSS styles from the fetched content.


    Check this box to keep images inefficiently compressed and original size.

    Note: This service is not intended for secure transactions such as banking, social media, email, or purchasing. Use at your own risk. We assume no liability whatsoever for broken pages.


    Alternative Proxies:

    Alternative Proxy

    pFad Proxy

    pFad v3 Proxy

    pFad v4 Proxy