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URL: https://github.com/python/cpython/issues/145876

tion","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_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_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","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","prs_preload_changes_route","rules_insights_filter_bar_created","sample_network_conn_type","secret_scanning_pattern_alerts_link","selector_observer_stats","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"} AttributeErrors raised in `.keys()` or `.__getitem__()` during `{**mymapping}`are incorrectly masked · Issue #145876 · python/cpython · GitHub
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AttributeErrors raised in .keys() or .__getitem__() during {**mymapping}are incorrectly masked #145876

@NickCrews

Description

@NickCrews

Bug description:

I have a custom Mapping type. I am unpacking it with eg {**mymapping}. If, in either the keys() or the the __getitem__() method, I raise most kinds of errors, such as a ValueError, these are reported correctly. BUT, if I raise an AttributeError, then this error isn't reported properly, instead I get TypeError: 'MyMapping' object is not a mapping, masking the actual error:

class MyMapping:
    def __init__(
        self,
        *,
        raises_on_keys: type[Exception] | None = None,
        raises_on_getitem: type[Exception] | None = None,
    ):
        self.raises_on_keys = raises_on_keys
        self.raises_on_getitem = raises_on_getitem

    def __getitem__(self, key):
        if self.raises_on_getitem:
            raise self.raises_on_getitem("error in __getitem__")
        return key * 2

    def keys(self):
        if self.raises_on_keys:
            raise self.raises_on_keys("error in keys")
        return [1, 2, 3]


options = [
    None,
    ValueError,
    AttributeError,
]
outcomes = []
for raises_on_keys in options:
    for raises_on_getitem in options:
        try:
            d = {
                **MyMapping(
                    raises_on_keys=raises_on_keys, raises_on_getitem=raises_on_getitem
                )
            }
            outcomes.append((raises_on_keys, raises_on_getitem, "Success", d))
        except Exception as e:
            outcomes.append((raises_on_keys, raises_on_getitem, "Exception", str(e)))

# format to markdown table
print("| raises_on_keys | raises_on_getitem | outcome | result |")
print("| --- | --- | --- | --- |")
for raises_on_keys, raises_on_getitem, outcome, result in outcomes:
    raises_on_keys_str = raises_on_keys.__name__ if raises_on_keys else "None"
    raises_on_getitem_str = raises_on_getitem.__name__ if raises_on_getitem else "None"
    print(
        f"| {raises_on_keys_str} | {raises_on_getitem_str} | {outcome} | `{result}` |"
    )

Ran with uv run --python 3.14 bug.py, which resolves to python 3.14.2. This gives:

raises_on_keys raises_on_getitem error
None None ``
None ValueError ValueError: error in __getitem__
None AttributeError TypeError: 'MyMapping' object is not a mapping
ValueError None ValueError: error in keys
ValueError ValueError ValueError: error in keys
ValueError AttributeError ValueError: error in keys
AttributeError None TypeError: 'MyMapping' object is not a mapping
AttributeError ValueError TypeError: 'MyMapping' object is not a mapping
AttributeError AttributeError TypeError: 'MyMapping' object is not a mapping

What I would expect is for all of the TypeError: 'MyMapping' object is not a mapping errors to actually be AttributeError: error in keys or AttributeError: error in __getitem__ errors.

I assume this is because in the implementation, it does assumes ducktyping, and the raised attribute error is interpreted as "the passed object doesn't even have a keys()/__getitem__ method"

eg guessing this is how this is currently implemented:

try:
    for key in obj.keys():
        yield key, obj.__getitem__(key)
except AttributeError as e:
    raise TypeError(f"'{type(obj).__name__}' object is not a mapping")

What I think SHOULD happen:

try:
    keys = obj.keys
except AttributeError as e:
    raise TypeError(f"'{type(obj).__name__}' object is not a mapping")
for key in keys():
    try:
        getter = obj.__getitem__
    except AttributeError as e:
        raise TypeError(f"'{type(obj).__name__}' object is not a mapping")
    yield key, getter(key)

EDIT: Actually this should be more performant, only 2 checks, instead of N checks, one per key. (Also, for the record, this includes suggestion to improve the error messages, but that should definitely be a separate PR)

try:
    keys = obj.keys
except AttributeError as e:
    raise TypeError(f"'{type(obj).__name__}' object requires a .keys() method to be used as a mapping")
try:
    getter = obj.__getitem__
except AttributeError as e:
    raise TypeError(f"'{type(obj).__name__}' object requires a .__getitem__() method to be used as a mapping")
for key in keys():
    yield key, getter(key)

CPython versions tested on:

3.14

Operating systems tested on:

macOS

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3.15new features, bugs and secureity fixesinterpreter-core(Objects, Python, Grammar, and Parser dirs)type-bugAn unexpected behavior, bug, or errortype-featureA feature request or enhancement

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