Why do are some events in a block missing an extrinsic_idx?

Question

Using py-substrate-interface to parse block data, we noticed that some events return None for the extrinsic_idx parameter.

Would someone please be so kind to explain why an event wouldn't have an extrinsic (idx) associated to it?

See below code and output for example:

Code:

from base64 import decode
from substrateinterface import SubstrateInterface

substrate = SubstrateInterface(
            url='wss://rpc.polkadot.io')

block_hash = substrate.get_block_hash(12449658)

test_block = substrate.get_block(block_hash=block_hash)

test_events = substrate.get_events(block_hash)

#visualising events in polkadot block 12449658
for idx,event in enumerate(test_events):
    decoded_event = event.decode()
    print("event_idx ",idx,
    "phase ",decoded_event['phase'],
    "module_id ",decoded_event['module_id'],
    "event_id ",decoded_event['event_id'],
    "extrinsic_idx", decoded_event['extrinsic_idx'])

Output:

event_idx  0 phase  Initialization module_id  ImOnline event_id  AllGood extrinsic_idx None
event_idx  1 phase  Initialization module_id  Staking event_id  EraPaid extrinsic_idx None
event_idx  2 phase  Initialization module_id  Balances event_id  Deposit extrinsic_idx None
event_idx  3 phase  Initialization module_id  Treasury event_id  Deposit extrinsic_idx None
event_idx  4 phase  Initialization module_id  Session event_id  NewSession extrinsic_idx None
event_idx  5 phase  ApplyExtrinsic module_id  System event_id  ExtrinsicSuccess extrinsic_idx 0
event_idx  6 phase  ApplyExtrinsic module_id  ParaInclusion event_id  CandidateIncluded extrinsic_idx 1
event_idx  7 phase  ApplyExtrinsic module_id  ParaInclusion event_id  CandidateIncluded extrinsic_idx 1
event_idx  8 phase  ApplyExtrinsic module_id  ParaInclusion event_id  CandidateIncluded extrinsic_idx 1
event_idx  9 phase  ApplyExtrinsic module_id  ParaInclusion event_id  CandidateIncluded extrinsic_idx 1
event_idx  10 phase  ApplyExtrinsic module_id  ParaInclusion event_id  CandidateIncluded extrinsic_idx 1
event_idx  11 phase  ApplyExtrinsic module_id  ParaInclusion event_id  CandidateIncluded extrinsic_idx 1
event_idx  12 phase  ApplyExtrinsic module_id  ParaInclusion event_id  CandidateIncluded extrinsic_idx 1
event_idx  13 phase  ApplyExtrinsic module_id  ParaInclusion event_id  CandidateIncluded extrinsic_idx 1
event_idx  14 phase  ApplyExtrinsic module_id  ParaInclusion event_id  CandidateIncluded extrinsic_idx 1
event_idx  15 phase  ApplyExtrinsic module_id  ParaInclusion event_id  CandidateIncluded extrinsic_idx 1
event_idx  16 phase  ApplyExtrinsic module_id  ParaInclusion event_id  CandidateBacked extrinsic_idx 1
event_idx  17 phase  ApplyExtrinsic module_id  ParaInclusion event_id  CandidateBacked extrinsic_idx 1
event_idx  18 phase  ApplyExtrinsic module_id  ParaInclusion event_id  CandidateBacked extrinsic_idx 1
event_idx  19 phase  ApplyExtrinsic module_id  ParaInclusion event_id  CandidateBacked extrinsic_idx 1
event_idx  20 phase  ApplyExtrinsic module_id  ParaInclusion event_id  CandidateBacked extrinsic_idx 1
event_idx  21 phase  ApplyExtrinsic module_id  ParaInclusion event_id  CandidateBacked extrinsic_idx 1
event_idx  22 phase  ApplyExtrinsic module_id  ParaInclusion event_id  CandidateBacked extrinsic_idx 1
event_idx  23 phase  ApplyExtrinsic module_id  ParaInclusion event_id  CandidateBacked extrinsic_idx 1
event_idx  24 phase  ApplyExtrinsic module_id  ParaInclusion event_id  CandidateBacked extrinsic_idx 1
event_idx  25 phase  ApplyExtrinsic module_id  ParaInclusion event_id  CandidateBacked extrinsic_idx 1
event_idx  26 phase  ApplyExtrinsic module_id  ParaInclusion event_id  CandidateBacked extrinsic_idx 1
event_idx  27 phase  ApplyExtrinsic module_id  ParaInclusion event_id  CandidateBacked extrinsic_idx 1
event_idx  28 phase  ApplyExtrinsic module_id  System event_id  ExtrinsicSuccess extrinsic_idx 1
event_idx  29 phase  Finalization module_id  Grandpa event_id  NewAuthorities extrinsic_idx None

Answer 1

Only the events in phase ApplyExtrinsic have an extrinsic index attached.
Everything that runs before is from on_initialize hooks and everything afterwards from on_finalize.
These always run in every block and cannot be triggered through an extrinsic, hence no index.