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When we invoke an event as follows :

event MyEvent(uint256 indexed _id, bytes32 _action, bytes32 _player, bytes32 _actionState,  address _owner);

emit MyEvent(uint256(_public_id), bytes32(_action), bytes32(_player), bytes32(_actionState), _owner);

In EtherScan, we see an output as follows :

EtherScan Event Log

What does Topics [0] 0x22b8025a23f25c3f52cdbc1f53a831dbd4e3b4aaeebe121d36c858abea974bc7 mean ?

In case of MyEvent, [1] 0x0000000000000000000000000000000000000000000000000000000000000001 means _public_id = 1

However, I do not know 0x22b8025a23f25c3f52cdbc1f53a831dbd4e3b4aaeebe121d36c858abea974bc7 comes from where ? and in general, the term of Topics what does it mean ?

Questioner
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1 Answers1

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In a transaction receipt the fied topics correspond to your event arguments which are indexed in your smart contract. For all events you will find as first argument the hash of the event name, and then the data which are indexed in hexabytes.

So in your case, in topics you will find 

'topics':[hash_event_name,// w3.sha3(text='myEvent(uint256)').hex() where Event name without input argumentsname
         0x000000...000000001//your first argument '_id' because is indexed 
         ]

All of your event arguments which are not indexed are stored in the data part of the transaction receipt

Lbrth_BoC
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  • Are you sure about this part? sha('MyEvent(uint256')) – mesqueeb May 25 '18 at 08:29
  • With web3.py the correct thing is : event_signature_hash = w3.sha3(text='MyEvent(uint256)').hex() #Event name without input arguments – Lbrth_BoC May 25 '18 at 08:33
  • I'm trying to use web3js 1.0 to get the sha of my event: web3.utils.sha3('TokenSold(uint256 tokenId,uint256 oldPrice,uint256 newPrice,address prevOwner,address winner,string name)') This gave me the wrong hash... – mesqueeb May 25 '18 at 08:35
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    Yes because you input the name of your arguments inside the sha3 functions. So, you should get the correct hash with : web3.utils.sha3('TokenSold(uint256, uint256, uint256, address, address, string)'). Add only the type of your variable – Lbrth_BoC May 25 '18 at 08:38
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    Thanks so much! I got the correct hash with your solution but without the spaces! Cheers mate!! – mesqueeb May 25 '18 at 08:39
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    you welcome ! I remember than I spent a couple of hours to get the correct hash too – Lbrth_BoC May 25 '18 at 08:42