I am trying to find the correct parameters to pass into the query function for the substrate interface module when interacting with the parallel api to interact with their AMM. I attempted to modify the code from the linked post:

How to determine which parameters are required when using substrate.query() to query a storage_function from polkascan's py-substrate-interface?

Below is my adapted code:

from substrateinterface import SubstrateInterface

paraurl = 'wss://rpc.parallel.fi'

substrate = SubstrateInterface(

storage_function = substrate.get_metadata_storage_function('AMM','Pools')

typeString = storage_function.get_params_type_string()
print('Storage Function params length:', len(typeString))

# Retrieve a SCALE object of the param type
param_type = substrate.create_scale_object(storage_function.get_params_type_string()[0])

# Possible options of Enum type

token_param = substrate.create_scale_object(param_type.type_mapping[0][1])

# Show possible options

However, I am receiving the error:

  File "/home/travnado300/Documents/Python Scripts/Substrate Interactions/Parallel.py", line 30, in <module>
    token_param = substrate.create_scale_object(param_type.type_mapping[0][1])
  TypeError: 'NoneType' object is not subscriptable

I am wondering if this is just an issue with the parallel API not having all of the information available or if there is a way to access the params needed to pass into the function:

  • You can check it manually with the PolkadotJS website. Or do you really need to figure this out dynamically? Aug 4, 2022 at 10:45
  • Can i check the required parameters there? I would like the code to be dynamic to pull the query on its own, but i do not need to check the parameters dynamically. Aug 4, 2022 at 20:59
  • Did you check the link? You can select Amm::pools there and see the signature of (u32, u32) -> Option<PalletTraitsPool>. Aug 5, 2022 at 10:07

2 Answers 2


You can use the storage_function.get_param_info() to get more information about the param types:

storage_function = substrate.get_metadata_storage_function('AMM', 'Pools')

param_info = storage_function.get_param_info()

# [{'primitive': 'u32'}, {'primitive': 'u32'}]

More information see: https://github.com/polkascan/py-substrate-interface#type-information-about-how-to-format-parameters


I am wondering if you are missing a step here, before using param_type. Possibly a decode() after create_scale_object, similar to https://github.com/polkascan/py-scale-codec/blob/master/README.md#encode-a-call

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