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Building an NFT indexer for Python

The article is written for Python programming language. Concepts, advices, and examples might differ from other programming languages
Source code for the tutorial

frolvanya/near-lake-nft-indexer: source code for this tutorial

The End

This tutorial ends with a working NFT indexer built on top NEAR Lake Framework for Python. The indexer is watching for nft_mint Events and prints some relevant data:

  • receipt_id of the Receipt where the mint has happened
  • Marketplace
  • NFT owner account name
  • Links to the NFTs on the marketplaces

The final source code is available on the GitHub frolvanya/near-lake-nft-indexer

Motivation

NEAR Protocol had introduced a nice feature Events. The Events allow a contract developer to add standardized logs to the ExecutionOutcomes thus allowing themselves or other developers to read those logs in more convenient manner via API or indexers.

The Events have a field standard which aligns with NEPs. In this tutorial we'll be talking about NEP-171 Non-Fungible Token standard.

In this tutorial our goal is to show you how you can "listen" to the Events contracts emit and how you can benefit from them.

As the example we will be building an indexer that watches all the NFTs minted following the NEP-171 Events standard, assuming we're collectors who don't want to miss a thing. Our indexer should notice every single NFT minted and give us a basic set of data like: in what Receipt it was minted, and show us the link to a marketplace (we'll cover Paras and Mintbase in our example).

We will use Python version of NEAR Lake Framework in this tutorial. Though the concept is the same for Rust, but we want to show more people that it's not that complex to build your own indexer.

Preparation

Credentials

Please, ensure you've the credentials set up as described on the Credentials page. Otherwise you won't be able to get the code working.

Let's create our project folder

mkdir lake-nft-indexer && cd lake-nft-indexer
touch main.py

Set up NEAR Lake Framework

In the main.py let's import stream function and near_primitives from near-lake-framework:

main.py
from near_lake_framework import near_primitives, LakeConfig, streamer

Add the main function

main.py
async def main():
print("Starting NFT indexer")

Add the instantiation of LakeConfig below:

main.py
config = LakeConfig.mainnet()
config.start_block_height = 69030747
config.aws_access_key_id = os.getenv("AWS_ACCESS_KEY_ID")
config.aws_secret_key = os.getenv("AWS_SECRET_ACCESS_KEY")

Just a few words on the config, function mainnet() has set s3_bucket_name, s3_region_name for mainnet. You can go to NEAR Explorer and get the most recent block height to set config.start_block_height.

Let's call streamer function with the config

main.py
stream_handle, streamer_messages_queue = streamer(config)
while True:
streamer_message = await streamer_messages_queue.get()

And an actual start of our indexer in the end of the main.py

main.py
loop = asyncio.get_event_loop()
loop.run_until_complete(main())

The final main.py at this moment should look like the following:

main.py
from near_lake_framework import LakeConfig, streamer, near_primitives
async def main():
print("Starting NFT indexer")

config = LakeConfig.mainnet()
config.start_block_height = 69030747
config.aws_access_key_id = os.getenv("AWS_ACCESS_KEY_ID")
config.aws_secret_key = os.getenv("AWS_SECRET_ACCESS_KEY")

stream_handle, streamer_messages_queue = streamer(config)
while True:
streamer_message = await streamer_messages_queue.get()

loop = asyncio.get_event_loop()
loop.run_until_complete(main())

Now we need to create a callback function that we'll be called to handle StreamerMessage our indexer receives.

main.py
async def handle_streamer_message(streamer_message: near_primitives.StreamerMessage):
pass

Events and where to catch them

First of all let's find out where we can catch the Events. We hope you are familiar with how the Data Flow in NEAR Blockchain, but let's revise our knowledge:

  • Mint an NFT is an action in an NFT contract (doesn't matter which one)
  • Actions are located in a Receipt
  • A result of the Receipt execution is ExecutionOutcome
  • ExecutionOutcome in turn, catches the logs a contract "prints"
  • Events built on top of the logs

This leads us to the realization that we can watch only for ExecutionOutcomes and ignore everything else StreamerMessage brings us.

Catching only the data we need

Inside the callback function handle_streamer_message we've prepared in the Preparation section let's start filtering the data we need:

main.py
async def handle_streamer_message(streamer_message: near_primitives.StreamerMessage):
for shard in streamer_message.shards:
for receipt_execution_outcome in shard.receipt_execution_outcomes:
for log in receipt_execution_outcome.execution_outcome.outcome.logs:
pass

We have iterated through the logs of all ExecutionOutcomes of Shards (in our case we don't care on which Shard did the mint happen)

Now we want to deal only with those ExecutionOutcomes that contain logs of Events format. Such logs start with EVENT_JSON: according to the Events docs.

main.py
async def handle_streamer_message(streamer_message: near_primitives.StreamerMessage):
for shard in streamer_message.shards:
for receipt_execution_outcome in shard.receipt_execution_outcomes:
for log in receipt_execution_outcome.execution_outcome.outcome.logs:
if not log.startswith("EVENT_JSON:"):
continue
try:
parsed_log = json.loads(log[len("EVENT_JSON:") :])
except json.JSONDecodeError:
print(
f"Receipt ID: `{receipt_execution_outcome.receipt.receipt_id}`\nError during parsing logs from JSON string to dict"
)
continue

Let us explain what we are doing here:

  1. We are walking through the logs in ExecutionOutcomes
  2. We are filtering ExecutionOutcomes that contain logs of Events format
  3. In order to collect the Events we are iterating through the ExecutionOutcome's logs trying to parse Event using json.loads

The goal for our indexer is to return the useful data about a minted NFT that follows NEP-171 standard. We need to drop irrelevant standard Events:

main.py
    if (
parsed_log.get("standard") != "nep171"
or parsed_log.get("event") != "nft_mint"
):
continue

Almost done

So far we have collected everything we need corresponding to our requirements.

The final look of the handle_streamer_message function:

main.py
async def handle_streamer_message(streamer_message: near_primitives.StreamerMessage):
for shard in streamer_message.shards:
for receipt_execution_outcome in shard.receipt_execution_outcomes:
for log in receipt_execution_outcome.execution_outcome.outcome.logs:
if not log.startswith("EVENT_JSON:"):
continue
try:
parsed_log = json.loads(log[len("EVENT_JSON:") :])
except json.JSONDecodeError:
print(
f"Receipt ID: `{receipt_execution_outcome.receipt.receipt_id}`\nError during parsing logs from JSON string to dict"
)
continue

if (
parsed_log.get("standard") != "nep171"
or parsed_log.get("event") != "nft_mint"
):
continue

print(parsed_log)

Now let's call handle_streamer_message inside the loop in main function

main.py
await handle_streamer_message(streamer_message)

And if we run our indexer we will be catching nft_mint event and print logs in the terminal.

python3 main.py
note

Having troubles running the indexer? Please, check you haven't skipped the Credentials part :)

Not so fast! Remember we were talking about having the links to the marketplaces to see the minted tokens? We're gonna extend our data with links whenever possible. At least we're gonna show you how to deal with the NFTs minted on Paras and Mintbase.

At this moment we have an access to logs that follows NEP-171 standard. We definitely know that all the data we have at this moment are relevant for us, and we want to extend it with the links to that minted NFTs at least for those marketplaces we know.

We know and love Paras and Mintbase.

Paras token URL

We did the research for you and here's how the URL to token on Paras is crafting:

https://paras.id/token/[1]::[2]/[3]

Where:

  • [1] - Paras contract address (x.paras.near)
  • [2] - First part of the token_id (parsed_log["data"] for Paras is an array of objects with token_ids key in it. Those IDs represented by numbers with column : between them)
  • [3] - token_id itself

Example:

https://paras.id/token/x.paras.near::387427/387427:373

Mintbase token URL

And again we did the research for you:

https://www.mintbase.io/thing/[1]:[2]

Where:

  • [1] - meta_id (parsed_log["data"] for Mintbase is an array of stringified JSON that contains meta_id)
  • [2] - Store contract account address (basically Receipt's receiver ID)

Example:

https://www.mintbase.io/thing/70eES-icwSw9iPIkUluMHOV055pKTTgQgTiXtwy3Xus:vnartistsdao.mintbase1.near

Let's start crafting the links:

main.py
def format_paras_nfts(data, receipt_execution_outcome):
links = []

for data_element in data:
for token_id in data_element.get("token_ids", []):
first_part_of_token_id = token_id.split(":")[0]
links.append(
f"https://paras.id/token/{receipt_execution_outcome.receipt.receiver_id}::{first_part_of_token_id}/{token_id}"
)

return {"owner": data[0].get("owner_id"), "links": links}

def format_mintbase_nfts(data, receipt_execution_outcome):
links = []
for data_block in data:
try:
memo = json.loads(data_block.get("memo"))
except json.JSONDecodeError:
print(
f"Receipt ID: `{receipt_execution_outcome.receipt.receipt_id}`\nMemo: `{memo}`\nError during parsing Mintbase memo from JSON string to dict"
)
return

meta_id = memo.get("meta_id")
links.append(
f"https://www.mintbase.io/thing/{meta_id}:{receipt_execution_outcome.receipt.receiver_id}"
)

return {"owner": data[0].get("owner_id"), "links": links}

We're going to print the receipt_id, so you would be able to search for it on NEAR Explorer, marketplace name and the list of links to the NFTs along with the owner account name.

main.py
if receipt_execution_outcome.receipt.receiver_id.endswith(
".paras.near"
):
output = {
"receipt_id": receipt_execution_outcome.receipt.receipt_id,
"marketplace": "Paras",
"nfts": format_paras_nfts(
parsed_log["data"], receipt_execution_outcome
),
}

A few words about what is going on here. If the Receipt's receiver account name ends with .paras.near (e.g. x.paras.near) we assume it's from Paras marketplace, so we are changing the corresponding variable.

Mintbase turn, we hope Nate and his team have migrated to NEAR Lake Framework already, saying "Hi!" and crafting the link:

main.py
    elif re.search(
".mintbase\d+.near", receipt_execution_outcome.receipt.receiver_id
):
output = {
"receipt_id": receipt_execution_outcome.receipt.receipt_id,
"marketplace": "Mintbase",
"nfts": format_mintbase_nfts(
parsed_log["data"], receipt_execution_outcome
),
}
else:
continue

Almost the same story as with Paras, but a little bit more complex. The nature of Mintbase marketplace is that it's not a single marketplace! Every Mintbase user has their own store and a separate contract. And it looks like those contract addresses follow the same principle they end with .mintbaseN.near where N is number (e.g. nate.mintbase1.near).

After we have defined that the ExecutionOutcome receiver is from Mintbase we are doing the same stuff as with Paras:

  1. Setting the marketplace variable to Mintbase
  2. Collecting the list of NFTs with owner and crafted links

And make it print the output to the terminal:

main.py
print(json.dumps(output, indent=4))

All together:

main.py
def format_paras_nfts(data, receipt_execution_outcome):
links = []

for data_element in data:
for token_id in data_element.get("token_ids", []):
first_part_of_token_id = token_id.split(":")[0]
links.append(
f"https://paras.id/token/{receipt_execution_outcome.receipt.receiver_id}::{first_part_of_token_id}/{token_id}"
)

return {"owner": data[0].get("owner_id"), "links": links}


def format_mintbase_nfts(data, receipt_execution_outcome):
links = []
for data_block in data:
try:
memo = json.loads(data_block.get("memo"))
except json.JSONDecodeError:
print(
f"Receipt ID: `{receipt_execution_outcome.receipt.receipt_id}`\nMemo: `{memo}`\nError during parsing Mintbase memo from JSON string to dict"
)
return

meta_id = memo.get("meta_id")
links.append(
f"https://www.mintbase.io/thing/{meta_id}:{receipt_execution_outcome.receipt.receiver_id}"
)

return {"owner": data[0].get("owner_id"), "links": links}


async def handle_streamer_message(streamer_message: near_primitives.StreamerMessage):
for shard in streamer_message.shards:
for receipt_execution_outcome in shard.receipt_execution_outcomes:
for log in receipt_execution_outcome.execution_outcome.outcome.logs:
if not log.startswith("EVENT_JSON:"):
continue
try:
parsed_log = json.loads(log[len("EVENT_JSON:") :])
except json.JSONDecodeError:
print(
f"Receipt ID: `{receipt_execution_outcome.receipt.receipt_id}`\nError during parsing logs from JSON string to dict"
)
continue

if (
parsed_log.get("standard") != "nep171"
or parsed_log.get("event") != "nft_mint"
):
continue

if receipt_execution_outcome.receipt.receiver_id.endswith(
".paras.near"
):
output = {
"receipt_id": receipt_execution_outcome.receipt.receipt_id,
"marketplace": "Paras",
"nfts": format_paras_nfts(
parsed_log["data"], receipt_execution_outcome
),
}
elif re.search(
".mintbase\d+.near", receipt_execution_outcome.receipt.receiver_id
):
output = {
"receipt_id": receipt_execution_outcome.receipt.receipt_id,
"marketplace": "Mintbase",
"nfts": format_mintbase_nfts(
parsed_log["data"], receipt_execution_outcome
),
}
else:
continue

print(json.dumps(output, indent=4))

And not that's it. Run the indexer to watch for NFT minting and never miss a thing.

python3 main.py
note

Having troubles running the indexer? Please, check you haven't skipped the Credentials part :)

Example output:

{
"receipt_id": "8rMK8rxb3WmFcSfM3ahFoeeoBF92pad3tpsqKoSWurr2",
"marketplace": "Mintbase",
"nfts": {
"owner": "vn-artists-dao.near",
"links": [
"https://www.mintbase.io/thing/aqdCBHB9_2XZY7pwXRRu5rGDeLQl7Q8KgNud1wKgnGo:vnartistsdao.mintbase1.near"
]
}
}
{
"receipt_id": "ArRh94Fe1LKF9yPrAdzrMozWoxMVQbEW2Z2Zf4fsSsce",
"marketplace": "Paras",
"nfts": {
"owner": "eeaeb516e0945893ac01eaf547f499abdbd344831c5fcbefa1a5c0a9f303cc5c",
"links": [
"https://paras.id/token/x.paras.near::432714/432714:1"
]
}
}

Conclusion

What a ride, yeah? Let's sum up what we have done:

  • You've learnt about Events
  • Now you understand how to follow for the Events
  • Knowing the fact that as a contract developer you can use Events and emit your own events, now you know how to create an indexer that follows those Events
  • We've had a closer look at NFT minting process, you can experiment further and find out how to follow nft_transfer Events

The material from this tutorial can be extrapolated for literally any event that follows the Events format

Not mentioning you have a dedicated indexer to find out about the newest NFTs minted out there and to be the earliest bird to collect them.

Let's go hunt doo, doo, doo 🦈

Source code for the tutorial

near-examples/near-lake-nft-indexer: source code for this tutorial