Web3 is instantiated with layers of middleware by default. They sit between the public Web3 methods and the providers, and are used to perform sanity checks, convert data types, enable ENS support, and more. Each layer can modify the request and/or response. While several middleware are enabled by default, others are available for optional use, and you're free to create your own!
Each middleware layer gets invoked before the request reaches the provider, and then processes the result after the provider returns, in reverse order. However, it is possible for a middleware to return early from a call without the request ever getting to the provider (or even reaching the middleware that are in deeper layers).
Middleware can be added, removed, replaced, and cleared at runtime. To make that easier, you can name the middleware for later reference.
Think of the middleware as being layered in an onion, where you initiate a web3.py request at the outermost layer of the onion, and the Ethereum node (like geth) receives and responds to the request inside the innermost layer of the onion. Here is a (simplified) diagram:
New request from web3.py
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Returned value in web3.py
The middleware are maintained in Web3.middleware_onion. See below for the API.
When specifying middleware in a list, or retrieving the list of middleware, they will be returned in the order of outermost layer first and innermost layer last. In the above example, that means that w3.middleware_onion.middleware would return the middleware in the order of: [2, 1, 0].
To add or remove items in different layers, use the following API:
Instead of working from the default list, you can specify a custom list of middleware when initializing Web3:
Web3(middleware=[my_middleware1, my_middleware2])Warning
This will replace the default middleware. To keep the default functionality, either use middleware_onion.add() from above, or add the default middleware to your list of new middleware.
The following middleware are included by default:
gas_price_strategyens_name_to_addressattrdictvalidationgas_estimate
The defaults are defined in the get_default_middleware() method in web3/manager.py.
Web3 includes optional middleware for common use cases. Below is a list of available middleware which are not enabled by default.
Note
It's important to inject the middleware at the 0th layer of the middleware onion: w3.middleware_onion.inject(ExtraDataToPOAMiddleware, layer=0)
ExtraDataToPOAMiddleware is required to connect to geth --dev and may also be needed for other EVM compatible blockchains like Polygon or BNB Chain (Binance Smart Chain).
If the middleware is not injected at the 0th layer of the middleware onion, you may get errors like the example below when interacting with your EVM node.
web3.exceptions.ExtraDataLengthError: The field extraData is 97 bytes, but should be
32. It is quite likely that you are connected to a POA chain. Refer to
http://web3py.readthedocs.io/en/stable/middleware.html#proof-of-authority
for more details. The full extraData is: HexBytes('...')The easiest way to connect to a default geth --dev instance which loads the middleware is:
>>> from web3.auto.gethdev import w3
# confirm that the connection succeeded
>>> w3.client_version
'Geth/v1.13.14-stable-4bb3c89d/linux-amd64/go1.20.2'This example connects to a local geth --dev instance on Linux with a unique IPC location and loads the middleware:
>>> from web3 import Web3, IPCProvider
# connect to the IPC location started with 'geth --dev --datadir ~/mynode'
>>> w3 = Web3(IPCProvider('~/mynode/geth.ipc'))
>>> from web3.middleware import ExtraDataToPOAMiddleware
# inject the poa compatibility middleware to the innermost layer (0th layer)
>>> w3.middleware_onion.inject(ExtraDataToPOAMiddleware, layer=0)
# confirm that the connection succeeded
>>> w3.client_version
'Geth/v1.7.3-stable-4bb3c89d/linux-amd64/go1.9'There is no strong community consensus on a single Proof-of-Authority (PoA) standard yet. Some nodes have successful experiments running though. One is go-ethereum (geth), which uses a prototype PoA for its development mode and the Goerli test network.
Unfortunately, it does deviate from the yellow paper specification, which constrains the extraData field in each block to a maximum of 32-bytes. Geth is one such example where PoA uses more than 32 bytes, so this middleware modifies the block data a bit before returning it.
This middleware provides an alternative to ethereum node managed filters. When used, Log and Block filter logic are handled locally while using the same web3 filter api. Filter results are retrieved using JSON-RPC endpoints that don't rely on server state.
>>> from web3 import Web3, EthereumTesterProvider >>> w3 = Web3(EthereumTesterProvider()) >>> from web3.middleware import LocalFilterMiddleware >>> w3.middleware_onion.add(LocalFilterMiddleware)
# Normal block and log filter apis behave as before.
>>> block_filter = w3.eth.filter("latest")
>>> log_filter = myContract.events.myEvent.build_filter().deploy()This middleware automatically captures transactions, signs them, and sends them as raw transactions. The from field on the transaction, or w3.eth.default_account must be set to the address of the private key for this middleware to have any effect.
The build method for this middleware builder takes a single argument:
private_key_or_accountA single private key or a tuple, list or set of private keys.Keys can be in any of the following formats:
- An
eth_account.LocalAccountobject- An
eth_keys.PrivateKeyobject- A raw private key as a hex string or byte string
>>> from web3 import Web3, EthereumTesterProvider
>>> w3 = Web3(EthereumTesterProvider)
>>> from web3.middleware import SignAndSendRawMiddlewareBuilder
>>> from eth_account import Account
>>> acct = Account.create('KEYSMASH FJAFJKLDSKF7JKFDJ 1530')
>>> w3.middleware_onion.add(SignAndSendRawMiddlewareBuilder.build(acct))
>>> w3.eth.default_account = acct.addressHosted nodes<local_vs_hosted> (like Infura or Alchemy) only support signed transactions. This often results in send_raw_transaction being used repeatedly. Instead, we can automate this process with SignAndSendRawMiddlewareBuilder.build(private_key_or_account).
>>> from web3 import Web3
>>> w3 = Web3(Web3.HTTPProvider('HTTP_ENDPOINT'))
>>> from web3.middleware import SignAndSendRawMiddlewareBuilder
>>> from eth_account import Account
>>> import os
>>> acct = w3.eth.account.from_key(os.environ.get('PRIVATE_KEY'))
>>> w3.middleware_onion.add(SignAndSendRawMiddlewareBuilder.build(acct))
>>> w3.eth.default_account = acct.address
>>> # use `eth_sendTransaction` to automatically sign and send the raw transaction
>>> w3.eth.send_transaction(tx_dict)
HexBytes('0x09511acf75918fd03de58141d2fd409af4fd6d3dce48eb3aa1656c8f3c2c5c21')Similarly, with AsyncWeb3:
>>> from web3 import AsyncWeb3
>>> async_w3 = AsyncWeb3(AsyncHTTPProvider('HTTP_ENDPOINT'))
>>> from web3.middleware import SignAndSendRawMiddlewareBuilder
>>> from eth_account import Account
>>> import os
>>> acct = async_w3.eth.account.from_key(os.environ.get('PRIVATE_KEY'))
>>> async_w3.middleware_onion.add(SignAndSendRawMiddlewareBuilder.build(acct))
>>> async_w3.eth.default_account = acct.address
>>> # use `eth_sendTransaction` to automatically sign and send the raw transaction
>>> await async_w3.eth.send_transaction(tx_dict)
HexBytes('0x09511acf75918fd03de58141d2fd409af4fd6d3dce48eb3aa1656c8f3c2c5c21')Now you can send a transaction from acct.address without having to build and sign each raw transaction.
When making use of this signing middleware, when sending dynamic fee transactions (recommended over legacy transactions), the transaction type of 2 (or '0x2') is necessary. This is because transaction signing is validated based on the transaction type parameter. This value defaults to '0x2' when maxFeePerGas and / or maxPriorityFeePerGas are present as parameters in the transaction as these params imply a dynamic fee transaction. Since these values effectively replace the legacy gasPrice value, do not set a gasPrice for dynamic fee transactions. Doing so will lead to validation issues.
# dynamic fee transaction, introduced by EIP-1559:
>>> dynamic_fee_transaction = {
... 'type': '0x2', # optional - defaults to '0x2' when dynamic fee transaction params are present
... 'from': acct.address, # optional if w3.eth.default_account was set with acct.address
... 'to': receiving_account_address,
... 'value': 22,
... 'maxFeePerGas': 2000000000, # required for dynamic fee transactions
... 'maxPriorityFeePerGas': 1000000000, # required for dynamic fee transactions
... }
>>> w3.eth.send_transaction(dynamic_fee_transaction)A legacy transaction still works in the same way as it did before EIP-1559 was introduced:
>>> legacy_transaction = {
... 'to': receiving_account_address,
... 'value': 22,
... 'gasPrice': 123456, # optional - if not provided, gas_price_strategy (if exists) or eth_gasPrice is used
... }
>>> w3.eth.send_transaction(legacy_transaction)To write your own middleware, create a class and extend from the base Web3Middleware class, then override only the parts of the middleware that make sense for your use case.
Note
The Middleware API borrows from the Django middleware API introduced in version 1.10.0.
If all you need is to modify the params before a request is made, you can override the request_processor method, make the necessary tweaks to the params, and pass the arguments to the next element in the middleware stack. Need to do some processing on the response? Override the response_processor method and return the modified response.
The pattern:
from web3.middleware import Web3Middleware
class CustomMiddleware(Web3Middleware):
def request_processor(self, method, params):
# Pre-request processing goes here before passing to the next middleware.
return (method, params)
def response_processor(self, method, response):
# Response processing goes here before passing to the next middleware.
return response
# If your provider is asynchronous, override the async methods instead:
async def async_request_processor(self, method, params):
# Pre-request processing goes here before passing to the next middleware.
return (method, params)
async def async_response_processor(self, method, response):
# Response processing goes here before passing to the next middleware.
return responseIf you wish to prevent making a call under certain conditions, you can override the wrap_make_request method. This allows for defining pre-request processing, skipping or making the request under certain conditions, as well as response processing before passing it to the next middleware.
from web3.middleware import Web3Middleware
class CustomMiddleware(Web3Middleware):
def wrap_make_request(self, make_request):
def middleware(method, params):
# pre-request processing goes here
response = make_request(method, params) # make the request
# response processing goes here
return response
return middleware
# If your provider is asynchronous, override the async method instead:
async def async_wrap_make_request(self, make_request):
async def middleware(method, params):
# pre-request processing goes here
response = await make_request(method, params)
# response processing goes here
return response
return middlewareCustom middleware can be added to the stack via the class itself, using the middleware_stack_api. The name kwarg is optional. For example:
from web3 import Web3
from my_module import (
CustomMiddleware,
)
w3 = Web3(HTTPProvider(endpoint_uri="..."))
# add the middleware to the stack as the class
w3.middleware_onion.add(CustomMiddleware, name="custom_middleware")