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).

Configuring Middleware

Middleware can be added, removed, replaced, and cleared at runtime. To make that easier, you can name the middleware for later reference.

Middleware Order

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


                                `````Layer 2``````
                         ```````                  ```````
                    `````               |                ````
                 ````                   v                    ````
              ```                                                ```
            `.               ````````Layer 1```````                `.`
          ``             ````                      `````              .`
        `.            ```               |               ```            `.`
       .`          ```                  v                  ```           `.
     `.          `.`                                         ```           .`
    ``          .`                  `Layer 0`                  ``           .`
   ``         `.               `````        ``````               .           .`
  `.         ``             ```         |        ```              .`          .
  .         ``            `.`           |           ``             .           .
 .         `.            ``       JSON-RPC call       .`            .          .`
 .         .            ``              |              .            ``          .
``         .            .               v               .            .          .
.         .`           .                                .            .          ``
.         .            .          Ethereum node         .`           .           .
.         .            .                                .            .           .
.         ``           `.               |               .            .           .
.          .            .`              |              .`            .          .
`.         .`            .`          Response         .`            .`          .
 .          .             `.`           |           `.`            `.           .
 `.          .              ```         |        ````             `.           .
  .          `.               `````     v     ````               `.           ``
   .           .`                 ```Layer 0``                  ``           `.
    .           `.                                            `.`           `.
     .            `.                    |                   `.`            `.
      .`            ```                 |                 ```             .`
       `.              ```              v             ````              `.`
         ``               ``````                 `````                 .`
           ``                   `````Layer 1`````                   `.`
             ```                                                  ```
               ````                     |                      ```
                  `````                 v                  ````
                      ``````                          `````
                            `````````Layer 2``````````


                             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].

Middleware Stack API

To add or remove items in different layers, use the following API:

Web3.middleware_onion.add(middleware, name=None)

Middleware will be added to the outermost layer. That means the new middleware will modify the request first, and the response last. You can optionally name it with any hashable object, typically a string.

>>> w3 = Web3(...)
>>> w3.middleware_onion.add(web3.middleware.GasPriceStrategyMiddleware)
# or
>>> w3.middleware_onion.add(web3.middleware.GasPriceStrategyMiddleware, 'gas_price_strategy')
Web3.middleware_onion.inject(middleware, name=None, layer=None)

Inject a named middleware to an arbitrary layer.

The current implementation only supports injection at the innermost or outermost layers. Note that injecting to the outermost layer is equivalent to calling Web3.middleware_onion.add() .

# Either of these will put the gas_price_strategy middleware at the innermost layer
>>> w3 = Web3(...)
>>> w3.middleware_onion.inject(web3.middleware.GasPriceStrategyMiddleware, layer=0)
# or
>>> w3.middleware_onion.inject(web3.middleware.GasPriceStrategyMiddleware, 'gas_price_strategy', layer=0)

Middleware will be removed from whatever layer it was in. If you added the middleware with a name, use the name to remove it. If you added the middleware as an object, use the object again later to remove it:

>>> w3 = Web3(...)
>>> w3.middleware_onion.remove(web3.middleware.GasPriceStrategyMiddleware)
# or
>>> w3.middleware_onion.remove('gas_price_strategy')
Web3.middleware_onion.replace(old_middleware, new_middleware)

Middleware will be replaced from whatever layer it was in. If the middleware was named, it will continue to have the same name. If it was un-named, then you will now reference it with the new middleware object.

>>> from web3.middleware import GasPriceStrategyMiddleware, AttributeDictMiddleware
>>> w3 = Web3(provider, middleware=[GasPriceStrategyMiddleware, AttributeDictMiddleware])

>>> w3.middleware_onion.replace(GasPriceStrategyMiddleware, AttributeDictMiddleware)
# this is now referenced by the new middleware object, so to remove it:
>>> w3.middleware_onion.remove(AttributeDictMiddleware)

# or, if it was named

>>> w3.middleware_onion.replace('gas_price_strategy', AttributeDictMiddleware)
# this is still referenced by the original name, so to remove it:
>>> w3.middleware_onion.remove('gas_price_strategy')

Empty all the middleware, including the default ones.

>>> w3 = Web3(...)
>>> w3.middleware_onion.clear()
>>> assert len(w3.middleware_onion) == 0

Return all the current middleware for the Web3 instance in the appropriate order for importing into a new Web3 instance.

>>> w3_1 = Web3(...)
# add uniquely named middleware:
>>> w3_1.middleware_onion.add(web3.middleware.GasPriceStrategyMiddleware, 'test_middleware')
# export middleware from first w3 instance
>>> middleware = w3_1.middleware_onion.middleware

# import into second instance
>>> w3_2 = Web3(..., middleware=middleware)
>>> assert w3_1.middleware_onion.middleware == w3_2.middleware_onion.middleware
>>> assert w3_2.middleware_onion.get('test_middleware')

Instantiate with Custom Middleware

Instead of working from the default list, you can specify a custom list of middleware when initializing Web3:

Web3(middleware=[my_middleware1, my_middleware2])


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.

Default Middleware

The following middleware are included by default:

  • gas_price_strategy

  • ens_name_to_address

  • attrdict

  • validation

  • gas_estimate

The defaults are defined in the get_default_middleware() method in web3/manager.py.


class web3.middleware.AttributeDictMiddleware

This middleware recursively converts any dictionary type in the result of a call to an AttributeDict. This enables dot-syntax access, like eth.get_block('latest').number in addition to eth.get_block('latest')['number'].


Accessing a property via attribute breaks type hinting. For this reason, this feature is available as a middleware, which may be removed if desired.

ENS Name to Address Resolution

class web3.middleware.ENSNameToAddressMiddleware

This middleware converts Ethereum Name Service (ENS) names into the address that the name points to. For example w3.eth.send_transaction will accept .eth names in the ‘from’ and ‘to’ fields.


This middleware only converts ENS names on chains where the proper ENS contracts are deployed to support this functionality. All other cases will result in a NameNotFound error.

Gas Price Strategy

class web3.middleware.GasPriceStrategyMiddleware


Gas price strategy is only supported for legacy transactions. The London fork introduced maxFeePerGas and maxPriorityFeePerGas transaction parameters which should be used over gasPrice whenever possible.

This adds a gasPrice to transactions if applicable and when a gas price strategy has been set. See Gas Price API for information about how gas price is derived.

Buffered Gas Estimate

class web3.middleware.BufferedGasEstimateMiddleware

This adds a gas estimate to transactions if gas is not present in the transaction parameters. Sets gas to: min(w3.eth.estimate_gas + gas_buffer, gas_limit) where the gas_buffer default is 100,000


class web3.middleware.ValidationMiddleware

This middleware includes block and transaction validators which perform validations for transaction parameters.

Optional Middleware

Web3 includes optional middleware for common use cases. Below is a list of available middleware which are not enabled by default.



This middleware checks how stale the blockchain is, and interrupts calls with a failure if the blockchain is too old.

  • allowable_delay is the length in seconds that the blockchain is allowed to be behind of time.time()

Because this middleware takes an argument, you must create the middleware with a method call.

two_day_stalecheck = StalecheckMiddlewareBuilder.build(60 * 60 * 24 * 2)

If the latest block in the blockchain is older than 2 days in this example, then the middleware will raise a StaleBlockchain exception on every call except web3.eth.get_block().

Proof of Authority

class web3.middleware.ExtraDataToPOAMiddleware


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
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

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

Why is ExtraDataToPOAMiddleware necessary?

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.

Locally Managed Log and Block Filters


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_account A single private key or a tuple, list or set of private keys.

    Keys can be in any of the following formats:

    • An eth_account.LocalAccount object

    • An eth_keys.PrivateKey object

    • 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.address

Hosted nodes (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)

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)

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)

Creating Custom Middleware

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.


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 response

If 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 middleware

Custom 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 (

w3 = Web3(HTTPProvider(endpoint_uri="..."))

# add the middleware to the stack as the class
w3.middleware_onion.add(CustomMiddleware, name="custom_middleware")