Web3 manages layers of middlewares by default. They sit between the public Web3 methods and the Providers, which handle native communication with the Ethereum client. Each layer can modify the request and/or response. Some middlewares are enabled by default, and others are available for optional use.

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 middlewares that are in deeper layers).

More information is available in the “Internals: Middlewares” section.

Default Middleware

Some middlewares are added by default if you do not supply any. The defaults are likely to change regularly, so this list may not include the latest version’s defaults. You can find the latest defaults in the constructor in web3/manager.py



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.

.eth Name Resolution


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 if invoked with the mainnet (where the ENS contract is deployed), for all other cases will result in an InvalidAddress error



This converts arguments and returned values to python primitives, where appropriate. For example, it converts the raw hex string returned by the RPC call eth_blockNumber into an int.

Gas Price Strategy



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


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



This middleware is a default specifically for HTTPProvider that retries failed requests that return the following errors: ConnectionError, HTTPError, Timeout, TooManyRedirects. Additionally there is a whitelist that only allows certain methods to be retried in order to not resend transactions, excluded methods are: eth_sendTransaction, personal_signAndSendTransaction, personal_sendTransaction.

Configuring Middleware

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

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 middlewares are maintained in Web3.middleware_onion. See below for the API.

When specifying middlewares in a list, or retrieving the list of middlewares, 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.middlewares would return the middlewares in the order of: [2, 1, 0].

See “Internals: Middlewares” for a deeper dive to how middlewares work.

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.pythonic_middleware)
# or
>>> w3.middleware_onion.add(web3.middleware.pythonic_middleware, 'pythonic')
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 pythonic middleware at the innermost layer
>>> w3 = Web3(...)
>>> w3.middleware_onion.inject(web3.middleware.pythonic_middleware, layer=0)
# or
>>> w3.middleware_onion.inject(web3.middleware.pythonic_middleware, 'pythonic', 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.pythonic_middleware)
# or
>>> w3.middleware_onion.remove('pythonic')
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 pythonic_middleware, attrdict_middleware
>>> w3 = Web3(...)

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

# or, if it was named

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

Empty all the middlewares, including the default ones.

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

Return all the current middlewares 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.pythonic_middleware, 'test_middleware')
# export middlewares from first w3 instance
>>> middlewares = w3_1.middleware_onion.middlewares

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

Optional Middleware

Web3 ships with non-default middleware, for your custom use. In addition to the other ways of Configuring Middleware, you can specify a list of middleware when initializing Web3, with:

Web3(middlewares=[my_middleware1, my_middleware2])


This will replace the default middlewares. To keep the default functionality, either use middleware_onion.add() from above, or add the default middlewares to your list of new middlewares.

Below is a list of built-in middleware, which is 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 = make_stalecheck_middleware(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().


All of the caching middlewares accept these common arguments.

  • cache_class must be a callable which returns an object which implements the dictionary API.

  • rpc_whitelist must be an iterable, preferably a set, of the RPC methods that may be cached.

  • should_cache_fn must be a callable with the signature fn(method, params, response) which returns whether the response should be cached.

web3.middleware.construct_simple_cache_middleware(cache_class, rpc_whitelist, should_cache_fn)

Constructs a middleware which will cache the return values for any RPC method in the rpc_whitelist.

A ready to use version of this middleware can be found at web3.middlewares.simple_cache_middleware.

web3.middleware.construct_time_based_cache_middleware(cache_class, cache_expire_seconds, rpc_whitelist, should_cache_fn)

Constructs a middleware which will cache the return values for any RPC method in the rpc_whitelist for an amount of time defined by cache_expire_seconds.

  • cache_expire_seconds should be the number of seconds a value may remain in the cache before being evicted.

A ready to use version of this middleware can be found at web3.middlewares.time_based_cache_middleware.

web3.middleware.construct_latest_block_based_cache_middleware(cache_class, average_block_time_sample_size, default_average_block_time, rpc_whitelist, should_cache_fn)

Constructs a middleware which will cache the return values for any RPC method in the rpc_whitelist for the latest block. It avoids re-fetching the current latest block for each request by tracking the current average block time and only requesting a new block when the last seen latest block is older than the average block time.

  • average_block_time_sample_size The number of blocks which should be sampled to determine the average block time.

  • default_average_block_time The initial average block time value to use for cases where there is not enough chain history to determine the average block time.

A ready to use version of this middleware can be found at web3.middlewares.latest_block_based_cache_middleware.

Proof of Authority


It’s important to inject the middleware at the 0th layer of the middleware onion: w3.middleware_onion.inject(geth_poa_middleware, layer=0)

The geth_poa_middleware is required to connect to geth --dev or the Goerli public network. It 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

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 geth_poa_middleware

# inject the poa compatibility middleware to the innermost layer (0th layer)
>>> w3.middleware_onion.inject(geth_poa_middleware, layer=0)

# confirm that the connection succeeded
>>> w3.client_version

Why is geth_poa_middleware 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’s 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 local_filter_middleware
>>> w3.middleware_onion.add(local_filter_middleware)
#  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.

  • 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 construct_sign_and_send_raw_middleware
>>> from eth_account import Account
>>> acct = Account.create('KEYSMASH FJAFJKLDSKF7JKFDJ 1530')
>>> w3.middleware_onion.add(construct_sign_and_send_raw_middleware(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 construct_sign_and_send_raw_middleware(private_key_or_account).

>>> from web3 import Web3
>>> w3 = Web3(Web3.HTTPProvider('HTTP_ENDPOINT'))
>>> from web3.middleware import construct_sign_and_send_raw_middleware
>>> from eth_account import Account
>>> import os
>>> acct = w3.eth.account.from_key(os.environ.get('PRIVATE_KEY'))
>>> w3.middleware_onion.add(construct_sign_and_send_raw_middleware(acct))
>>> w3.eth.default_account = acct.address

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)