Middleware
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
|
|
v
`````Layer 2``````
``````` ```````
````` | ````
```` v ````
``` ```
`. ````````Layer 1``````` `.`
`` ```` ````` .`
`. ``` | ``` `.`
.` ``` v ``` `.
`. `.` ``` .`
`` .` `Layer 0` `` .`
`` `. ````` `````` . .`
`. `` ``` | ``` .` .
. `` `.` | `` . .
. `. `` JSON-RPC call .` . .`
. . `` | . `` .
`` . . v . . .
. .` . . . ``
. . . Ethereum node .` . .
. . . . . .
. `` `. | . . .
. . .` | .` . .
`. .` .` Response .` .` .
. . `.` | `.` `. .
`. . ``` | ```` `. .
. `. ````` v ```` `. ``
. .` ```Layer 0`` `` `.
. `. `.` `.
. `. | `.` `.
.` ``` | ``` .`
`. ``` v ```` `.`
`` `````` ````` .`
`` `````Layer 1````` `.`
``` ```
```` | ```
````` v ````
`````` `````
`````````Layer 2``````````
|
v
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)
- Web3.middleware_onion.remove(middleware)
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')
- Web3.middleware_onion.clear()
Empty all the middleware, including the default ones.
>>> w3 = Web3(...) >>> w3.middleware_onion.clear() >>> assert len(w3.middleware_onion) == 0
- Web3.middleware_onion.middleware
Return all the current middleware for the
Web3
instance in the appropriate order for importing into a newWeb3
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])
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.
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
.
AttributeDict
- 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, likeeth.get_block('latest').number
in addition toeth.get_block('latest')['number']
.Note
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.Note
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
Warning
Gas price strategy is only supported for legacy transactions. The London fork introduced
maxFeePerGas
andmaxPriorityFeePerGas
transaction parameters which should be used overgasPrice
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
Validation
- 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.
Stalecheck
- web3.middleware.StalecheckMiddlewareBuilder()
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 oftime.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) web3.middleware_onion.add(two_day_stalecheck)
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 exceptweb3.eth.get_block()
.
Locally Managed Log and Block Filters
- web3.middleware.LocalFilterMiddleware()
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()
Signing
- web3.middleware.SignAndSendRawMiddlewareBuilder()
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
objectAn
eth_keys.PrivateKey
objectA 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)
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)
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.
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 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 (
CustomMiddleware,
)
w3 = Web3(HTTPProvider(endpoint_uri="..."))
# add the middleware to the stack as the class
w3.middleware_onion.add(CustomMiddleware, name="custom_middleware")