Middleware

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

AttributeDict

web3.middleware.attrdict_middleware()

This middleware converts the output of a function from a dictionary to an AttributeDict which enables dot-syntax access, like eth.getBlock('latest').number in addition to eth.getBlock('latest')['number'].

.eth Name Resolution

web3.middleware.name_to_address_middleware()

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

Pythonic

web3.middleware.pythonic_middleware()

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

web3.middleware.gas_price_strategy_middleware()

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.

HTTPRequestRetry

web3.middleware.http_retry_request_middleware()

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 or parity) 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 middlewares are maintained in Web3.middleware_stack. 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 list(w3.middleware_stack) 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_stack.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_stack.add(web3.middleware.pythonic_middleware)
# or
>>> w3.middleware_stack.add(web3.middleware.pythonic_middleware, 'pythonic')
Web3.middleware_stack.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_stack.add() .

# Either of these will put the pythonic middleware at the innermost layer
>>> w3 = Web3(...)
>>> w3.middleware_stack.inject(web3.middleware.pythonic_middleware, layer=0)
# or
>>> w3.middleware_stack.inject(web3.middleware.pythonic_middleware, 'pythonic', layer=0)
Web3.middleware_stack.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_stack.remove(web3.middleware.pythonic_middleware)
# or
>>> w3.middleware_stack.remove('pythonic')
Web3.middleware_stack.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_stack.replace(pythonic_middleware, attrdict_middleware)
# this is now referenced by the new middleware object, so to remove it:
>>> w3.middleware_stack.remove(attrdict_middleware)

# or, if it was named

>>> w3.middleware_stack.replace('pythonic', attrdict_middleware)
# this is still referenced by the original name, so to remove it:
>>> w3.middleware_stack.remove('pythonic')
Web3.middleware_stack.clear()

Empty all the middlewares, including the default ones.

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

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])

Warning

This will replace the default middlewares. To keep the default functionality, either use middleware_stack.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.

Stalecheck

web3.middleware.make_stalecheck_middleware(allowable_delay)

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)
web3.middleware_stack.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 except web3.eth.getBlock().

Cache

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 an amount of time defined by cache_expire_seconds.

  • 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.

Geth-style Proof of Authority

This middleware is required to connect to geth --dev or the Rinkeby public network.

For example, to connect to a local geth --dev instance on Linux:

>>> from web3 import Web3, IPCProvider

# connect to the default geth --dev IPC location
>>> w3 = Web3(IPCProvider('/tmp/geth.ipc'))

>>> from web3.middleware import geth_poa_middleware

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

# confirm that the connection succeeded
>>> w3.version.node
'Geth/v1.7.3-stable-4bb3c89d/linux-amd64/go1.9'

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 it’s development mode and the Rinkeby 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.