There is a stack of middlewares managed by Web3. 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 in the stack 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 further down the stack).

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 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


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.



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.

Modifying 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.

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

Middleware will be added to the top of the stack. 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')

Middleware will be removed from wherever it sat in the stack. 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 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 wherever it sat in the stack. 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')

Empty all the middlewares, including the default ones.

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

Built-in Middleware

Web3 ships with non-default middleware, for your custom use. In addition to the other ways of Modifying 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_stack.add() from above, or add the default middlewares to your list of new middlewares.

Below is a list of built-in middleware.



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.getBlock().