web3.py: Optional compatibility with geth PoA dev chain, to fix: Error could not format value as field 'extraData'

What’s Wrong

When using geth --dev, it uses a proof-of-authority chain for instant mining. Unfortunately, the mechanism for PoA is to add a bunch of bytes to extraData, which is not yellow-paper-compliant. We would like to have the default web3.py strictly reject non-compliant clients, but still enable people to use this popular development alternative.

Proposed Solution

Add a new middleware, called something like truncate_extra_data_middleware. It would trim the extraData field down to 32 bytes or fewer. Note that this would be best to go earlier in the middleware stack than the pythonic middleware, which raises the exception.

To be complete:

• A test showing that a long extradata does not raise an error, after adding the new middleware
• A test showing that a long extradata does raise an error in the default configuration
• A new middleware truncate_extra_data_middleware (probably constructed with a more general construct_truncate_result_middleware(whitelist_methods, field, truncate_to))
• Add to the API for the middleware stack (probably). Either something like .bury(obj) which puts a new middleware to the bottom of the stack, or a more general .insert(pos, obj) which inserts middleware to an arbitrary location. Up for discussion.
• Some basic documentation describing why and how to use the new middleware

Original Issue

Getting the following error message when trying to send a transaction via web3py:

Error Could not format value ‘0xd783010800846765746887676f312e392e32856c696e7578000000000000000086ddf484c77c385bf8ec5c04427ccb3b2624efc39c966ae16858213df5f87ca453022c474a9346faec0be34f6ec2c16da2a987fd08670465c3b70bb361848a8a00’ as field ‘extraData’

Here is the script im using

https://gist.github.com/postables/8b634de55033c27a7d870aaeb5f02103