Update contract.py

[leafyoung]

What was wrong?

contract_function.call() returns the latest result when w3.eth.default_block has been set.

The default value "latest" for block_identifier parameter masks w3.eth.default_block even when it is set to non-None.

w3.eth.default_block is by default set to 'latest'. Changing the default value to None will not change its default behaviour.

If we do not change this, we have to add block_identifier=past_block_number to every .call(). This breaks existing codes using web3.py and it's very tedious to do so.

Related to Issue #

I found this wrong behaviour when running regression test with an earlier version of web3.py

How was it fixed?

Change the default value to None to make parse_block_identifier do the work.

Todo:

• Add entry to the release notes

Set default block_identifier in ContractFunction.call() to None

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[kclowes]

Thanks for raising this issue @leafyoung. This looks like a good change to me. I found a couple other places where we're doing the same thing, so I'll update those and push it up.

[kclowes]

Thanks @leafyoung!

[leafyoung]

Thanks @kclowes