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Thanks.
I'm not sure this is a correct change. And in fact, I would say that the current quoting of | is also incorrect.
& and | (and ^ and perhaps several others) have special meaning to cmd.exe. list2cmdline is documented as applying the quoting rules which the "MS C runtime" uses: cmd.exe and the C runtime are different and have different rules.
It seems to me that whoever added the | handling to list2cmdline was confused about the purpose of this function, or failed to properly document the function.
It would make more sense to document list2cmdline as applying cmd.exe-style quoting rules, if those are the rules it is actually going to implement.
A better option, though, would probably be to implement the cmd.exe quoting rules in a different function from the MS C runtime rules.
This all might benefit from a sanity check from someone who's actually worked with the subprocess module before, though (ie, not me).