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So far at Plurk we have struggled to overcome scalability issues - - and none of them are due to Python, even thought we use Python a lot.

Most of the problems we have seen have been related to the database. It was and still is the biggest bottleneck. And all the people I know that drive big sites will confirm this.

This all said, one should evaluate problems and not blindly use Python for everything. Python is a great language, but some other language is a better choice for some problems. And here Java and specially Java's NIO library is a much better choice for doing a server that should handle tons of open connections.