[issue28850] Regression in Python 3: Subclassing PrettyPrinter.format doesn't work anymore

[Xiang Zhang]

Changes by Xiang Zhang <angwerzx at 126.com>:


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nosy: +xiang.zhang
stage:  -> needs patch
versions: +Python 3.7 -Python 3.3, Python 3.4

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue28850>
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