[issue19640] Drop _source attribute of namedtuple

[STINNER Victor]

STINNER Victor added the comment:

I found this issue while using my tracemalloc module to analyze the memory consumption of Python. On the Python test suite, the _source attribute is the 5th line allocating the memory memory:

/usr/lib/python3.4/collections/__init__.py: 676.2 kB

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue19640>
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