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Dictionary unpacking doesn't create a copy #3227

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malcolmvr opened this issue Nov 8, 2019 · 0 comments

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@malcolmvr
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@malcolmvr malcolmvr commented Nov 8, 2019

With Python, executing the following code:

o = {'a': 1, 'b': 2}
o2 = {**o}
o2['a'] = 3
print(o['a'], o2['a'])

results in (1, 3) being printed.

When compiled with Cython (3,3) is printed.

If the code is changed to:

o = {'a': 1, 'b': 2}
o2 = {**o, 'c': 4} # changed
o2['a'] = 3
print(o['a'], o2['a'])

Then the Cython compiled version behaves the same as the Python-interpreted version. Cython seems to be taking a shortcut and not creating a copy of the object.

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