Hi list, I just stumbled accross NPY_WRITEABLE flag. Now I'd like to know if there are ways either from Python or C to make an array temporarily immutable. Thanks, Martin.
Martin Wiechert wrote:
Hi list,
I just stumbled accross NPY_WRITEABLE flag. Now I'd like to know if there are ways either from Python or C to make an array temporarily immutable.
Just setting the flag Python: make immutable: a.flags.writeable = False make mutable again: a.flags.writeable = True C: make immutable: a->flags &= ~NPY_WRITEABLE make mutable again: a->flags |= NPY_WRITEABLE In C you can play with immutability all you want. In Python you can only make something writeable if you either 1) own the data or 2) the object that owns the data is itself "writeable" -Travis
Thanks Travis. Do I understand correctly that the only way to be really safe is to make a copy and not to export a reference to it? Because anybody having a reference to the owner of the data can override the flag? Cheers, Martin On Wednesday 20 September 2006 20:18, Travis Oliphant wrote:
Martin Wiechert wrote:
Hi list,
I just stumbled accross NPY_WRITEABLE flag. Now I'd like to know if there are ways either from Python or C to make an array temporarily immutable.
Just setting the flag
Python:
make immutable: a.flags.writeable = False
make mutable again: a.flags.writeable = True
C:
make immutable: a->flags &= ~NPY_WRITEABLE
make mutable again: a->flags |= NPY_WRITEABLE
In C you can play with immutability all you want. In Python you can only make something writeable if you either 1) own the data or 2) the object that owns the data is itself "writeable"
-Travis
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Martin Wiechert wrote:
Thanks Travis.
Do I understand correctly that the only way to be really safe is to make a copy and not to export a reference to it? Because anybody having a reference to the owner of the data can override the flag?
No, that's not quite correct. Of course in C, anybody can do anything they want to the flags. In Python, only the owner of the object itself can change the writeable flag once it is set to False. So, if you only return a "view" of the array (a.view()) then the Python user will not be able to change the flags. Example: a = array([1,2,3]) a.flags.writeable = False b = a.view() b.flags.writeable = True # raises an error. c = a c.flags.writeable = True # can be done because c is a direct alias to a. Hopefully, that explains the situation a bit better. -Travis
On Thursday 21 September 2006 18:24, Travis Oliphant wrote:
Martin Wiechert wrote:
Thanks Travis.
Do I understand correctly that the only way to be really safe is to make a copy and not to export a reference to it? Because anybody having a reference to the owner of the data can override the flag?
No, that's not quite correct. Of course in C, anybody can do anything they want to the flags.
In Python, only the owner of the object itself can change the writeable flag once it is set to False. So, if you only return a "view" of the array (a.view()) then the Python user will not be able to change the flags.
Example:
a = array([1,2,3]) a.flags.writeable = False
b = a.view()
b.flags.writeable = True # raises an error.
c = a c.flags.writeable = True # can be done because c is a direct alias to a.
Hopefully, that explains the situation a bit better.
It does. Thanks Travis.
-Travis
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participants (3)
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Martin Wiechert -
Martin Wiechert -
Travis Oliphant