Can I make a read-only list using Python's property system?
I have created a Python class that has a list as a member. Internally, I would like it to do something every time the list is modified. If this were C++, I would create getters and setters that would allow me to do my bookkeeping whenever the setter was called, and I would have the getter return a const reference, so that the compiler would yell at me if I tried to do modify the list through the getter. In Python, we have the property system, so that writing vanilla getters and setters for every data member is (thankfully) no longer necessary.
However, consider the following script:
def main():
foo = Foo()
print('foo.myList:', foo.myList)
# Here, I'm modifying the list without doing any bookkeeping.
foo.myList.append(4)
print('foo.myList:', foo.myList)
# Here, I'm modifying my "read-only" list.
foo.readOnlyList.append(8)
print('foo.readOnlyList:', foo.readOnlyList)
class Foo:
def __init__(self):
self._myList = [1, 2, 3]
self._readOnlyList = [5, 6, 7]
@property
def myList(self):
return self._myList
@myList.setter
def myList(self, rhs):
print("Insert bookkeeping here")
self._myList = rhs
@property
def readOnlyList(self):
return self._readOnlyList
if __name__ == '__main__':
main()
Output:
foo.myList: [1, 2, 3]
# Note there's no "Insert bookkeeping here" message.
foo.myList: [1, 2, 3, 4]
foo.readOnlyList: [5, 6, 7, 8]
This illustrates that the absence of the concept of const in Python allows me to modify my list using the append() method, despite the fact that I've made it a property. This can bypass my bookkeeping mechanism (_myList), or it can be used to modify lists that one might like to be read-only (_readOnlyList).
One workaround would be to return a deep copy of the list in the getter method (i.e. return self._myList[:]). This could mean a lot of extra copying, if the list is large or if the copy is done in an inner loop. (But premature optimization is the root of all evil, anyway.) In addition, while a deep copy would prevent the bookkeeping mechanism from being bypassed, if someone were to call .myList.append() , their changes would be silently discarded, which could generate some painful debugging. It would be nice if an exception were raised, so that they'd know they were working against the class' design.
A fix for this last problem would be not to use the property system, and make "normal" getter and setter methods:
def myList(self):
# No property decorator.
return self._myList[:]
def setMyList(self, myList):
print('Insert bookkeeping here')
self._myList = myList
If the user tried to call append(), it would look like foo.myList().append(8), and those extra parentheses would clue them in that they might be getting a copy, rather than a reference to the internal list's data. The negative thing about this is that it is kind of un-Pythonic to write getters and setters like this, and if the class has other list members, I would have to either write getters and setters for those (eww), or make the interface inconsistent. (I think a slightly inconsistent interface might be the least of all evils.)
Is there another solution I am missing? Can one make a read-only list using Python's property system?
You could have method return a wrapper around your original list -- collections.Sequence might be of help for writing it. Or, you could return a tuple -- The overhead of copying a list into a tuple is often negligible.
Ultimately though, if a user wants to change the underlying list, they can and there's really nothing you can do to stop them. (After all, they have direct access to self._myList if they want it).
I think that the pythonic way to do something like this is to document that they shouldn't change the list and that if the do, then it's their fault when their program crashes and burns.