This snippet is easy: it defines an immutable wrapper around a sequence type (list, deque, etc) which allows a class, function or module to expose a read only version of a sequence to the outer world. A constant wrapper around mapping types could also be defined in a similar manner.
Expose an immutable sequence
class ConstSequence(object):
"""Immutable wrapper around a sequence type instance."""
_SliceType = type(slice(None)) # <-- this should work for all versions of python
def __init__(self, seq):
if isinstance(seq, ConstSequence):
seq = seq._adaptee
self._adaptee = seq
def __getitem__(self, key):
if isinstance(key, self._SliceType):
return ConstSequence(self._adaptee[key])
else:
return self._adaptee[key]
def __len__(self):
return len(self._adaptee)
def __contains__(self, key):
return key in self._adaptee
def __iter__(self):
return (x for x in self._adaptee)
def __reversed__(self):
return (x for x in reversed(self._adaptee))
def __str__(self):
return 'ConstSequence(%r)' % self._adaptee
__repr__ = __str__
if __name__ == "__main__":
# example use
class A(object):
"""Class A stores a list internally, but exposes a read-only version
of this list.
"""
def __init__(self):
self._items = list()
self.items = ConstSequence(self._items)
def add_string(self, s):
"Methods in class A can alter the list's content"
self._items.extend(s)
a = A()
a.add_string("hello")
print a.items[1] # prints 'e' : client code can read the list
a.items[1] = "z" # raises TypeError : client code can't alter the list's content
TrustyTony 888 pyMod Team Colleague Featured Poster
But:
class ConstSequence(object):
"Read-only wrapper around a sequence type instance"
def __init__(self, seq):
if isinstance(seq, ConstSequence):
seq = seq._adaptee
self._adaptee = seq
def __getitem__(self, key):
import types
if isinstance(key, types.SliceType):
return ConstSequence(self._adaptee[key])
else:
return self._adaptee[key]
def __len__(self):
return len(self._adaptee)
def __contains__(self, key):
return key in self._adaptee
def __iter__(self):
return (x for x in self._adaptee)
def __reversed__(self):
return (x for x in reversed(self._adaptee))
def __str__(self):
return 'ConstSequence(%s)' % self._adaptee
__repr__ = __str__
if __name__ == "__main__":
# example use
class A(object):
"""Class A stores a list internally, but exposes a read-only version
of this list.
"""
def __init__(self):
self._items = list()
self.items = ConstSequence(self._items)
def add_string(self, s):
"Methods in class A can alter the list's content"
self._items.extend(s)
def __str__(self):
return 'A(%s)' % self.items
__repr__ = __str__
a = A()
a.add_string("hello")
a.add_string([[1, 2, 3]])
print a
print a.items[1] # prints 'e' : client code can read the list
a.items[-1][-1] = 'Gaboom!'
print a # list in ConstSequence is not constant
a.items[1] = "z" # raises TypeError : client code can't alter the list's content Gribouillis commented: good remark +13
Gribouillis 1,391 Programming Explorer Team Colleague
Yes, it's the same problem with tuples
t = (1, 2, ["hello", "world"])
print(t)
t[-1][-1] = "daniweb"
print(t)
""" my output -->
(1, 2, ['hello', 'world'])
(1, 2, ['hello', 'daniweb'])
"""Immutability never meant true immutability in python. If ConstSequences are immutable like tuples are, it's ok for me.
I approve adding a __str__ method. You could have included the missing 'import types' out of the __getitem__ method.
Ene Uran 638 Posting Virtuoso
Line 9if isinstance(key, types.SliceType):
can be changed toif isinstance(key, slice):
to work with Python27 and Python33
Gribouillis commented: thanks, changed this in the code. +13
Gribouillis 1,391 Programming Explorer Team Colleague
Thanks. I don't know why they removed types.SliceType in python 3.
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