With Python being a language for consenting adults, it does not have constant or read-only data types to protect one from oneself, but you can fake it with the property decorator.
class Foo(object):
@property
def bar(self):
return 'hello'
foo = Foo()
foo
<__main__.Foo at 0xb4af328c>
foo.bar
'hello'
foo.bar = 'world'
--------------------------------------------------------------------------- AttributeError Traceback (most recent call last) <ipython-input-4-19fdb2839390> in <module>() ----> 1 foo.bar = 'world' AttributeError: can't set attribute