Raymond Hettinger added the comment:
FWIW, the learners in my Python classes seem to find the words "read-only" to
be helpful. Also, I think "not visible" conveys the wrong mental model
("shadowed" being a little more accurate).
I also disagree with saying that "variables are never read-only". In fact,
unless declared "nonlocal", the namespace for the nested scope can't be written
to; likewise, we also have dict proxies in classes that are specifically
designed to be read-only; further, there are many read-only attributes in
Python.
The current paragraph is succinct and accurate. That said, there is no doubt
that creative people can find ways to misread it, so I think there is room to
add an extra paragraph that elaborates on the rules:
1) variable *lookups* will search from locals to nested scopes to globals to
builtins, stopping at the first match or raising NameError if not found; and,
2) variable *writes* always go into locals unless explicitly declared as being
in another namespace with "nonlocal" or "global".
The docs can't smooth this over by changing a single misinterpreted word. One
way or another, either in the tutorial or in a FAQ, users need to learn why x=1
doesn't write to an enclosing scope without a namespace declaration and why
self.x+=1 can read a class variable, increment the value, and then write to an
instance variable. Learning this is fundamental to understanding the language
and it can't be glossed over by saying that the word "read-only" was confusing.
Everyone gets what "read-only" means; instead, the challenge is to grapple
with what the rules are and why the language behaves this way.
----------
nosy: +rhettinger
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Python tracker <rep...@bugs.python.org>
<http://bugs.python.org/issue26449>
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