This Engineering Notebook post discusses a mypy complaint that probably
should never be eliminated. Feel free to ignore, unless you are Félix :-)
Two Position methods, p.moveToNthChild and p.moveToParent, set p.v = None
if there is no such position. This is a fundamental feature of *all* of
the p.moveTo* methods! All these methods change p *in place*. They do *not*
return a new position.
Here is p.moveToNthChild:
def moveToNthChild(self, n: int):
p = self
if p.v and len(p.v.children) > n:
p.stack.append((p.v, p._childIndex))
p.v = p.v.children[n]
p._childIndex = n
else:
# mypy rightly doesn't like setting p.v to None.
# Leo's code must use the test `if p:` as appropriate.
p.v = None # type: ignore
return p
As noted in the comment, the *only* valid way to test whether any of the
p.moveTo methods yields a valid position is to test `if p:` or `if not
p:`. The p.__bool__ special method enforces this convention:
def __bool__(self):
"""
Return True if a position is valid.
The tests 'if p' or 'if not p' are the _only_ correct ways
to test whether a position p is valid.
Tests like 'if p is None' or 'if p is not None' will not
work properly.
"""
return self.v is not None
To repeat, mypy *rightly* complains that assigning None to p.v is a type
error. Changing the declaration of p.v in p.__init__ from:
self.v: "VNode" = v
to
self.v: Optional["VNode"] = v
creates a cascade of (valid!) mypy complaints. Rather than attempt a
(possibly heroic) fix, I think it best to leave the two `type: ignore`
statements in place, with comments explaining their significance.
*Summary*
Setting p.v to None *is a real type error.* Imo, it would not be helpful
to pretend otherwise. Setting p.v to None works only because:
1. p.__bool__ tests p.v.
2. Leonine scripts must test `if p:` or `if not p:` after calling p.moveTo*
methods.
There is *no way* to change this special case now. It affects all Leonine
scripts. Any anyway, I would not likely change this coding convention even
if I could.
All comments welcome. I would especially like to know how leojs handles
this issue.
Edward
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