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Comment #9 on issue 3025 by [email protected]: Piecewise evaluate=False
does not work when conditions are boolean
http://code.google.com/p/sympy/issues/detail?id=3025
To summarize a discussion from the pull request:
Piecewise((x, x > 1), (0, True), nan) in Sean's branch evaluates to 0,
because (0, True) is parsed as an expr, cond pair, and since the condition
is True, it returns the expression. evaluate=False would allow to keep
this from evaluating, which would be desirable if x is later evaluated to
something greater than 1, in which case you would get x (whatever it is),
not 0 (or at least that was my opinion; Ronan hasn't stated his yet).
In other words, you can consider Piecewise((x, x > 1), (0, True), nan) as
if x > 1: return x
elif True: return 0
else: return nan
If x is given, this should return x, not 0.
On the other hand, if Piecewise does not evaluate to explicitly True
conditions, it will be useless.
Let me know if I've left anything out. I don't know if you wanted to
discuss the IfElse here as well.
#6 describes current behaviour in master. I think that it's wrong, and
that Piecewise((x, x>1), (0, True), (1, False), (2, True)) should always
be canonicalised to Piecewise((x, x>1), (0, True)), irrespective of issue
2626.
I definitely agree that it's wrong, and was probably never intentioned to
do that, but rather is just a side effect of the way the current
parser/printer works.
It doesn't make sense to say "Piecewise((x, x>1), (0, True)), irrespective
of issue 2626." because those mean different things in master and in Sean's
branch (or technically, master and Sean's branch after we end the
deprecation cycle). That's why I think we should just discuss things in
terms of the new syntax, so that we are clear about what we mean (hence, we
could talk about Piecewise((x, x>1), (0, True), nan), which is unambiguous
in the new syntax due to redundancy, even with deprecation).
This is what I meant when I said "let's do all discussions there," by the
way, that we should discuss things using the new syntax, not necessarily
that the discussion itself should be in the pull request. Sorry if there
was ambiguity there.
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