You can use infinite recursion productively in python. It even evaluates
pretty quickly:
sage: def a():
try:
return a()+1
except Exception:
return 1
sage: a()
991
sage: %time a()
CPU times: user 106 µs, sys: 1.04 ms, total: 1.15 ms
Wall time: 929 µs
983
We've used it in the sage library in at least two places (although one was
rewritten a while ago to use a different mechanism).
Seriously, though:
Any time you write an "except" that can catch RuntimeError you run the risk
of masking a "RuntimeError: maximum recursion depth exceeded" and because
python's maximum recursion depth is normally set pretty low, you will not
catch things that are logically infinite recursions by walltime.
If you have to use a try/except, make sure you catch a very specific error.
Unfortunately error types in python aren't very fine grained, so it can be
hard to make your test sufficiently specific using the type mention in
"except". The painful and expensive alternative is to examine the error
object before deciding to pass or raise.
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