Make and test some small improvements to lazy

[wchargin]

Follow-up to #1781. See individual commit messages for details. To be
merged with rebase.

[nfelt]

Optional but seems like you might as well put this logic in _memoize itself.

[wchargin]

But _memoize takes a function that accepts a single hashable argument,
so recursion is perfectly fine. If _memoize took a niladic function,
the argument would be more convincing, I think.

[nfelt]

It'd be per-argument, e.g.

def _memoize(f):
  """Memoizing decorator for f, which must have exactly 1 hashable argument."""
  nothing = object()  # Unique "no value" sentinel object.
  loading = object()  # Unique "loading" sentinel object.
  cache = {}
  # Use a reentrant lock to allow f() to reference itself.
  lock = threading.RLock()
  @functools.wraps(f)
  def wrapper(arg):
    if cache.get(arg, nothing) == nothing:
      with lock:
        current = cache.get(arg, nothing)
        if current == loading:
          raise RuntimeException("circular reference")
        elif current == nothing:
          cache[arg] = loading
          cache[arg] = f(arg)
    return cache[arg]
  return wrapper

[wchargin]

If you don’t mind, I’ll keep it as written in this PR, because that
enables us to give a better error message.

[nfelt]

Update comment?

[wchargin]

Yes; thanks.

[nfelt]

six.assertRaisesRegex()?

[wchargin]

Sure; will change.

[nfelt]

IMO self.fail("...") slightly more idiomatic for this

[wchargin]

Agreed; will change.