[Data] Replace lambda mutable default arguments

[peytondmurray]

Why are these changes needed?

Across the code base there are a number of instances where lambda functions were being used as default arguments. Our documentation build system doesn't handle these correctly, resulting in rendering issues with docs. Additionally, functions are mutable, which means that these can have really weird/unintended behavior that is hard to debug.

This PR changes all the instances I could find where we were using a lambda function as a default argument and swaps it out according to the usual if arg is None: arg = <lambda function> pattern.

@angelinalg Can you confirm this fixes the documentation side of things for the functions that are part of the public API?

Related issue number

Partially addresses #45129.

Checks

• I've signed off every commit(by using the -s flag, i.e., git commit -s) in this PR.
• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
  • I've added any new APIs to the API Reference. For example, if I added a
    method in Tune, I've added it in doc/source/tune/api/ under the
    corresponding .rst file.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(

[peytondmurray]

Before:

After

[angelinalg]

Wow! I didn't even know how broken it was when I reported it. Much better! Confirmed! Thanks.
cc: @can-anyscale

[bveeramani]

Nice

[bveeramani]

@peytondmurray for my own understanding, could you help me understand how functions are mutable? I understand they're objects and you can assign attributes on them, but I don't think you can change the actual function behavior, can you?

[peytondmurray]

@peytondmurray for my own understanding, could you help me understand how functions are mutable? I understand they're objects and you can assign attributes on them, but I don't think you can change the actual function behavior, can you?

No, you can't change function behavior, but functions are objects that have __setattr__, so in this case you'd have to be doing something pretty weird for it to affect anything, e.g.

def foo(x = lambda y: y):
    if hasattr(x, 'bar'):
       x.bar += 3
    else:
       x.bar = 0

    # do other stuff...

So the mutability is a little more hidden from the user than for example a list, since it's pretty unusual to be setting attributes on functions in the first place. For the record I'm not sure if any of these affect parts of the code do anything like this, but in any case it's better to just be safe since it fixes documentation rendering anyway.