Consider --allow-redefinition for mypy

[mpharrigan]

See python/mypy#1174 for full context, but there are common idioms where you may want to re-use a variable name

items: Set[Item] = set()
items.add(...)
items = frozenset(items)

Mypy currently complains. They added a flag: python/mypy#6197 which may or may not become the default at some point. I say we add this to our mypy configuration.

[balopat]

ref #3767

[maffoo]

I have mixed feelings about this. Sometimes it's nice to redefine variables (I often encounter this when a function accepts an arg with a union type but we then convert it to a single type and want the rest of the code to use this "narrowed" type). But other times a redefinition is inadvertent and mypy flagging it can actually detect bugs (sometimes bugs like this only surface when refactoring). It looks like --allow-redefinition only applies in certain limited situations, which is good, but I'd still be a bit leery about adding it.

[mpharrigan]

I'm going through the numpy stuff and there are a non-zero number of places where redefinitions are used, especially when constructing arrays:

arr: List[float]
for thing in iterator:
  arr.append(complicated_function(thing))
arr: np.ndarray = np.asarray(arr)

[95-martin-orion]

Discussion in Cirq sync:

• particularly nice for e.g. redefining Optional values to non-optional type
• not allowing redefinition enforces cleaner code, but mostly helps with redefinition in multiple scopes

From mypy: --allow_redefinition only allows redefinition within the same block and nesting depth.

[mhucka]

This is a tough call, because there are pros and cons for both options (add, or don't add). Based on the fact that (a) the default of not adding --allow-redefinition does not seem to have been problematic enough for people to need to change it in the last 4 years, (b) the option can be added manually on the command line to check/mypy if/when desired, and (c) it seems safer to leave this option off, I'm going ahead and closing this issue. If that's the wrong call and people want to revisit this, please feel free to leave comments here and reopen this issue.