Unclear why --namespace-packages isn't enabled by default

[russelldavis]

Why isn't the behavior enabled by --namespace-packages on by default? Is it because it's new/unstable, and will eventually become the default? (If so, can the docs be updated to reflect that?)

[gvanrossum]

It has some downsides too, e.g. it means mypy can't resolve the package to which a file belongs when it's a namespace package (because there's no __init__.py to recognize it as a package). So it's not going to become the default.

[russelldavis]

Got it, thanks. Can the docs be updated to explain the downsides?

[gvanrossum]

Can you suggest a PR?

[russelldavis]

For posterity, here's the proposal for enabling this by default: #8584