Vendored modules (hypothesis/vendor/pretty.py) produce type errors with Mypy

[PiDelport]

As of the 0.62.0 release, when type-checking a Hypothesis-using project with Mypy, at least the pretty vendored module raises (valid) type errors:

…/hypothesis/vendor/pretty.py:501: error: Module has no attribute "InstanceType"
…/hypothesis/vendor/pretty.py:503: error: Incompatible types in assignment (expression has type "Tuple[Callable[[object], str]]", variable has type "Tuple[Callable[[object], str], Any]")
…/hypothesis/vendor/pretty.py:764: error: Cannot assign to a type
…/hypothesis/vendor/pretty.py:764: error: Cannot assign multiple types to name "_exception_base" without an explicit "Type[...]" annotation
…/hypothesis/vendor/pretty.py:791: error: Module has no attribute "DictProxyType"
…/hypothesis/vendor/pretty.py:792: error: Module has no attribute "DictProxyType"
…/hypothesis/vendor/pretty.py:794: error: Module has no attribute "ClassType"
…/hypothesis/vendor/pretty.py:795: error: Module has no attribute "SliceType"
…/hypothesis/vendor/pretty.py:800: error: Name 'xrange' is not defined
…/hypothesis/vendor/pretty.py:801: error: Name 'long' is not defined
…/hypothesis/vendor/pretty.py:802: error: Name 'unicode' is not defined
…/hypothesis/vendor/pretty.py:808: error: Need type annotation for '_deferred_type_pprinters'

(The errors are things like Python 2 compatibility code that isn't valid under Python 3.)

I'm guessing this is due to the py.typed marker, which counts everything under the hypothesis
package to be type-checked.

The workaround is to explicitly ignore the module's errors in the project's mypy.ini:

[mypy-hypothesis.vendor.pretty]
ignore_errors = True

However, projects should not be expected to do this in general: the fix should be to explicitly annotate the code in question (or the whole module, pending python/mypy#626) with # type: ignore.

[PiDelport]

As an aside, the Hypothesis CI should probably run Mypy too, to avoid this kind of problem.

[Zac-HD]

Ack! You're entirely right, thanks for reporting this. We actually do run Mypy in CI... but with the vendor directory ignored. Whoops.

I'll fix this as a matter of priority.