Setup MyPy config and enforce types for build-support scripts

build-support/githooks/pre-commit

@@ -58,6 +58,12 @@ if git rev-parse --verify "${MERGE_BASE}" &>/dev/null; then
   # quite often with a pexrc available.
   echo "* Checking lint" && ./pants --exclude-target-regexp='testprojects/.*' --changed-parent="${MERGE_BASE}" lint || exit 1
 
+  echo "* Checking types (for build-support)"
+  # NB: This task requires Python 3, so we must temporarily override any intepreter constraints
+  # set by ci.sh, specifically it setting constraints to Python 2, because those constraints may
+  # cause a select-interpreter failure.
+  PANTS_PYTHON_SETUP_INTERPRETER_CONSTRAINTS="['CPython>=3.6']" ./pants mypy 'build-support::' || exit 1
+
   if git diff "${MERGE_BASE}" --name-only | grep '\.rs$' > /dev/null; then
     echo "* Checking formatting of rust files" && ./build-support/bin/check_rust_formatting.sh || exit 1
     # Clippy happens on a different Travis CI shard because of separate caching concerns.

build-support/mypy/mypy.ini

@@ -0,0 +1,50 @@
+[mypy]
+# Refer to https://mypy.readthedocs.io/en/latest/command_line.html for the definition of each
+# of these options. MyPy is frequently updated, so this file should be periodically reviewed
+# for any new behavior that we can opt-in to.
+#
+# In general, we would like to be as strict as possible, but currently keep most checks turned off
+# because our codebase has so little type coverage. As we add more types, these options should be
+# re-evaluated and made more strict where possible.
+
+# Optionals
+no_implicit_optional = True
+
+# Untyped. Eventually we should turn on as many of these as possible, but for now we turn them
+# off to allow an incremental migration.
+check_untyped_defs = False
+disallow_untyped_calls = False
+disallow_untyped_defs = False
+disallow_untyped_decorators = False
+disallow_incomplete_defs = False
+
+# Dynamic typing
+disallow_any_unimported = False
+disallow_any_expr = False
+disallow_any_decorated = False
+disallow_any_explicit = False
+disallow_any_generics = False
+disallow_subclassing_any = False
+
+# Strictness
+strict_equality = False
+
+# Warnings
+warn_unused_ignores = True
+warn_no_return = True
+warn_return_any = True
+warn_redundant_casts = True
+
+# Error output
+show_column_numbers = True
+show_traceback = True
+
+# Imports. See https://mypy.readthedocs.io/en/latest/running_mypy.html#missing-imports.
+ignore_missing_imports = True
+follow_imports = normal
+follow_imports_for_stubs = False
+
+# Ignore complaints about our dynamic programming
+# TODO: write a plugin that teaches MyPy how these work!
+# See https://mypy-lang.blogspot.com/2019/03/extending-mypy-with-plugins.html.
+always_true = enum,datatype

pants.ini

@@ -299,6 +299,10 @@ skip: True
 [pycheck-context-manager]
 skip: True
 
+[mypy]
+mypy_version: 0.701
+config_file: build-support/mypy/mypy.ini
+
 [scala]
 version: 2.12