Dmypy crashes when adding import and rechecking

Crash Report

Daemon crashes after adding an import flask statement and re-checking.

Traceback

Daemon crashed!
Traceback (most recent call last):
  File "mypy/dmypy_server.py", line 221, in serve
  File "mypy/dmypy_server.py", line 264, in run_command
  File "mypy/dmypy_server.py", line 323, in cmd_run
  File "mypy/dmypy_server.py", line 385, in check
  File "mypy/dmypy_server.py", line 564, in fine_grained_increment_follow_imports
  File "mypy/dmypy_server.py", line 679, in find_reachable_changed_modules
KeyError: 'jinja2'

To Reproduce

mkdir test && cd test
touch hi.py
dmypy run -- . # works
echo "import flask" > hi.py
dmypy run -- . # crashes
dmypy run -- . # works

Note that flask is not installed.

Your Environment

• Mypy version used: 0.800
• Mypy command-line flags: dmypy run -- .
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.9.1
• Operating system and version: macOS 10.15.7

mypy/dmypy_server.py から始め、提供された再現手順を使って cmd_run、check、fine_grained_increment_follow_imports、find_reachable_changed_modules を追跡します。利用できない flask の import を追加した後に発生する 'jinja2' の検索欠落を調査します。繰り返し dmypy を実行してもクラッシュしなくなれば完了です。