Background syntax check broken with last PyCharm version
Answered
Seems like syntax check is broken since last update to 2016.3. Errors are reported where there is no error. The only way to remove those false positives is to run File / Invalidate caches and restart PyCharm. Please fix this, it's quite annoying to have code full of false errors.
Here's the screenshot:

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Please, file an issue about it to PyCharm bug-tracker: https://youtrack.jetbrains.com/issues/PY with your idea.log (https://intellij-support.jetbrains.com/hc/en-us/articles/207241085-Locating-IDE-log-files), thank you.