&gt; What does mypy do if you make a call from code with type annotations into code without? Or vice-versa?<p>It&#x27;s not able to analyze across such boundaries, but as you expand the set of typed code you get better and better coverage.<p>&gt; Has anyone ever gotten paid to add annotations to code that works?<p>At Dropbox, we&#x27;re starting to add type annotations to code. We&#x27;re confident it will catch many bugs at lint time.
Source: I work at Dropbox.<p>&gt; Personally, I view type systems as like a safety line when doing work on a roof, and optional typing as having a line that might or might not be tied off.<p>At least it has the possibility of being tied off and catching you. Better than definitely not having a line.