> 1. It opens the source file one more time.  This is probably acceptable
> because existing code already opens it at least four times when -m (show
> missing) option is selected.  (Twice in find_executable_linenos() and
> twice in linecache.getlines().  Fixing that would require refactoring of
> linecache code.

Create a function like linecache.getencoding() seems to be overkill.

I created issue #10335 to add a function tokenize.open_python(): open a Python 
script in read mode without opening the file twice and get the encoding with 
detect_encoding(). This issue is more generic than trying to optimize the 
trace module.

> 2. This will not work for source code not stored in a file, but provided by
> a __loader__.get_source() method.  However it looks like trace will not
> work at all in this case, so fixing that is a separate issue.

For this case, I think that we can add a try/except IOError with a fallback to 
encoding = 'utf-8'.