fastest way to test file for string?

[kj]

Hi.  I need to implement, within a Python script, the same
functionality as that of Unix's

   grep -rl some_string some_directory

I.e. find all the files under some_directory that contain the string
"some_string".

I imagine that I can always resort to the shell for this, but is
there an efficient way of doing it within Python?

(BTW, portability is not high on my list here; this will run on a
Unix system, and non-portable Unix-specific solutions are fine with
me.)

TIA!
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