import os
replacement = """some
multi-line string"""
for dname, dirs, files in os.walk("some_dir"):
    for fname in files:
        fpath = os.path.join(dname, fname)
        with open(fpath) as f:
            s = f.read()
        s = s.replace("{$replace}", replacement)
        with open(fpath, "w") as f:
            f.write(s)

The above solution has flaws, such as the fact that it opens literally every file it finds, or the fact that each file is read entirely into memory (which would be bad if you had a 1GB text file), but it should be a good starting point.

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You also may want to look into the re module if you want to do a more complex find/replace than looking for a specific string.