Maybe it's my preference, but I am much more used to the idiom in which on initialization, you do
f = open('hisc.txt','r')
# do some exception handling so if the file is empty, hiScore is 0 unless you wanted to start with a default higher than 0
hiScore = int(f.read())
f.close()
And then at the end of the game:
if myScore > hiScore:
f = open('hisc.txt', 'w')
f.write(str(myScore))
f.close()
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