Why does the ";" exist when it goes against formatting standards?

Question

As you know, the ; symbol is used to separate lines like this:

print "Hai"; print "Bai"

From what I've heard, this goes against formatting standards for python. So, if this goes against formatting standards, why was it implemented? How is it used properly?

Answer 1

";" are used in cases, you have no other choice. E.g. when calling Python from command line and willing to execute those two statements.

$ python -c "print 'Hai'; print 'Bai'"
Hai
Bai

Or more complex cases:

$ python -c 'a=1; b=2; print "a + b = {a} + {b} = {res}".format(a=a, b=b, res=a+b)'
a + b = 1 + 2 = 3

note: Do not take my "no other choice" literally, you will find nice alternatives to some cases in comments.

Answer 2

To extend on Jan Vlcinsky's answer, if you're trying to quickly determine the efficiency of something, timeit comes in handy. For example, (from the docs):

python -m timeit -s 'text = "sample string"; char = "g"'  'char in text'