[Python] Any way to extract the sign of a number in Python?

I have am coding the algorithm of Bolzano in Python. This is my code for now:

def Bolzano(fonction, a, b, tol=0.000001):
while abs(b-a)>tol:
m=(a+b)/2
if cmp(fonction(m))==cmp(fonction(a)):
a=m
else:
b=m
return a, b


It works until it encounters cmp, which it doesn't recognise. However I don't see another way to do it, since Python doesn't have a sign function. Is there any other way to extract the sign of a number?

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