Fix Python – Why doesn’t Python have a sign function?

I can’t understand why Python doesn’t have a sign function. It has an abs builtin (which I consider sign’s sister), but no sign.
In python 2.6 there is even a copysign function (in math), but no sign. Why bother to write a copysign(x,y) when you could just write a sign and then get the copysign directly from abs(x) * sign(y)? The latter would be m….

[Fixed] python – “Least Astonishment” and the Mutable Default Argument

Anyone tinkering with Python long enough has been bitten (or torn to pieces) by the following issue:
def foo(a=[]):
return a

Python novices would expect this function to always return a list with only one element: [5]. The result is instead very different, and very astonishing (for a novice):
>>> foo()
>>> foo()
[5, 5]