e2.py

import sys
import functools

class der(object):

    def __init__(self, h):
        if isinstance(h, float):
            self.h = h
        else:
            self.h = sys.float_info.epsilon*1000
            self.fun = h

    def __call__(self, f):
        if isinstance(f, float):
            x = f
            return (self.fun(x+self.h)-self.fun(x))/self.h
        @functools.wraps(f)
        def wrapped_f(*args):
            x = float(args[0])
            return (f(x+self.h)-f(x))/self.h
        return wrapped_f

@der
def f(x):
    return x**2

print f(0.001)