Question

Here is my class Bar:

class Bar:
    def __init__(self, start, end, open, volume, high=open, low=open, last=open):
        self.start = start
        self.end = end
        self.open = open
        self.high = high
        self.low = low
        self.last = last
        self.volume = int(volume)

    def __str__(self):
        return self.start.strftime("%m/%d/%Y\t%H:%M:%S") + "\t" + self.end.strftime("%H:%M:%S") + "\t" +str(self.open) + "\t" + str(self.high) + "\t" + str(self.low) + "\t" + str(self.last) + "\t" + str(self.volume)

1) I am trying to initialize the high, low, and last for whatever open is. Is this the right way to do this?

2) When I do print(str(bar)) I get funny output such as…

03/13/2012 12:30:00 13:30:00 138.91 <built-in function open> 138.7 <built-in function open> 13177656

Answer 1

If you had written your function as

def __init__(self, start, end, foo, volume, high=foo, low=foo, last=foo):
    self.start = start
    self.end = end
    self.open = foo
    self.high = high
    self.low = low
    self.last = last
    self.volume = int(volume)

you would have gotten a NameError complaining that the name foo is not defined. This is because the default value for a parameter cannot refer to another parameter, which isn’t yet defined when you try to take its value. (Default values are set when the method is defined, not when it is called.)

However, open is a built-in function in Python, so it was defined; it just wasn’t the open you meant it to be. The right way to do this is probably

class Bar:
    def __init__(self, start, end, open, volume, high=None, low=None, last=None):
        self.start = start
        self.end = end
        self.open = open
        self.high = open if high is None else high
        self.low = open if low is None else low
        self.last = open if last is None else last
        self.volume = int(volume)

In addition, you might want to use ‘open_’ instead of ‘open’ as the parameter name, just to avoid confusion (and temporarily shadowing) the built-in function.