gistfile1.txt

class Foo(object):

    def __init__(self):
        self.history = {"var1": [], "var2": [], "var3": [], "var4": []}

    def update(self, to_update):
        for k, v in self.history.iteritems():
            if k == to_update:
                v.append(1)
            else:
                v.append(float('nan'))

if __name__ == "__main__":
    foo = Foo()
    vars = ["var1", "var2", "var1", "var4"]
    print foo.history
    for var in vars:
        foo.update(var)
        print foo.history

rory@clotho:~$ python foo.py
{'var4': [], 'var1': [], 'var3': [], 'var2': []}
{'var4': [], 'var1': [], 'var3': [], 'var2': []}
{'var4': [nan], 'var1': [1], 'var3': [nan], 'var2': [nan]}
{'var4': [nan, nan], 'var1': [1, nan], 'var3': [nan, nan], 'var2': [nan, 1]}
{'var4': [nan, nan, nan], 'var1': [1, nan, 1], 'var3': [nan, nan, nan], 'var2': [nan, 1, nan]}
{'var4': [nan, nan, nan, 1], 'var1': [1, nan, 1, nan], 'var3': [nan, nan, nan, nan], 'var2': [nan, 1, nan, nan]}