Example of why you shouldn't use a mutable as a default parameter in python

bad_idea.py

class BadIdea():
    def __init__(self, history=[]):
        self.history = history

    def get_history(self):
        return self.history

    def append_history(self, input):
        self.history.append(input)


a = BadIdea()
b = BadIdea()

print "a.history = {}".format(a.get_history())
print "b.history = {}".format(b.get_history())
# output:
# a.history = []
# b.history = []

print "\n\na.append_history('abc')"
a.append_history('abc')
print "a.history = {}".format(a.get_history())
print "b.history = {}".format(b.get_history())
# output:
# a.append_history('abc')
# a.history = ['abc']
# b.history = ['abc']

print "\n\nb.append_history('123')"
b.append_history('123')
print "a.history = {}".format(a.get_history())
print "b.history = {}".format(b.get_history())
# output:
# b.append_history('123')
# a.history = ['abc', '123']
# b.history = ['abc', '123']

print "\n\n\n"


class BetterIdea():
    def __init__(self, history=None):
        if history is None:
            self.history = []
        else:
            self.history = history

    def get_history(self):
        return self.history

    def append_history(self, input):
        self.history.append(input)


a = BetterIdea()
b = BetterIdea()

print "a.history = {}".format(a.get_history())
print "b.history = {}".format(b.get_history())
# output:
# a.history = []
# b.history = []

print "\n\na.append_history('abc')"
a.append_history('abc')
print "a.history = {}".format(a.get_history())
print "b.history = {}".format(b.get_history())
# output:
# a.append_history('abc')
# a.history = ['abc']
# b.history = []

print "\n\nb.append_history('123')"
b.append_history('123')
print "a.history = {}".format(a.get_history())
print "b.history = {}".format(b.get_history())
# output:
# b.append_history('123')
# a.history = ['abc']
# b.history = ['123']