Question

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“Least Astonishment” in Python: The Mutable Default Argument

Okay, so basically I have this code:

 # Our Rule type.

class Rule(object):
    """An object to represent a rule and include
    methods to apply these rules."""

    def __init__(self,bind="F",rule=["F"]):
        self.bind = bind
        self.rule = rule

    def show(self):
        try:
            print self.bind
            #print self.rule
            for i in range(len(self.rule)):
                print self.rule[i]
        except:
            print "Incomplete"

    def inflate(self,seq):
        for i in range(len(seq)):
            if seq[i] == self.bind:
                seq[i] = self.rule
            elif type(seq) == type(["X"]):
                seq[i] = self.inflate(seq[i])
        return seq

    def inflate_depth(self,seq,depth):
        while depth > 0:
            seq = self.inflate(seq)
            depth = depth - 1
            print self.rule
        return seq

I call it from another file with this code:

pity = Rule(rule=["R","F","L","F","L","F","R","F"])

seq = ["F"]

inf = pity.inflate_depth(seq,2)

So, I should end up with a list that looks like this:

[['R', [...], 'L', [...], 'L', [...], 'R', [...]]]

Which seems to work fine, but there is one fundamental error.

self.rule

has been altered to contain ['R', [...], 'L', [...], 'L', [...], 'R', [...]]

Why? There is no where whatsoever I assign a new value to that variable, but it changes nonetheless.

Answer 1

I’m not 100% sure about this but I suspect the problem is that variables in Python are references and not values. That is to say, when you set seq[i] = self.rule, seq[i] points to where self.rule is stored in memory and any changes made to seq[i]‘s value results in a change to self.rule since their values are stored at the same location in memory.

What you probably should do is deep copy self.rule‘s value into seq[i] rather than simply assign it so that they use two different memory addresses to store their values (see http://docs.python.org/library/copy.html for more information), that way assigning to seq[i] won’t affect self.rule.