Python dictionary deepcopy

21
votes

I was wondering in how does exactly deepcopy work in the following context:

from copy import deepcopy

def copyExample:
    self.myDict = {}
    firstPosition = "First"
    firstPositionContent = ["first", "primero"]
    secondPosition = "Second"
    secondPositionContent = ["second"]
    self.myDict[firstPosition] = firstPositionContent 
    self.myDict[secondPosition] = secondPositionContent
    return deepcopy(self.myDict)

def addExample(self):
    copy =  self.copyExample()
    copy["Second"].add("segundo")

Does it return the reference to the lists I have in the dictionary? Or does it work as I expect and copy every list in a new list with a different reference?

I know what a deep copy is (so there is no need to explain the difference between deep and shallow) but I am wondering if it works as I expect it to do and therefore do not change the instance variable when I use addExample().

2
pythondictionarydeep-copy
Did you print the two dictionaries to see if they're different? What did you see? Were they different? If so, the copy has a new list to which you added an element. - S.Lott

2 Answers

15
votes

The documentation makes it pretty clear that you're getting new copies, not references. Deepcopy creates deep copies for built in types, with various exceptions and that you can add custom copy operations to your user-defined objects to get deep copy support for them as well. If you're not sure, well that's what unit testing is for.

2
votes

I know it isn't answering your question but I think it's noteworthy for people looking at this question.

If the data you're copying is simple in nature deepcopy might be overkill. With simple in nature I mean if your data is representable as Json. Let me illustrate with code:

I've used http://www.json-generator.com/ to get some sample json data.

def deepCopyList(inp):
    for vl in inp:
        if isinstance(vl, list):
            yield list(deepCopyList(vl))
        elif isinstance(vl, dict):
            yield deepCopyDict(vl)

def deepCopyDict(inp):
    outp = inp.copy()
    for ky, vl in outp.iteritems():
        if isinstance(vl, dict):
            outp[ky] = deepCopyDict(vl)      
        elif isinstance(vl, list):
            outp[ky] = list(deepCopyList(vl))  
    return outp

def simpleDeepCopy(inp):
    if isinstance(inp, dict):
        return deepCopyDict(inp)
    elif isinstance(inp, list):
        return deepCopyList(inp)
    else:
        return inp

if __name__ == '__main__':
    import simplejson as json
    import time
    from copy import deepcopy
    fl = open('sample.json', 'r')
    sample = json.load(fl)
    start = time.time()
    for _ in xrange(10000):
        tmp = simpleDeepCopy(sample)
    end = time.time()
    print 'simpleDeepCopy: ' + str(end - start)
    start = time.time()
    for _ in xrange(10000):
        tmp = deepcopy(sample)
    end = time.time()
    print 'copy.deepcopy: ' + str(end - start)

output:

simpleDeepCopy: 0.0132050514221
copy.deepcopy: 2.66142916679

simpleDeepCopy: 0.0128579139709
copy.deepcopy: 2.60736298561