Inconsistency using map in python

Question

I have a constants file constants.py like so:

querystring = {
    "limit":"10000",
    "sort":"desc"
}

dummy1 = {
    "a": "22",
    "q": "*",
}

dummy2 = {
    "a": "24",
    "q": "**",
}

streams = [dummy1, dummy2]

I'm trying to initialise a list by manipulating the values from the constants.py file.

from constants import querystring, streams
def setParams(dummy, querystring):
    ld = {}
    ld["query"] = setQuerystring( dummy, querystring)
    print ld
    return ld

def setQuerystring( dummy, querystring):
    query = querystring
    query["filter"] = "stream:" + dummy["a"]
    query["query"] = dummy["q"]
    return query

l = map(lambda x: setParams(x, querystring), streams)
print l[0]
print l[1]

While the lambda function is running, the output is correctly printed, but when I see the final value returned by the map, the values are different. Why this inconsistency?

The program output:

{'query': {'sort': 'desc', 'filter': 'stream:22', 'limit': '10000', 'query': '*'}} # l[0] -> during lambda execution
{'query': {'sort': 'desc', 'filter': 'stream:24', 'limit': '10000', 'query': '**'}} # l[1] -> during lambda execution
{'query': {'sort': 'desc', 'filter': 'stream:24', 'limit': '10000', 'query': '**'}} # l[0] -> from map 
{'query': {'sort': 'desc', 'filter': 'stream:24', 'limit': '10000', 'query': '**'}} # l[1] -> from map

querystring is in fact mutable dict. query = querystring does not create a copy of dict, but uses reference instead. In setQuerystring you modify same dict instance over and over again. Try query = querystring.copy() — Łukasz Rogalski
– Łukasz Rogalski, Commented Jul 17, 2017 at 13:58

Moses Koledoye · Accepted Answer · 2017-07-17 13:59:02Z

3

You're reusing and modifying the querystring dict across iterations. Later changes from successive iterations are therefore propagated to previously attached copies. You should consider attaching a copy to each dict instead, and modifying that copy:

def setQuerystring( dummy, querystring):
    query = querystring.copy()
    ...

{'query': {'sort': 'desc', 'filter': 'stream:22', 'limit': '10000', 'query': '*'}}
{'query': {'sort': 'desc', 'filter': 'stream:24', 'limit': '10000', 'query': '**'}}
{'query': {'sort': 'desc', 'filter': 'stream:22', 'limit': '10000', 'query': '*'}}
{'query': {'sort': 'desc', 'filter': 'stream:24', 'limit': '10000', 'query': '**'}}

answered Jul 17, 2017 at 13:59

Moses Koledoye

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1 Comment

TheChetan Over a year ago

I was under the assumption that the default behaviour was to copy, not change the existing object.

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