Merging two dic in python3 using Unpacking Generalisations in Python3.5

Question

There are two dictionaries as follow which I want to merge them, my point is to select those keys that I am interested in, for example I am interested in all the keys except county. Solution I 've used is using del function after creating your new dictionary, however I am sure there are more ways that are more efficient to my solution. How can I solve this problem without del function using UNPACKING ARGUMENT.

    >>> d1 =  {'avgUserperDay': '12', 'avgPurchaseperDay': '1', 'country': 'Japan'}
    >>> d2 = {'tUser': 1, 'tPurchase': 0, 'country': 'Japan'}
    >>> d ={**d1,**d2}
    >>>{'tUser': 1, 'tPurchase': 0, 'avgPurchaseperDay': '1', 'avgUserperDay': '12', 'country': 'Japan'}
    >>> del d['country']
    >>> d
    {'tUser': 1, 'tPurchase': 0, 'avgPurchaseperDay': '1', 'avgUserperDay': '12'}

AFTER DISCUSSION,

This command works with 3.5.1,

>>> {**{k:v for k, v in chain(d1.items(), d2.items()) if k != 'country'}}
{'tUser': 1, 'tPurchase': 0, 'avgPurchaseperDay': '1', 'avgUserperDay': '12'}

Alex · Answer 1 · 2016-10-05T13:34:32.910

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why would you want to do it using argument unpacking? just do:

from itertools import chain
d = {key:value for key, value in chain(d1.iteritems(), d2.iteritems())
     if key not in keys_to_ignore}

where keys_to_ignore is list/set/tuple of keys you want to ignore

edited Oct 05 '16 at 13:34

answered Oct 05 '16 at 13:29

Alex

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score 1 · Answer 2 · edited May 23 '17 at 12:15

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How can I solve this problem without del function using UNPACKING ARGUMENT?

Yes, you can. But you should not. Ideal way should be:

d1.update(d2)   # values will be updated in d1 dict
del d1['country']

There no direct way to meet your conditions:

create new dict using argument unpacking
not using del to remove country.

But there are work around if this is what you want. For example using itertools.chain() with dict comprehension as:

{**{k:v for k, v in chain(d1.items(), d2.items()) if k != 'country'}}

Note: For those curios about how {**dict1, **dict2} even works. It is new syntax supported since from python 3.5+, proposed in PEP 448. Check: What's New in Python 3.5 document.

Since, {**dict1, **dict2} is only supported in Python 3.5+, for earlier version of Python, you can do (as mentioned by S. De Melo):

d = dict(d1, **d2)
d.pop("country")

OR without using ** as:

{k, v for k, v in chain(d1.iteritems(), d2.iteritems()) if k != 'country'}

edited May 23 '17 at 12:15

Community

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answered Oct 05 '16 at 13:31

Moinuddin Quadri

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Does this work in Python 2? The `{**{` is a syntax error in Python 3 – Patrick Haugh Oct 05 '16 at 13:31
It is supported only in Python 3.5+. But since user has specified in the sample code that it is working. I edited question with the 3.5 tag – Moinuddin Quadri Oct 05 '16 at 13:32
@MoinuddinQuadri in python3 i've got SyntaxError: invalid syntax. – pm1359 Oct 05 '16 at 13:35
@PatrickHaugh : Check [What's new in Python 3.5](https://docs.python.org/dev/whatsnew/3.5.html#pep-448-additional-unpacking-generalizations) – Moinuddin Quadri Oct 05 '16 at 13:47
@MoinuddinQuadri I am using version 3.5.1, also in my question you can see i have used unpacking argument and it works. – pm1359 Oct 05 '16 at 14:12
thx, I have added the correct one in description, u should use as above with 3.5.1, u can't use iteritems here.. – pm1359 Oct 05 '16 at 14:16
error with this command is `ValueError: too many values to unpack (expected 2)`, by the way thx for your explanations – pm1359 Oct 05 '16 at 14:25

S. de Melo · Accepted Answer · 2016-10-05T13:37:47.093

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If you don't want to use del, you can replace it by .pop(key). For example, using unpacking argument too:

d = dict(d1, **d2)
d.pop("country")

Notice that .pop returns the value too (here "Japan").

edited Oct 05 '16 at 13:37

answered Oct 05 '16 at 13:34

S. de Melo

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what is its preference to del? – pm1359 Oct 05 '16 at 13:37
It is useful if you want to retrieve the value. But it must be slower. See http://stackoverflow.com/a/5713303/6919239 – S. de Melo Oct 05 '16 at 13:40

Merging two dic in python3 using Unpacking Generalisations in Python3.5

3 Answers3