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pythonpython-3.xdictionarycopying
aretor
aretor

Reputation: 2569

copying a dict leaving out one (key, val) pair

In Python 3, how do I copy a dictionary leaving out just one element? Data sharing among the two dicts is fine for me. Currently I have this code in mind:

def copy_leaving_out(dictionary, key):
    return {k: v for k, v in dictionary if k != key}

Is there a better way to achieve this?

EDIT: I forgot to use dictionary.items() instead of dictionary, please consider the following code instead of the previous one:

def copy_leaving_out(dictionary, key):
    return {k: v for k, v in dictionary.items() if k != key}

Upvotes: 0

Views: 251

Answers (3)

Eugene Yarmash
Eugene Yarmash

Reputation: 149776

Using a dictionary comprehension is fine (and pythonic). However, to iterate over the key/value pairs you need to call dictionary.items():

def copy_leaving_out(dictionary, key):
    return {k: v for k, v in dictionary.items() if k != key}

If you want to be more explicit, you can also use dictionary.copy() to create a shallow copy of the dictionary, then remove the needed key:

def copy_leaving_out(dictionary, key):
    copy = dictionary.copy()
    del copy[key]
    return copy

Performance-wise, the second version appears to be noticeably faster, probably because it doesn't involve key comparisons:

In [14]: d = {k: k for k in range(200)}

In [15]: %timeit copy_leaving_out_dc(d, 100)
13.9 µs ± 724 ns per loop (mean ± std. dev. of 7 runs, 100000 loops each)

In [16]: %timeit copy_leaving_out_copy(d, 100)
738 ns ± 4.45 ns per loop (mean ± std. dev. of 7 runs, 1000000 loopseach)

Upvotes: 5

Alex Tereshenkov
Alex Tereshenkov

Reputation: 3620

I don't think you need to use a dictionary comprehension. It would suffice just to drop this key. 

d = {1: 10, 2:20, 3:30}
res = d.copy()
res.pop(1)
10
res
{2: 20, 3: 30}

Upvotes: 1

cph_sto
cph_sto

Reputation: 7585

Use items() functions in dictionary comprehensions to get key and value and finally using if clause -

d={'a':1,'b':2,'c':3}
key_remove = 'a'
d_out = {k:v for k,v in d.items() if k != key_remove }
print(d_out)
    {'b': 2, 'c': 3}

Upvotes: 1

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