Python: Dictionary merge by updating but not overwriting if value exists

问题

If I have 2 dicts as follows:

d1 = {('unit1','test1'):2,('unit1','test2'):4}
d2 = {('unit1','test1'):2,('unit1','test2'):''}

In order to 'merge' them:

z = dict(d1.items() + d2.items())
z = {('unit1','test1'):2,('unit1','test2'):''}

Works fine. Additionally what to be done, if i would like to compare each value of two dictionaries and only update d2 into d1 if values in d1 are empty/None/''?

[EDIT] Question: When updating d2 into d1, when the same key exists, I would like to only maintain the numerical value (either from d1 or d2) instead of empty value. If both values are empty, then no problems maintaining empty value. If both have values, then d1-value should stay. :) (lotsa if-else .. i'd try myself in the meantime)

i.e.

d1 = {('unit1','test1'):2,('unit1','test2'):8,('unit1','test3'):''}
d2 = {('unit1','test1'):2,('unit1','test2'):'',('unit1','test3'):''}

#compare & update codes

z = {('unit1','test1'):2,('unit1','test2'):8, ('unit1','test2'):''} # 8 not overwritten by empty.

please help to suggest.

Thanks.

回答1:

Just switch the order:

z = dict(d2.items() + d1.items())

By the way, you may also be interested in the potentially faster update method.

In Python 3, you have to cast the view objects to lists first:

z = dict(list(d2.items()) + list(d1.items()))

If you want to special-case empty strings, you can do the following:

def mergeDictsOverwriteEmpty(d1, d2):
    res = d2.copy()
    for k,v in d2.items():
        if k not in d1 or d1[k] == '':
            res[k] = v
    return res

回答2:

Python 2.7. Updates d2 with d1 key/value pairs, but only if d1 value is not None,'' (False):

>>> d1 = dict(a=1,b=None,c=2)
>>> d2 = dict(a=None,b=2,c=1)
>>> d2.update({k:v for k,v in d1.iteritems() if v})
>>> d2
{'a': 1, 'c': 2, 'b': 2}

回答3:

To add to d2 keys/values from d1 which do not exist in d2 without overwriting any existing keys/values in d2:

temp = d2.copy()
d2.update(d1)
d2.update(temp)

回答4:

d2.update(d1) instead of dict(d2.items() + d1.items())

回答5:

Here's an in-place solution (it modifies d2):

# assumptions: d2 is a temporary dict that can be discarded
# d1 is a dict that must be modified in place
# the modification is adding keys from d2 into d1 that do not exist in d1.

def update_non_existing_inplace(original_dict, to_add):
    to_add.update(original_dict) # to_add now holds the "final result" (O(n))
    original_dict.clear() # erase original_dict in-place (O(1))
    original_dict.update(to_add) # original_dict now holds the "final result" (O(n))
    return

Here's another in-place solution, which is less elegant but potentially more efficient, as well as leaving d2 unmodified:

# assumptions: d2 is can not be modified
# d1 is a dict that must be modified in place
# the modification is adding keys from d2 into d1 that do not exist in d1.

def update_non_existing_inplace(original_dict, to_add):
    for key in to_add.iterkeys():
        if key not in original_dict:
            original_dict[key] = to_add[key]

回答6:

In case when you have dictionaries with the same size and keys you can use the following code:

dict((k,v if k in d2 and d2[k] in [None, ''] else d2[k]) for k,v in d1.iteritems())

来源：https://stackoverflow.com/questions/6354436/python-dictionary-merge-by-updating-but-not-overwriting-if-value-exists

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