Add an element in each dictionary of a list (list comprehension)

Question

I have a list of dictionaries, and want to add a key for each element of this list. I tried:

result = [ item.update({"elem":"value"}) for item in mylist ] 

but the update method returns None, so my result list is full of None.

result = [ item["elem"]="value" for item in mylist ] 

returns a syntax error.

Answer 1

If you want to use list comprehension, there's a great answer here: https://stackoverflow.com/a/3197365/4403872

In your case, it would be like this:

result = [dict(item, **{'elem':'value'}) for item in myList] 

Eg.:

myList = [{'a': 'A'}, {'b': 'B'}, {'c': 'C', 'cc': 'CC'}] 

Then use either

result = [dict(item, **{'elem':'value'}) for item in myList] 

or

result = [dict(item, elem='value') for item in myList] 

Finally,

>>> result [{'a': 'A', 'elem': 'value'},  {'b': 'B', 'elem': 'value'},  {'c': 'C', 'cc': 'CC', 'elem': 'value'}] 

Answer 2

You don't need to worry about constructing a new list of dictionaries, since the references to your updated dictionaries are the same as the references to your old dictionaries:

 for item in mylist:     item.update( {"elem":"value"})