There is a dict,
dlist = [{'Bilbo' : 'Ian', 'Frodo' : 'Elijah'}, {'Bilbo' : 'Martin', 'Thorin' : 'Richard'}]
And let k = 'Frodo'
I want to extract the value corresponding to k when it exists and make a list.
I wrote
value_list = [dlist[i][k] for i in range(len(dlist)) if k in dlist[i] else "NOT PRESENT"].
But computer says else is wrong. I don't know why.
When you have else part in the list comprehension , bring it before the for loop , Example -
value_list = [dlist[i][k] if k in dlist[i] else "NOT PRESENT" for i in range(len(dlist))]
value_list
>>> ['Elijah', 'NOT PRESENT']
if after the for in a list comprehension is for filtering the list: when the condition is false you get no element at all.
if..else before the list comprehension is simply a ternary operator, no different than in any other expression.
A couple of other points: for i in range(len(...)) is almost always wrong in Python. If you are only iterating over a single list just iterate over that.
Also, dictionaries have a method .get() that you can use to avoid the if altogether. So:
value_list = [d.get(k, "NOT PRESENT") for d in dlist]
or if you prefer the if:
value_list = [d[k] if k in d else "NOT PRESENT" for d in dlist]
dlist[i][k] for i in range(len(dlist)) to just be d[k] for d in dlist, since d will refer to each element of the list as it iterates, rather than using i as an index.
[dlist[i][k] if k in dlist[i] else "NOT PRESENT" for i in range(len(dlist)) ].