CODEWITHSUNDEEP
pythonlistif-statementstring-formattingdictionary-comprehension
cwyjm
cwyjm

Reputation: 61

Dictionary returning only the Last Item in List Value

I have 2 lists as inputs:

A = ['A', 'B', 'C', 'D', 'E']
sessionid = [1, 1, 3, 2, 2]

I want to use dictionary comprehension without importing any libraries to generate the following output, with the key as 'Session x' by using string formatting:

{'Session 1': ['A','B'], 'Session 2': ['D','E'], 'Session 3': ['C']}

I tried with this code:

dictb = {'Session {}'.format(session): [a if session not in dictb.keys() else [a]] for a,session in list(zip(A,sessionid))}

but it returns me this:

{'Session 1': ['B'], 'Session 3': ['C'], 'Session 2': ['E']}

I think there is a problem with if-else statement in the comprehension. How should I improve this?

Upvotes: 1

Views: 557

Answers (2)

K1DV5
K1DV5

Reputation: 61

This is because you're generating a dictionary with string keys that have the format Session{} and checking if the dictionary has integer keys, which it always doesn't and thus takes the part after else. When the same integer comes, it updates the value and that's why you're seeing the last updated value. I wouldn't recommend doing what you want using a dict comprehension, for both performance and readability reasons but if you really want to, try (Not tested):

dictb = {'Session {}'.format(session): [dictb['Session {}'.format(session)] + [a] if 'Session {}'.format(session) not in dictb.keys() else [a]] for a,session in list(zip(A,sessionid))}

The recommended approach would be:

dictb = {}
for a, session in zip(A, sessionid):
    key = 'Session {}'.format(session)
    if key in dictb:
        dictb[key].append(a)
    else:
        dictb[key] = [a]

Upvotes: 0

Riccardo Bucco
Riccardo Bucco

Reputation: 15364

Here is a possible solution:

dictb = {f'Session {k}': [v for v, i in zip(A, sessionid) if k == i]
         for k in sorted(set(sessionid))}

Output:

{'Session 1': ['A', 'B'], 'Session 2': ['D', 'E'], 'Session 3': ['C']}

Upvotes: 1

Related Questions