Select value from list of tuples where condition

Question

I have a list of tuples. Every tuple has 5 elements (corresponding to 5 database columns) and I'd like to make a query

select attribute1 from mylist where attribute2 = something

e.g.

personAge = select age from mylist where person_id = 10

Is it possible to query the list of tuples in some way?

Balikpapan

Answer 1

If you have named tuples you can do this:

results = [t.age for t in mylist if t.person_id == 10]

Otherwise use indexes:

results = [t[1] for t in mylist if t[0] == 10]

Or use tuple unpacking as per Nate's answer. Note that you don't have to give a meaningful name to every item you unpack. You can do (person_id, age, _, _, _, _) to unpack a six item tuple.

Jurassic

Answer 2

One solution to this would be a list comprehension, with pattern matching inside your tuple:

>>> mylist = [(25,7),(26,9),(55,10)]
>>> [age for (age,person_id) in mylist if person_id == 10]
[55]

Another way would be using map and filter:

>>> map( lambda (age,_): age, filter( lambda (_,person_id): person_id == 10, mylist) )
[55]

Stephanestephani

Answer 3

Yes, you can use filter if you know at which position in the tuple the desired column resides. If the case is that the id is the first element of the tuple then you can filter the list like so:

filter(lambda t: t[0]==10, mylist)

This will return the list of corresponding tuples. If you want the age, just pick the element you want. Instead of filter you could also use list comprehension and pick the element in the first go. You could even unpack it right away (if there is only one result):

[age] = [t[1] for t in mylist if t[0]==10]

But I would strongly recommend to use dictionaries or named tuples for this purpose.

Shadchan

But what if the OP wants to use a different qualifier? E.g., a range instead of a value? This would preclude the use of dictionaries. – Stephanestephani

@Stephanestephani As I undrestood it he has a list of homogenous tuples, e.g: [(1,'name', 20, 'bar'),(2,'name2', 33, 'foo'), ...] and if I would want to work with this I would map each tuple into dictionary, e.g: [{'id': 1, 'name': 'name', 'age': 20, 'etc': 'bar'}, ..]. Querying it would became much more readable. If I only query by the id I could even use it as a dictionary key (so it would be a dictionary of dictionaries). – Shadchan

I guess I meant that if he wanted to select where person_id > 5 and person_id < 15 or something like that, dictionaries are ill-equipped to perform this kind of search; Additionally, what if he wanted to select by age instead? You'd have to perform some sort of a dictionary comprehension in both of those cases. – Stephanestephani

In the case of list of dictionaries it would be simple as [t for t in data where t['id'] > 5 and t['id'] < 15]. You can select basically anything this way (as long as it takes only one line into the perspective). – Shadchan

Answer 4

Building on Nate's answer: If your list has tuples with non-unique values, this might produce a boolean ValueError (truth value of a Series is ambiguous). To circument that:

mylist = [(25,7),(26,9),(55,10)]
[age for (age,person_id) in mylist if any(person_id == 10)]

Obivously for a "person_id" this less intuitive, but imagine you switch the query: finding all person_IDs for people of a given age.

Statehood