How to convert lists of class objects to a list of their attributes

user_10713538 0
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guys! So I recently started learning about python classes and objects. For instance, I have a following list of strings:

alist = ["Four", "Three", "Five", "One", "Two"]

Which is comparable to a class of Numbers I have:

class Numbers(object):
   One=1
   Two=2
   Three=3
   Four=4
   Five=5

How could I convert alist into 

alist = [4, 3, 5, 1, 2]

based on the class above?

My initial thought was to create a new (empty) list and use a for loop that adds the corresponding object value (e.g. Numbers.One) to the empty list as it goes through alist. But I'm unsure whether that'd be the most efficient solution.

Therefore, I was wondering if there was a simpler way of completing this task using Python Classes / Inheritance.

I hope someone can help me and explain to me what way would work better and why!

Thank you!!

inheritance class python
4 Answers

If you are set on using the class, one way would be to use __getattribute__()

print([Numbers().__getattribute__(a) for a in alist])
#[4, 3, 5, 1, 2]

But a much better (and more pythonic IMO) way would be to use a dict:

NumbersDict = dict(
    One=1,
    Two=2,
    Three=3,
    Four=4,
    Five=5
)
print([NumbersDict[a] for a in alist])
#[4, 3, 5, 1, 2]
user_58588510

Most objects (and hence classes) in python have the __dict__ field, which is a mapping from attribute names to their values. You can access this field using the built-in vars, so

values = [vars(Numbers)[a] for a in alist]

will give you what you want.

user_9891210

While I totally agree that using a dict for Numbers would be easier and straight forward, but showing you the Enum way as your class involves magic numbers and sort of a valid use case for using enums.

A similar implementation using Enum would be:

from enum import Enum

class Numbers(Enum): 
    One = 1 
    Two = 2 
    Three = 3 
    Four = 4 
    Five = 5

Then you can use getattr and Numbers.<attr>.value to get the constant numbers:

In [592]: alist = ["Four", "Three", "Five", "One", "Two"]                                                                                                                                                   

In [593]: [getattr(Numbers, n).value for n in alist]                                                                                                                                                        
Out[593]: [4, 3, 5, 1, 2]

Edit based on comment:

If you want to get the names back from a number list:

In [952]: l = [4, 3, 5, 1, 2]                                                                                                                                                                               

In [953]: [Numbers(num).name for num in l]                                                                                                                                                                  
Out[953]: ['Four', 'Three', 'Five', 'One', 'Two']
user_37895500

EDIT: I suppose that the words and numbers are just a trivial example, a dictionary is the right way to do it if that's not the case as written in the comments.

Your assumptions are correct - either create an empty list and populate it using for loop, or use list comprehension with a for loop to create a new list with the required elements.

Empty list with for loop

#... Numbers class defined above
alist = ["Four", "Three", "Five", "One", "Two"]
nlist = []
numbers = Numbers()
for anumber in alist:
    nlist.append(getattr(numbers, anumber))

print(nlist)
[4, 3, 5, 1, 2]

List comprehension with for loop

#... Numbers class defined above
alist = ["Four", "Three", "Five", "One", "Two"]
numbers = Numbers()

nlist = [getattr(numbers, anumber) for anumber in alist]

print(nlist)
[4, 3, 5, 1, 2]
Admin1
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