Trying to pass parameters as functions in Python

Question

I have this:

class Student:
    def __init__(self, FirstName, LastName):
        self.FirstName = FirstName
        self.LastName = LastName

    def __iter__(self):                                            
        return self

Students = [
#       FirstName   LastName   
Student("Mary",     "Smith"),
Student("Joe",      "Jones"),    
Student("Jane",     "Doe"  )]

I make a Student class. And then I make a list of 3 Student objects. Then I can print a list of the names like this:

for item in Students:
    print item.FirstName, item.LastName

And it obediently outputs this:

Mary Smith
Joe Jones
Jane Doe

Then I googled how to sort a list of objects by an attribute and found this:

Students.sort(key = lambda x: x.LastName)

And it works. That line sort the list by the LastName attribute and I can print it the same way as above and the names are in order by LastName :

Jane Doe
Joe Jones
Mary Smith

But what I want to happen is have user inputs for what list to sort ( sortlist ) and by which attribute to sort by ( sortby ). So I tried:

def sort(sortlist, sortby):
    sortlist.sort(key = lambda x: x.sortby)

sort(Students, FirstName) 

And that doesn't work. First it says:

NameError: "name 'FirstName' is not defined"

Which is it's own problem… How do I tell it that FirstName is an attribute of the list I want it to sort?

Then I made FirstName a string just to try that…

AttributeError: "Student instance has no attribute 'sortby'"

Which I know there's no attribute sortby , that's supposed to be a parameter… but it can't use a parameter like that? Why not? How can I make it do this?

Answer 1

Python is actually looking for an attribute called 'sortby' which of course would not exist. Use the getattr builtin to fetch the attribute from the object using a string instead of the identifier.

def sort(sortlist, sortby):
    sortlist.sort(key = lambda x: getattr(x, sortby))

Answer 2

You can also use attrgetter from the operator module:

from operator import attrgetter
def sort(sortlist, sortby):
    sortlist.sort(key=attrgetter(sortby))