Python list to store class instance?

Question

Given a python class class Student(): and a list names = []; then I want to create several instances of Student() and add them into the list names,

names = [] # For storing the student instances
class Student():
    def __init__(self, score, gender):
        self.score = score
        self.gender = gender

And now I want to check out the scores of all the male students, can I do it like this?

scores = []
for i in names:
    if i.gender ==  "Male":
        scores.append(i.score)

My question is: How to create a list that can (if could be done by any statement) store the instance of Student? Or rather, when I write names = [], how could I state every element in names is an instance of Student so that I can use the attributs of this element despite python is weak type? I hope I made myself clear ;)

Can I write like:

    for i in range(len(names)):
        student = Student()
        student = names[i]
        if student.gender == "Male":
            # Whatever

I guess not...

Answer 1

Did you try your code above? It should work fine. You can condense it into:

scores = [ student.name for student in names if student.gender == "Male" ]

Note that calling the list names is misleading, since it is a list of Student instances.

You can't define the list to be a list of Student instances; that's not how Python works.

Are you asking how to create the list that you've called names?

names = [ ]
for ( score, gender ) in <some-data-source>:
    names.append( Student( score, gender ) )

which is of course equivalent to

names = [ Student( score, gender ) for score, gender in <some-data-source> ]

and in turn to

names = [ Student( *row ) for row in <some-data-source> ]

If you need to do a lot of processing for each row then you can either move the processing into a separate function or use a for loop.

def process_row( row ):
    ...
    return score, gender

names = [ Student( *process_row( row ) ) for row in <some-data-source> ]

---

Responding to your edit, I think you are trying to declare the types of variables in Python. You wrote:

for i in range(len(names)):
    student = Student()
    student = names[i]
    if student.gender == "Male":
        # Whatever

What is the purpose of the line student = Student() -- are you trying to declare the type of the variable student? Don't do that. The following will do what you intended:

for student in students:
   if student.gender == "Male":
       # Whatever

Notice several things:

1. We don't need to iterate over range(n) and then look up each instance in names; iterating over every element of a container is the purpose of a for loop.
2. You don't need to make any claims about what student is -- it could be a string, a boolean, a list, a Student, whatever. This is dynamic typing. Likewise, students doesn't have to be a list; you can iterate over any iterable.
3. When you write student.gender, Python will get the gender attribute of student, or raise an exception if it doesn't have one.

Answer 2

First of all python is not weakly typed. It is however dynamically typed so you can't specify an element type for your list.

However this does not prevent you from accessing an object's attributes. This works just fine:

names = [Student(1,"Male"), Student(2,"Female")]
scores = []
for i in names:
    if i.gender ==  "Male":
        scores.append(i.score)

It is however more pythonic to write this using a list comprehension:

names = [Student(1,"Male"), Student(2,"Female")]
scores = [i.score for i in names if i.gender == "Male"]