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> ]

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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.