python 如何跟踪可变数据类型？

Question

I am mostly familiar with Java and am in the process of learning Python. I have been playing around with OOP in Python and am slightly confused how it keeps track of data types. What I am confused about is the get_average_grade() function in the image below. In the for loop how does Python know that student is an object of type Student? Note: the list self.students was assigned to be an empty list in the class' init .

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Answer 1

Python variables don't actually have data types. Python variables are simply symbolic names for a reference (or pointer) to an object. That object has a value (for example 23 or "cat") and a type (for example str or int or Student ). A name can be associated with an int one second and with a Student the next second.

You can call get_grade() on any variable. If that variable doesn't have a get_grade() method then the result is an AttributeError exception.

Some helpful background that all Python programmers should read:

• python variables
• duck typing
• names and values

Answer 2

Python is smart enough to know what type you want a variable to be when you assign it. Check this out.

student = 'Hi Mom!'
print (student)

student = 7
print (student)

student = [1, 2, 3]
print (student)

At any time 'student' can be a string, integer, list, dictionary etc, etc...