输出格式化Python 3.3

Question

So this is probably a very basic question about output formatting in python using '.format' and since I'm a beginner, I can't figure this out for the life of me. I've tried to be as detailed as possible, just to make sure that there's no confusion.

Let me give you an example so that you can better understand my dilemma. Consider the following program

list = (['wer', 'werwe', 'werwe' ,'wer we'])   # list[0], list[1], list[2], list[3]
list.append(['vbcv', 'cvnc', 'bnfhn', 'mjyh']) # list[4]
list.append(['yth', 'rnhn', 'mjyu', 'mujym'])  # list[5]
list.append(['cxzz', 'bncz', 'nhrt', 'qweq'])  # list[6]

first = 'bill'
last = 'gates'

print ('{:10} {:10} {:10} {:10}'.format(first,last,list[5], list[6]))     

Understandably that would give the output:

bill       gates      ['yth', 'rnhn', 'mjyu', 'mujym'] ['cxzz', 'bncz', 'nhrt', 'qweq']

So here's my real question. I was doing this practice problem from the book and I don't understand the answer. The program below will give you a good idea of what kind of output we are going for:

students = []
students.append(['DeMoines', 'Jim', 'Sophomore', 3.45]) #students[0]
students.append(['Pierre', 'Sophie', 'Sophomore', 4.0]) #students[1]
students.append(['Columbus', 'Maria', 'Senior', 2.5])   #students[2]
students.append(['Phoenix', 'River', 'Junior', 2.45])   #students[3]
students.append(['Olympis', 'Edgar',  'Junior', 3.99])  #students[4]
students.append(['van','john', 'junior', 3.56])         #students[5]
def Grades(students):
    print ('Last       First      Standing        GPA')
    for students in students:
        print('{0:10} {1:10} {2:10} {3:8.2f}'.format(students[0],students[1],students[2],students[3]))

The output we're trying to get is a kind of a table that gives all the stats for all the students -

Last       First      Standing        GPA
DeMoines   Jim        Sophomore      3.45
Pierre     Sophie     Sophomore      4.00
Columbus   Maria      Senior         2.50
Phoenix    River      Junior         2.45
Olympis    Edgar      Junior         3.99
van        john       junior         3.56

So here's what I don't understand. We are working with basically the same thing in the two examples ie a list inside a list. For my first example, the print statement was:

print('{:10} {:10} {:10} {:10}'.format(first, last, list[5], list[6]))  

where list[5] and list[6] are lists themselves and they are printed in entirety, as you can see from the output. But that doesn't happen in the book problem . There, the print statement says

print('{0:10} {1:10} {2:10} {3:8.2f}'.format(students[0], students[1], students[2], students[3])) 

As you can see from the table output, here students[0] refers only to 'DeMoines' . But if you just run the statement students[0] in the Python interpreter, it gives the whole sub list, as it should.

['DeMoines', 'Jim', 'Sophomore', 3.45]

So, basically, I've got two questions, why does students[0] have two different meanings and why does students[0] not print the whole list like we did with list[5] and list[6] ?

Answer 1

Look at the for loop :

for students in students:
#   ^^^^^^^^

So, students (inside loop) does not actually refers to list of list . And students[0] refers to first element from element from list of lists , as expected.

I suggest replace students from function argument, say, with all_students or something like that.

Answer 2

Try renaming the variable list into something that's not a reserved word or built-in function or type.

What's confusing to beginners - and it happens to everyone sooner or later - is what happens if you redefine or use in unintended ways a reserved word or a builtin.

If you do

list = [1, 2, 3, 4]

you re-bind the name list to no longer point to the builtin list data type but to the actual list [1, 2, 3, 4] in the current scope. That is almost always not what you intend to do. Using a variable dir is a similar pitfall.

Also do not use additional parantheses () around the square brackets of the list assignment. Something like words = ['wer', 'werwe', 'werwe' ,'wer we'] suffices.

Generally consider which names you choose for a variable. students is descriptive, helpful commentary, list is not. Also if list currently holds a list, your algorithm might be changed later on with the variable holding a set or any other container type. Then a type-based variable name will be even misleading.