Python 3.4中的数据组织3

Question

In Python I am using this file Criteria.txt and it looks like this

Homework,5,10,10

Quiz,3,20,20

Midterm,1,50,30

Final,1,100,40

I typed in this code to open and manipulate the Criteria.txt file

criteria=open('Criteria.txt','r')
criteria_line=criteria.readline()



for criteria_line in criteria:
      print (criteria_line),
      criteria_myline = criteria_line.strip().split(',')
      print ("Assignment Type: "+criteria_myline[0])
      print ("Number: "+ criteria_myline [1])
      print ("Points: "+ criteria_myline [2])
      print ("Weight: "+ criteria_myline [3])
      print ()

The result was this

(None,)

Assignment Type: Quiz

Number: 3

Points: 20

Weight: 20

Midterm,1,50,30

(None,)

Assignment Type: Midterm

Number: 1

Points: 50

Weight: 30

Final,1,100,40

(None,)

Assignment Type: Final

Number: 1

Points: 100

Weight: 40

This was all correct except Homework didn't show up. You can see in the Criteria.txt file that Homework was the first assignment type in the list. What is wrong with this code? Why did it leave Homework out?

Answer 1

The line

criteria_line=criteria.readline()

reads the first line, the one containing Homework . In the following for loop, you immediately discard this first result.

Answer 2

This happens because you read the first line of the file before your for loop starts. This causes the cursor in the file object to move to the next line, so when you iterate through for criteria_line in criteria , you're excluding the very first line (the one that includes Homework ). If you remove the line

criteria_line = criteria.readline()

it should work.

This all works because a file object is an iterable and python will iterate over the lines in the file by default.

Answer 3

When you criteria_line=criteria.readline() , you've already consumed one line from your file object criteria which is Homework,5,10,10 , a fix is to read whenever you need the data in the for loop and as side note use with context opening file for better & safer coding:

with open('Criteria.txt') as criteria    
    for criteria_line in criteria:
        #print (criteria_line) #for debugging
        criteria_myline = criteria_line.strip().split(',')
        print ("Assignment Type: "+criteria_myline[0])
        print ("Number: "+ criteria_myline [1])
        print ("Points: "+ criteria_myline [2])
        print ("Weight: "+ criteria_myline [3])
        print ()