为什么我的代码读取CSV文件，打印字典而不是列表？

Question

import unicodecsv

def read_csv(filename):
    with open(filename, 'rb') as f:
        reader = unicodecsv.DictReader(f)
        return list(reader)

enrollments = read_csv('enrollments.csv')
daily_engagement = read_csv('daily-engagement.csv')
project_submissions = read_csv('project-submissions.csv')

In the previous lesson I was learning how to read a CSV file and convert it to an editable format. The code works great but when I was trying to understand the code, and I don't understand why, when I print enrollments[0] , that the result is a dictionary.

The first part of the function uses with open(filename, 'rb') as f: I understand that it is used to open the file.

The next part is reader = unicodecsv.DictReader(f) , I understand that it is used for mapping the information read, into a dictionary.

The third part is where I'm having difficulties understanding, the return list(reader) line. Doesn't this line mean that the reader variable is returned as a list() ?

So why it is returning a dictionary when I use print?

print enrollments[0]

{u'account_key': u'448',
 u'cancel_date': u'2015-01-14',
 u'days_to_cancel': u'65',
 u'is_canceled': u'True',
 u'is_udacity': u'True',
 u'join_date': u'2014-11-10',
 u'status': u'canceled'}

Answer 1

enrollments is a list

enrollments[0] is a dict

So enrollments is a list of dictionaries.

Everything worked as expected, read_csv returns a list (of dictionaties)

Answer 2

the second part reader = unicodecsv.DictReader(f) I understand that it is used for maping the information read into a dict.

You are close , but not quite correct.

DictReader() puts each row in the input CSV file into a dictionary. So you get a series of dictionaries, no a single dictionary. You got those rows by iterating , like using a for loop ( for row in reader: ... ). list() uses reader as an iterator to create a list of all the rows.

return list(reader) In here ain't I telling the computer to return the variable reader as a list?

Yes, you are passing the reader object to list() , which produces a list object from all the rows in the file . Each row is converted to a dictionary, so the end result is a list of dictionaries . You could see this as result = [] for row in reader: result.append(row) , and the result list is what is returned from the function.

You then take that list, and index the list with 0 , so you ask for the first entry in the list:

print enrollments[0]

This doesn't print the whole list, this prints one element from the list . And that one element is a dictionary because that's the data for the first row of data in the file.

If you wanted to print the list itself, use

print enrollment