Python的csv.reader（filename）真的会返回一个列表吗？ 似乎不是这样

Question

So I am still learning Python and I am on learning about reading files, csv files today. The lesson I just watched tells me that using

csv.reader(filename)

returns a list.

So I wrote the following code:

import csv
my_file = open(file_name.csv, mode='r')
parsed_data = csv.reader(my_file)
print(parsed_data)

and what it prints is

<_csv.reader object at 0x0000000002838118>

If what it outputs is a list, shouldn't I be getting a list, ie, something like this?

[value1, value2, value3]

Answer 1

What you get is an iterable , ie an object which will give you a sequence of other objects (in this case, strings). You can pass it to a for loop, or use list() to get an actual list:

parsed_data = list(csv.reader(my_file))

The reason it is designed this way is that it allows you to work with files that are larger than the amount of memory you have on your computer (or simply files that are large enough to consume inconvenient amounts of memory if you were to load all of its contents into a list). With an iterable, you may choose to look at one element at a time and eg throw it out of memory again before reading the next.

Answer 2

The return value of the csv.reader is an iterator (reader object).

You need to iterate it, to get lists:

import csv
my_file = open(file_name.csv, mode='r')
parsed_data = csv.reader(my_file)
for row in parsed_data:
    print(row)   # <--- a list of strings