io.StringIO vs open() in Python 3

Question

All I could find is this statement:

The easiest way to create a text stream is with open(), optionally specifying an encoding:

f = open("myfile.txt", "r", encoding="utf-8")

In-memory text streams are also available as StringIO objects:

f = io.StringIO("some initial text data")

But this gives no insight at all on when I should use open() over io.StringIO and vice-versa. I know that they do not work exactly the same behind the scene. But why would someone go for open() in Python 3 ?

Answer 1

The difference is: open takes a file name (and some other arguments like mode or encoding), io.StringIO takes a plain string and both return file-like objects.

Hence:

• Use open to read files ;
• Use StringIO when you need a file-like object and you want to pass the content of a string.

An example with StringIO:

import csv
import io

reader = csv.reader(io.StringIO("a,b,c\n1,2,3"))
print ([r for r in reader])
# output [['a', 'b', 'c'], ['1', '2', '3']]

It's very useful because you can use a string where a file was expected.

In the usual case, with a csv file on your disk, you would write something like:

with open(<path/to/file.csv>, ...) as f:
    reader = csv.reader(f, ...)