File IO

Usually the result of your program is important not only now but in the future as well. For this it is a good practice to store the results in file so that you can work with them later on if necessary. This is called file input and output, or in short: file IO.

Reading from a file

The best way to open a file in Python is by using the with statement. This automatically opens the file, keeps a lock on it so that no one is able to modify it while you are working with it, and closes it afterwards. Interacting with the file is then done by working with the variable that you specify after as. This is called a file object. To get the complete content of a file use its read() method like this:

>>> with open('csv_file.txt', 'r') as f:
...     file_content = f.read()
...
>>> print(file_content)
1, 2, 3
4, 5, 6
7, 8, 9

where the first parameter is the path to the file and the second parameter is the so-called file mode. r is for read-only.

Note

In some tutorials and also production code you may find something along the lines of this to interact with a file:

>>> f = open('csv_file.txt', 'r')
>>> file_content = f.read()
>>> f.close()
>>> print(file_content)
1, 2, 3
4, 5, 6
7, 8, 9

This is kind of the old style of working with files when the with statement did not exist yet. It has the huge downside that you have to take care about closing the file. And if an error is raised between open and close it is not closed at all. The with statement takes care of this for you even in the case of an exception and is subsequently the preferred way.

You can also iterate over f as if it is some form of container:

>>> with open('csv_file.txt', 'r') as f:
...     for i, line in enumerate(f):
...         print('Line', i, '--', line)
...
Line 0 -- 1, 2, 3

Line 1 -- 4, 5, 6

Line 2 -- 7, 8, 9

As you can see you get some additional white lines. The reason for this is that each line still contains its newline character \n in the end. To this one print() adds an additional one by default so you end up with an empty line. To circumvent this use the strip() method of the line string:

>>> with open('csv_file.txt', 'r') as f:
...     for i, line in enumerate(f):
...         stripped_line = line.strip()
...         print('Line', i, '--', stripped_line)
...
Line 0 -- 1, 2, 3
Line 1 -- 4, 5, 6
Line 2 -- 7, 8, 9

Writing to a file

To write to a file you have to open it first, this time with w as file opening mode, to indicate that you want to write to the file. Then you can use the write() method of the file object to write to the file:

>>> with open('my_first_file.txt', 'w') as f:
...     f.write('This is smart.')
...     f.write('This is even smarter.')
...
14
21

Now the content of your file would be

This is smart.This is even smarter.

Which is not nicely formatted. So you have to take care that you add the newline character \n and spaces accordingly:

>>> with open('my_first_file.txt', 'w') as f:
...     f.write('This is smart.\n')
...     f.write('This is even smarter.\n')
...
15
22

Subsequently the content of your file would be

This is smart.
This is even smarter.

Summary

• You can open a file using the open() function in a with statement.
• To merely read from a file open it with the filemode r and use the read() method of the file object.
• To write from a file open it with the filemode w and use the write method of the file object.