print() function is typically used to display text either in the command-line or in the interactive interpreter, depending on how the Python program is executed. However, we can change its behavior to write text to a file instead of to the console.
In this article, we'll examine the many ways we can write to a file with the
Redirecting a Python's Script Output in the Terminal
The quick and dirty way to redirect a Python script's output is directly from the command-line while executing the script.
For example, if we had a Python file called
hello.py with the following contents:
print("Hallo") # Deliberately in German
We can redirect the output of the file in the shell using a single right angle bracket:
$ python3 hello.py > output.txt
If we open our newly created
output.txt, we'll see the following contents:
However, with this method, all output of the script is written to a file. It is often more flexible to perform this redirection from within the Python script itself.
Redirecting the Standard Output Stream
In Python, the
print() function is more flexible than you might think. It was not hard-coded in such a way that specified text can only be written to the display. Instead, it sends text to a location called the standard output stream, also known as
All UNIX systems have three main pipes - standard input pipe (
stdin), standard output pipe (
stdout) and standard error pipe (
By default, the standard output pipe points to the interactive window used to execute the program, so we normally see text printed out on the screen. However, the standard output can be redirected to other locations, such as files, for convenience.
If the standard output is redirected to a specific file, the text specified in the
print() function will be written to that file instead of being displayed on the screen.
In Python, a reference to the standard output can be obtained using the
stdout object of the
sys module. It is a file-like object, meaning it has methods that allow Python to read and write from it like an actual file.
Let's see an example where we change
stdout to be a file:
import sys print('This message will be displayed on the screen.') original_stdout = sys.stdout # Save a reference to the original standard output with open('filename.txt', 'w') as f: sys.stdout = f # Change the standard output to the file we created. print('This message will be written to a file.') sys.stdout = original_stdout # Reset the standard output to its original value
print() function takes the supplied string argument, appends a newline character to the end, and calls the
stdout.write() method to write it to standard output.
In the example above, we first print a line of text as we're accustomed to, which will be displayed in the console when we run the file. We then reassigned
stdout to our custom file object -
f. Since a file object has a perfectly valid
write() method, our printed value gets written to the file without a problem.
Note that it is good practice to store the original value of the standard output in a variable before changing it. This way we can reset the standard output to its original value after we're done, which can help avoid confusion.
Let's save the code to a new file,
printToFile.py. And then, let's execute it:
$ python3 printToFile.py
We'll see the following output in the Terminal:
This message will be displayed on the screen.
And the script will create a new file called
filename.txt with the following contents:
This message will be written to a file.
You successfully redirected data from the standard output stream to a file. Let's see how we can do this to another popular file-like object that's dedicated to programming errors.
Redirecting the Standard Error Stream
In Python, errors are written to the standard error stream, also known as
stderr. This also defaults to the interactive window but can be changed via the
sys.stderr object. If we wanted to print values to the
stderr, we could simply redirect the
sys.stdout to point to the
Create a file called
printToStderr.py and add the following code:
import sys print('This message will be displayed via standard output.') original_stdout = sys.stdout # Save a reference to the original standard output sys.stdout = sys.stderr # Redirect the standard output to the standard error. print('This message will be displayed via the standard error.') sys.stdout = original_stdout # Reset the standard output to its original value
This example is almost identical to the previous one, except that instead of redirecting the standard output stream to a file, we redirect it to the standard error stream. If the standard error stream was also redirected somewhere else, the output would be sent to that location instead of to the screen.
Let's run this file:
$ python3 printToStderr.py
Our output would show:
This message will be displayed via standard output. This message will be displayed via the standard error.
While it may appear like regular output to us, for the computer it is displayed through different pipelines.
Print Using the "file" Parameter
In the previous examples, we explicitly redirected the standard output to another file-like object by changing the
stdout object. However, for convenience we can do this directly from within the
print() function by specifying the output location with the
For example, if we wanted to print directly to a file without changing the entire script's
stdout, we would write:
import sys print('This message will be displayed on the screen.') with open('filename.txt', 'w') as f: print('This message will be written to a file.', file=f)
As we did not fiddle with redirecting the standard output explicitly, we no longer have to reset it to its initial value. As a result, this is the preferred way to write to a file with the
Note: Although the parameter's name is
file, remember that it works with any file-like object. If you wanted to print to
stderr, for example, you would change the
print() statement to:
print('This message will be written to stderr.', file=sys.stderr)
In this article, we discussed redirecting Python's
print() function output using various methods. These methods included redirecting the output of a Python script from the command-line, redirecting the standard output within Python scripts, and specifying a file-like object in the
file parameter directly in the
About the Author