How to Capture Standard Output in Python?

Capturing Standard Output in Python

In Python, it's often useful to capture the output printed to the standard output (stdout) for various reasons, such as saving it for later processing or testing. One common scenario involves calling a function from a library, such as do_something(my_object), that prints information to stdout.

To capture the output of this function call, the following approach can be used:

1. Implement a Concatenating Context Manager (Capturing):

   • Define a class named Capturing that inherits from list.
   • Implement its __enter__ and __exit__ methods to capture stdout output.
   • In its __enter__ method, store the current stdout into self._stdout and redirect stdout to an in-memory StringIO buffer (self._stringio).
   • In its __exit__ method, append the captured output to its internal list (self) after restoring stdout to its original state.

2. Use the Context Manager:

   • Use a with statement to create an instance of Capturing.
   • Pass the instance as an argument to the __init__ method of Capturing to append its captured output to the instance created in the previous step.

3. Obtain the Captured Output:

   • After exiting the with block, the output variable now contains the captured stdout output as a list of strings.

Example Usage:

<code class="python">from contextlib import redirect_stdout  # redirect_stdout is only available in Python 3.4 or later

def do_something(my_object):
    print('stdout info')

with redirect_stdout(StringIO()) as stdout:
    do_something(my_object)
    captured_output = stdout.getvalue()</code>

Using the Python 3.4 syntax:

<code class="python">with StringIO() as output:
    with redirect_stdout(output):
        do_something(my_object)
    captured_output = output.getvalue()</code>

By following these steps, you can easily capture the stdout output from a Python function call, enabling you to utilize the printed information for further processing, debugging, or testing purposes.

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