How to capture output with `%%capture` in IPython

Using `%%capture` to manage output in IPython notebooks.

In IPython, the %%capture cell magic is a powerful tool for controlling the output of a cell. It can either suppress or store the output, including standard output (stdout), standard error (stderr), and even rich display outputs like plots. Here's a guide on how to use %%capture effectively.

Basics of %%capture

1. Suppressing Output: By default, %%capture suppresses the output from a cell. This is useful for hiding output that is not needed.

%%capture
print('hi, stdout')
print('hi, stderr', file=sys.stderr)

2. Storing Output You can store the output in a variable by specifying a name after %%capture.

%%capture captured
print('hi, stdout')
print('hi, stderr', file=sys.stderr)

captured

<IPython.utils.capture.CapturedIO at 0x2c06a2914>

Retrieving and Using Captured Output

Accessing Captured Output: After capturing the output in a variable, you can access it:

# Display captured output
captured()

hi, stdout
hi, stderr

Access stdout and stderr separately:

print(captured.stdout)

hi, stdout

print(captured.stderr)

hi, stderr

Captured Output Object: The captured output is stored in an IPython.utils.capture.CapturedIO object, allowing separate access to stdout and stderr.

Advanced Uses of %%capture

Capturing Rich Output: %%capture is not limited to text output; it can also capture rich outputs like plots.

%%capture capture

print("setting up X")
x = np.linspace(0,5,1000)
y = np.sin(x)
print("hello")
plt.plot(x,y)

captured2()

hello

Selective Capturing: You can choose to capture only stdout, stderr, or rich display outputs by using flags like --no-stdout, --no-stderr, and --no-display.

%%capture cap --no-stderr
print('hi, stdout')
print("hello, stderr", file=sys.stderr)

Summary

%%capture in IPython is a versatile tool for controlling cell output. It's useful for suppressing unwanted output, storing output for later use, and even capturing rich display outputs like plots. By understanding how to use %%capture effectively, you can manage the output of your Jupyter notebooks more efficiently, making them cleaner and more focused on the results that matter.