Want to automate testing of the actual console input & output of your user-facing components?

stdio-mgr is a context manager for mocking/managing all three standard I/O streams: stdout, stderr, and stdin. While some functionality here is more or less duplicative of redirect_stdout and redirect_stderr in contextlib within the standard library, it provides (i) a much more concise way to mock both stdout and stderr at the same time, and (ii) a mechanism for mocking stdin, which is not available in contextlib.

First, install:

$ pip install stdio-mgr

Then use!

All of the below examples assume stdio_mgr has already been imported via:

from stdio_mgr import stdio_mgr

Mock stdout:

>>> with stdio_mgr() as (in_, out_, err_): ... print('foobar') ... out_cap = out_.getvalue() >>> out_cap 'foobar\n' >>> in_.closed and out_.closed and err_.closed True

By default print appends a newline after each argument, which is why out_cap is 'foobar\n' and not just 'foobar'.

As currently implemented, stdio_mgr closes all three mocked streams upon exiting the managed context.

Mock stderr:

>>> import warnings >>> with stdio_mgr() as (in_, out_, err_): ... warnings.warn("'foo' has no 'bar'") ... err_cap = err_.getvalue() >>> err_cap '... UserWarning: \'foo\' has no \'bar\'\n...'

Mock stdin:

The simulated user input has to be pre-loaded to the mocked stream. Be sure to include newlines in the input to correspond to each mocked Enter keypress! Otherwise, input will hang, waiting for a newline that will never come.

If the entirety of the input is known in advance, it can just be provided as an argument to stdio_mgr. Otherwise, .append() mocked input to in_ within the managed context as needed:

>>> with stdio_mgr('foobar\n') as (in_, out_, err_): ... print('baz') ... in_cap = input('??? ') ... ... _ = in_.append(in_cap[:3] + '\n') ... in_cap2 = input('??? ') ... ... out_cap = out_.getvalue() >>> in_cap 'foobar' >>> in_cap2 'foo' >>> out_cap 'baz\n??? foobar\n??? foo\n'

The _ = assignment suppresses printing of the return value from the in_.append() call—otherwise, it would be interleaved in out_cap, since this example is shown for an interactive context. For non-interactive execution, as with unittest, pytest, etc., these 'muting' assignments should not be necessary.

Both the '??? ' prompts for input and the mocked input strings are echoed to out_, mimicking what a CLI user would see.

A subtlety: While the trailing newline on, e.g., 'foobar\n' is stripped by input, it is retained in out_. This is because in_ tees the content read from it to out_ before that content is passed to input.

In addition to mocking, stdio_mgr can also be used to wrap functions that directly output to stdout/stderr. A stdout example:

>>> def emboxen(func): ... def func_wrapper(s): ... from stdio_mgr import stdio_mgr ... ... with stdio_mgr() as (in_, out_, err_): ... func(s) ... content = out_.getvalue() ... ... max_len = max(map(len, content.splitlines())) ... fmt_str = '| {{: <{0}}} |\n'.format(max_len) ... ... newcontent = '=' * (max_len + 4) + '\n' ... for line in content.splitlines(): ... newcontent += fmt_str.format(line) ... newcontent += '=' * (max_len + 4) ... ... print(newcontent) ... ... return func_wrapper >>> @emboxen ... def testfunc(s): ... print(s) >>> testfunc("""\ ... Foo bar baz quux. ... Lorem ipsum dolor sit amet.""") =============================== | Foo bar baz quux. | | Lorem ipsum dolor sit amet. | ===============================

Available on PyPI (pip install stdio-mgr).

Source on GitHub. Bug reports and feature requests are welcomed at the Issues page there.

Copyright (c) 2018-2025 Brian Skinn

The stdio-mgr documentation (currently docstrings and README) is licensed under a Creative Commons Attribution 4.0 International License (CC-BY). The stdio-mgr codebase is released under the MIT License. See LICENSE.txt for full license terms.