stdio-mgr: Context manager for mocking/wrapping stdin/stdout/stderr

Current Development Version:

Most Recent Stable Release

Info

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Have a CLI Python application?

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

stdio-mgr can help

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.

Want to modify internal print calls within a function or method?

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. |
===============================

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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.