When you just create a magick mock and then call `__str__` on it, it never
throws an exception and always return a string.
But... when you create a magick mock and then call `__str__` from two
threads concurrently, one of them will raise an exception.

Race is as follows:
1. Thread 1 creates magick mock m.
2. Thread 1 enters `m.__str__` it starts some internal preparations. It
does not exit `__str__` yet.
3. Thread 2 enters `m.__str__`.
In effect one of this threads will try to return `MagickMock` instance
instead of `str` object, which if forbidden by python.
Some low-level python check will raise following exception: `TypeError:
__str__ returned non-string (type MagicMock)`.

Stanisław Skonieczny

On Mon, Feb 22, 2021 at 10:56 AM Irit Katriel <report@bugs.python.org>
wrote:

>
> Irit Katriel <iritkatriel@yahoo.com> added the comment:
>
> Why is this only relevant to multiple thread?
>
> ----------
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> <https://bugs.python.org/issue26600>
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