Mocking properties in Python
It doesn’t happen all that often, but sometimes when writing unit tests you want to mock a property and specify a return value. The
mock library provides a
PropertyMock for that, but using it probably doesn’t work the way you would initially think it would.
I had this exact problem earlier this week and had to explain it to two different people in the two days after that. So why not write it down in a blog post?
Suppose you have this silly implementation of a class that takes a file path and reads the file’s content into a variable that can be accessed through a property:
You also have this equally silly function that takes an instance of the silly file reader and returns the reversed content:
Of course you want to test this silly function. Maybe you come up with something like this:
Unfortunately, that won’t work:
The thing with a
PropertyMock is that you need to set it for the type of an object, not for the object itself. To get things working, you need to do it like this: