Monkeypatch a vendor package that is not available when running tests

[wigging]

I am using a Python package that is provided by a vendor that makes laboratory equipment. The vendor package provides an interface to control an instrument in the lab. Due to licensing, the vendor package is only on the computer that the instrument is connected to.

I made another Python package that relies on the vender package for some functionality. However, the computer where the vendor package resides is not always accessible which hampers the development of my own package. So the only time I can develop my package is when I have access to the lab computer.

I tried to monkeypatch functions from the vendor package. But it doesn't work because pytest complains that the vendor package can't be found. Below is some code to demonstrate the problem.

# This represents a module in `mypackage`. The imported `calc` package is a
# vendor package that is not available when I run the tests.

import calc  # <-- represents third-party vendor package

def divider(x: float, y: float) -> float:
    """Divide two numbers."""
    z = calc.divide(x, y)
    return z

# This test fails because pytest cannot find the vendor package which the
# `mypackage` relies on.

import mypackage


def test_divider(monkeypatch):
    """Test divider function."""

    def mock_divider(a, b):
        return 0.8

    monkeypatch.setattr(mypackage, "divider", mock_divider)

    z = mypackage.divider(9, 3)
    assert z == 0.8

Is there a way I can use monkeypatch or some other feature of pytest to run tests for my package when the vendor package is not available? I basically want to mock the vendor package so that I can develop my own package when I don't have access to the computer where the vendor package resides.

[RonnyPfannschmidt]

Create a own package that fakes the import names

Also reach out to the vendor about test automation

Some venors provide extra license specifically for development or ci

[webknjaz]

You may want to start here: https://hynek.me/articles/what-to-mock-in-5-mins/

[wigging]

@webknjaz Can you put your comments in a separate answer so it's easier to track your response?

[webknjaz]

I don't think of it as a standalone response, just augmenting what Ronny said.

[wigging]

A more elegant way would be to create a wrapper api you use and can safely mock/ control

How would this apply to my example shown below? Are you suggesting I create something like a CalcAPI class that acts as a wrapper API to the calc package?

# This represents a module in `mypackage`. The imported `calc` package is a
# vendor package that is not available when I run the tests.

import calc  # <-- represents third-party vendor package

def divider(x: float, y: float) -> float:
    """Divide two numbers."""
    z = calc.divide(x, y)
    return z

[webknjaz]

Yes, except put your things in a class and pass that module into the initializer. In tests, you'll be able to pass a fake instead.
Look up facade/proxy patterns to understand more.