How to monkeypatch dynamic class attribute in pytest

Question

I use Pytest and I want to test a class which has a dynamic attribute set by a function

Here is an example

file_1.py

def fn():
    return 'foo'

class Cls(object):
    cls_attr = fn()

test_file_1.py

import file_1

def test_cl1(monkeypatch):
    monkeypatch.setattr('file_1.fn', lambda: 'bar')
    assert file_1.fn() == 'bar'
    cls = file_1.Cls()
    assert cls.cls_attr == 'bar' # <-- fail here

I think that python "compile"s the class before, then monkeypatch is run after.

Is there a way to "reload" the class with the monkeypatched function?

pfctdayelise · Accepted Answer · 2015-03-03 17:59:36Z

2

I don't think so... but why not monkeypatch the Cls.cls_attr instead?

ETA: Perhaps what you want to use instead of monkeypatch is mock. There is a pytest-mock plugin that may be useful with it too.

edited Mar 3, 2015 at 17:59

answered Feb 28, 2015 at 0:04

pfctdayelise

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2 Comments

IxDay Over a year ago

This is an extremely simplified example. In the code I use functools.partials and some nested stuff I want to test either with this.

pfctdayelise Over a year ago

On reflection I think perhaps you are after mock rather than monkeypatch.

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