py.test's monkeypatch.setattr(...) not working in some cases

Question

In conftest (in an autouse fixture):

monkeypatch.setattr('collector.util.download_data', lambda url:"Winning" )

In collector/util.py:

def download_data(url):
    assert False

In the_caller.py:

from collector.util import download_data
def some_function():
    download_data("blah")

When I call some_function(), I get the assert. However, if I change the_caller.py to:

import collector
def some_function():
    collector.util.download_data("blah")

then I get "Winning".

Why is this behaving differently, and how can I make the monkeypatch work for both scenarios?

This seems related: stackoverflow.com/questions/5286390/…

EmpireJones
– EmpireJones

2015-02-09 07:11:33 +00:00
Commented Feb 9, 2015 at 7:11 — EmpireJones
– EmpireJones, Commented Feb 9, 2015 at 7:11

anthony sottile · Accepted Answer · 2019-11-14 23:19:58Z

17

In general, it seems that the issue relates to how imports work in python. I'm not sure that there is a good solution.

The best work-around that I have found so far is the following:

monkeypatch.setattr('collector.util.download_data.__code__', replacement_function.__code__)

This works with both import types. One limitation is that this doesn't work with closures.

This functionality can be added to the framework via:

from _pytest.monkeypatch import monkeypatch

monkeypatch.setcode = lambda self, func_str, replacement_func: \
    monkeypatch.setattr(self, func_str + ".__code__", replacement_func.__code__)

Reference: https://mail.python.org/pipermail/pytest-dev/2013-October/002386.html

edited Nov 14, 2019 at 23:19

anthony sottile

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answered Feb 9, 2015 at 8:45

EmpireJones

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4 Comments

underyx Over a year ago

I think it's worth to note that that code up there is monkeypatching a monkeypatch library.

Alex Over a year ago

See the unittest docs for a good explanation of this.

cjauvin Over a year ago

Wow, I was deeply lost in an unpleasant monkeypatching rabbit hole, and this is the only thing that works. Unfortunately, understanding why (it works) will need to wait for another day..

Serhii Kushchenko Over a year ago

It turned out to be the only viable way to make monkeypatch work.

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