How can I use mocking/patching for changing the local directory variable inside a class method to avoid writing test output files into it?

Question

I have an object as follows with a to_json method that I'd like to write a test for.

import networkx as nx

class Example:
    def __init__(self, graph):
        self.graph = graph

    def to_json(self):
        data = nx.readwrite.node_link_data(self.graph)
        filepath = os.path.join('~/.cache/product', 'some_file_name.txt')
        with open(filepath, "w") as outfile:
            json.dump(data, outfile)
        return filepath

When running the unit test for to_json, I don't want to write the output file to the ~/.cache/product directory but rather /tmp/product directory. How can I use achieve this using mocking/patching?

My current test setup looks something like:

def test_example_to_json():
    ex = Example(graph={}) # Some graph
    filepath = ex.to_json() # This is the problematic step since the file is being written to ~/.cache/product
    with open(filepath, "r") as infile:
        assert json.load(infile) == {}

Any advice here would be appreciated.

Answer 1

It is not possible to change the filepath variable and then have the function continue with the rest of the computation via mocking/patching.

One solution here that worked for me is as follows: I defined an environment variable called CACHE_DIR and used that to get the cache directory. Monkeypatch does allow you to use monkeypatch.setenv("CACHE_DIR", "/tmp/product").

import networkx as nx


class Example:
    def __init__(self, graph):
        self.graph = graph

    def to_json(self):
        data = nx.readwrite.node_link_data(self.graph)
        filepath = os.path.join(os.getenv('CACHE_DIR'), 'some_file_name.txt')
        with open(filepath, "w") as outfile:
            json.dump(data, outfile)
        return filepath

with the following unittest:

import pytest


def test_example_to_json(monkeypatch):
    ex = Example(graph={})

    monkeypatch.setenv("CACHE_DIR", "/tmp/product")
    filepath = ex.to_json() 
    with open(filepath, "r") as infile:
        assert json.load(infile) == {}