mymodule.py
def write_df_to_csv(self, df, modified_fn):
new_csv = self.path + "/" + modified_fn
df.to_csv(new_csv, sep=";", encoding='utf-8', index=False)
test_mymodule.py
class TestMyModule(unittest.TestCase):
def setUp(self):
args = parse_args(["-f", "test1"])
self.mm = MyModule(args)
self.mm.path = "Random/path"
self.test_df = pd.DataFrame(
[
["bob", "a"],
["sue", "b"],
["sue", "c"],
["joe", "c"],
["bill", "d"],
["max", "b"],
],
columns=["A", "B"],
)
def test_write_df_to_csv(self):
to_csv_mock = mock.MagicMock()
with mock.patch("project.mymodule.to_csv", to_csv_mock, create=True):
self.mm.write_df_to_csv(self.test_df, "Stuff.csv")
to_csv_mock.assert_called_with(self.mm.path + "/" + "Stuff.csv")
When I run this test, I get:
FileNotFoundError: [Errno 2] No such file or directory: 'Random/path/Stuff.csv'
I'm trying to mock the to_csv
in my method. My other tests run as expected, however I'm not sure where I am going wrong with this test. Is my use of MagicMock correct, or am I overlooking something else?
You didn't provide a minimal, reproducible example, so I had to strip some things out to make this work. I suppose you can fill in the missing bits on your own.
One problem was with mock.patch("project.mymodule.to_csv", ...)
which tries to mock a class named to_csv
in the module at the import path project.mymodule
. This only "worked" because you passed create=True
, but of course mocking something that didn't exist before has no effect because nobody will call it.
You could mock out the entire DataFrame
class using mock.patch("pandas.DataFrame", ...)
. Note: it's not pd
regardless of how (or even whether) you imported pandas
in the current module.
But then your unit test will be asserting that to_csv
was called on any DataFrame
, not necessarily the one you passed in. By mocking just the to_csv
method on the one DataFrame
object that we are passing into write_df_to_csv
, the test becomes a bit more comprehensive and also easier to understand. We can do this using mock.patch.object
.
mock.patch.object
returns the mock function, on which we can subsequently call assertions. Because it's a method mock, not a free function, we don't need to pass the self
argument in the assertion.
project/mymodule.py
def write_df_to_csv(df, file_name):
df.to_csv(file_name, sep=";", encoding='utf-8', index=False)
project/test_mymodule.py
import unittest.mock as mock
import unittest
import pandas as pd
import project.mymodule as mm
class TestMyModule(unittest.TestCase):
def test_write_df_to_csv(self):
test_df = pd.DataFrame(...)
with mock.patch.object(test_df, "to_csv") as to_csv_mock:
mm.write_df_to_csv(test_df, "Stuff.csv")
to_csv_mock.assert_called_with("Stuff.csv")
if __name__ == '__main__':
unittest.main()
Output
The test fails in a proper way now, because the arguments don't actually match!
$ python -m project.test_mymodule
F
======================================================================
FAIL: test_write_df_to_csv (__main__.TestMyModule)
----------------------------------------------------------------------
Traceback (most recent call last):
File "/tmp/project/test_mymodule.py", line 25, in test_write_df_to_csv
to_csv_mock.assert_called_with("Stuff.csv")
File "/usr/lib/python3.8/unittest/mock.py", line 913, in assert_called_with
raise AssertionError(_error_message()) from cause
AssertionError: expected call not found.
Expected: to_csv('Stuff.csv')
Actual: to_csv('Stuff.csv', sep=';', encoding='utf-8', index=False)
----------------------------------------------------------------------
Ran 1 test in 0.003s
FAILED (failures=1)