How to mock the read file function using unittest

Question

I want to mock the read file method using unittest. Need to test read_file() method from the read class. having below class:

class read:
     def read_file():
        result = None
        if os.path.isfile("c:/user/file.txt"):
            result = []
            try:
                file = open("c:/user/file.txt", "r")
                file.close()
            except Exception as e:
                raise error(f"reading file error{"c:/user/file.txt"} - {e}")
        else:
            raise error(f"no such file error{"c:/user/file.txt"}""")
        return result

Tried test for read_file() method:

def test_read_file():

    read_file=[
        "20000, 00000000, 10000, xyz"
    ]
    mock_open = mock.mock_open(read_data=read_file)
    with mock.patch("builtins.open", mock_open):
        reader = read("filename")
        exp_result = read_file()
    assert read_file == exp_result

Answer 1

The class read

There are some errors in your class read, so I have made the minimum corrections to it and after that I have saved the class read in a file called read.py. The file read.py is as below:

import os

class read:

    #++++++++++++++++++++++++++++++
    # Add this method to your class
    #++++++++++++++++++++++++++++++
    def __init__(self, filename):
        self.filename = filename

    # ---> I have add the argument self in the method because it is necessary
    # when you define the method of a class
    def read_file(self):
        result = None
        # ---> the path of the file 'file.txt' is saved in
        # the attribute self.filename
        if os.path.isfile(self.filename):
            result = []
            try:
                file = open(self.filename, "r")
                file.close()
            except Exception as e:
                # ----> raise Exception and not 'error'
                raise Exception(f"reading file error{self.filename} - {e}")
        else:
            # ----> raise Exception and not 'error'
            raise Exception(f"no such file error {self.filename}")
        return result

Note that I have add the method __init__ to it.
To try to execute the method read_file() you can use the following code (write it in the same file read.py):

try:
    file_content = read("c:/user/file.txt").read_file()
    print(file_content)
except Exception as ex:
    print(ex)

If the file c:/user/file.txt exists you will obtain the following output (empty list because you have set result = []):

[]

If the file c:/user/file.txt doesn't exist you will obtain the following error message (print by the instruction print(ex)):

no such file error c:/user/file.txt

The test file

I write a possible test for the method read_file() of the class read in a file called test_read.py saved in the same folder which contains the file read.py. The content of the file test_read.py is:

import unittest
from unittest.mock import patch
from read import read

class MyTestCase(unittest.TestCase):

    # ---> here yor test with a bit changes <---
    def test_read_file(self):
        read_file = [
            "20000, 00000000, 10000, xyz"
        ]
        # instantiation of the class read
        reader = read("filename")
        # patching of the method 'read_file'
        with patch.object(reader, 'read_file') as mock_read_file:
            # set the return value of the read_file
            mock_read_file.return_value = read_file
            # call the method read_file()
            exp_result = reader.read_file()
            # check the result
            assert read_file == exp_result

if __name__ == '__main__':
    unittest.main()

The test patches the method read_file() setting its return value to the list ["20000, 00000000, 10000, xyz"], so when the method read_file() is executed by the instruction reader.read_file() it returns exactly that list as the test is expecting.

To execute the previous test do the following steps:

# go to the folder where are present the 2 files (read.py and test_read.py)
cd path/to/folder/with/file/test_read.py

# execute the followed command
python -m unittest discover -v

The output of the test execution is:

no such file error c:/user/file.txt
.
----------------------------------------------------------------------
Ran 1 test in 0.001s

OK

I hope I have guessed your intentions.