Python Language Unit Testing Unit tests with pytest


installing pytest:

pip install pytest

getting the tests ready:

mkdir tests
touch tests/

Functions to test in docker_something/

from subprocess import Popen, PIPE 
# this Popen is monkeypatched with the fixture `all_popens`

def copy_file_to_docker(src, dest):
        result = Popen(['docker','cp', src, 'something_cont:{}'.format(dest)], stdout=PIPE, stderr=PIPE)
        err =
        if err:
            raise Exception(err)
    except Exception as e:
    return result

def docker_exec_something(something_file_string):
    fl = Popen(["docker", "exec", "-i", "something_cont", "something"], stdin=PIPE, stdout=PIPE, stderr=PIPE)
    err =
    if err:
    result =

The test imports

import os
from tempfile import NamedTemporaryFile
import pytest
from subprocess import Popen, PIPE

from docker_something import helpers
copy_file_to_docker = helpers.copy_file_to_docker
docker_exec_something = helpers.docker_exec_something

mocking a file like object in

class MockBytes():
    '''Used to collect bytes
    all_read = []
    all_write = []
    all_close = []

    def read(self, *args, **kwargs):
        # print('read', args, kwargs, dir(self))
        self.all_read.append((self, args, kwargs))

    def write(self, *args, **kwargs):
        # print('wrote', args, kwargs)
        self.all_write.append((self, args, kwargs))

    def close(self, *args, **kwargs):
        # print('closed', self, args, kwargs)
        self.all_close.append((self, args, kwargs))

    def get_all_mock_bytes(self):
        return self.all_read, self.all_write, self.all_close

Monkey patching with pytest in

def all_popens(monkeypatch):
    '''This fixture overrides / mocks the builtin Popen
        and replaces stdin, stdout, stderr with a MockBytes object

        note: monkeypatch is magically imported
    all_popens = []
    class MockPopen(object):
        def __init__(self, args, stdout=None, stdin=None, stderr=None):
            self.args = args
            self.byte_collection = MockBytes()
            self.stdin = self.byte_collection
            self.stdout = self.byte_collection
            self.stderr = self.byte_collection
    monkeypatch.setattr(helpers, 'Popen', MockPopen)

    return all_popens

Example tests, must start with the prefix test_ in the file:

def test_docker_install():
    p = Popen(['which', 'docker'], stdout=PIPE, stderr=PIPE)
    result =
    assert 'bin/docker' in result

def test_copy_file_to_docker(all_popens):    
    result = copy_file_to_docker('asdf', 'asdf')
    collected_popen = all_popens.pop()
    mock_read, mock_write, mock_close = collected_popen.byte_collection.get_all_mock_bytes()
    assert mock_read
    assert result.args == ['docker', 'cp', 'asdf', 'something_cont:asdf']

def test_docker_exec_something(all_popens):

    collected_popen = all_popens.pop()
    mock_read, mock_write, mock_close = collected_popen.byte_collection.get_all_mock_bytes()
    assert len(mock_read) == 3
    something_template_stdin = mock_write[0][1][0]
    these = [os.environ['USER'], os.environ['password_prod'], 'table_name_here', 'test_vdm', 'col_a', 'col_b', '/tmp/test.tsv']
    assert all([x in something_template_stdin for x in these])

running the tests one at a time:

py.test -k test_docker_install tests
py.test -k test_copy_file_to_docker tests
py.test -k test_docker_exec_something tests

running all the tests in the tests folder:

py.test -k test_ tests