This is an example of patching/mocking 3rd party libs. Some libs touch the underlying system and thus we need a way to isolate their usage in the code we're testing.

hook.py

from flask import Flask
from flask import request
import os

'''
Notice we're not using the 'from' statement here and notice
the calls using these modules. Ex. git.Repo.
This will allow the mocking to work within the write context.
'''

import git
import subprocess

app = Flask(__name__)
app.config['DEBUG'] = True

BASE_DIRS = ['/var/www', '/apps']


def _find_site(site_slug):
    location = None
    for base in BASE_DIRS:
        l = os.path.join(base, site_slug)
        if os.path.isdir(l):
            location = l
    if location is None:
        location = os.path.join(BASE_DIRS[0], site_slug)
    return location

@app.route("/deploy", methods=['POST'])
def deploy():

    data = request.json
    real_repo = data['repository']['absolute_url']
    real_repo = real_repo[1:-1]+".git"

    dir_name = _find_site(data['repository']['slug'])

    if not os.path.isdir(dir_name):
        '''
        Do something smart here like check folder permissions and such
        '''
        try:
            subprocess.check_call(["git", "clone", "git@bitbucket.org:"+real_repo, dir_name])
        except subprocess.CalledProcessError as e:
            return "Error Occured"+e.message+"\n"
        repo = git.Repo(dir_name)
    else:
        repo = git.Repo(dir_name)
        origin = repo.remotes.origin
        origin.fetch()
        origin.pull()

    return "Deployed "+repo.head.commit.hexsha+"\n"

hook_test.py

import hook
import json
from mock import patch

import git
import subprocess

import unittest


class DeployTest(unittest.TestCase):

    def setUp(self):
        hook.app.config['TESTING'] = True
        self.app = hook.app.test_client()

    def test_client_is_active(self):
        res = self.app.get('/')
        self.assertEqual("Hello World!", res.data)

    @unittest.skip
    def test_post_deploy(self):
        headers = [('Content-Type', 'application/json')]
        j = json.dumps(dict(repository=dict(absolute_url='s', slug='blah')))
        with patch('git.Repo', spec=True, side_effect=None) as git.Repo:
            git.Repo.return_value.head.commit.hexsha = 'abc123'
            res = self.app.post('/deploy', headers=headers, data=j)
        self.assertEqual("Deployed abc123\n", res.data)
    
    def test_deploy_failure_clone(self):
        headers = [('Content-Type', 'application/json')]
        j = json.dumps(dict(repository=dict(absolute_url='s', slug='blah')))
        with patch('subprocess.check_call', side_effect=subprocess.CalledProcessError) as subprocess.check_call:
            subprocess.check_call.return_value = ''
            try:
                res = self.app.post('/deploy', headers=headers, data=j)
            except subprocess.CalledProcessError:
                assert False  # fail if exception isn't caught
        self.assertEqual("Cannot Create Directory\n", res.data)