title: Python - InvocationError in tox running py.test and coverage test
body: <p>My project has the following structure:</p>
  
  <pre><code>my_project
  |
   setup.py
   tox.ini
   src
     |
     core_functions.py 
     client.py
     server.py
     sql_database.py
     tests.py
  </code></pre>
  
  <p>The <code>tests.py</code> file contains the tests for all the function in files <code>client.py, server.py, sql_database.py, core_functions.py</code></p>
  
  <p>Next, I created the <code>tox.ini</code> file which I want to use for automation of <code>py.tests</code> and <code>coverage tests</code>. The structure of the tox.ini file is as follows:</p>
  
  <pre><code>[tox]
  envlist = py27,cov
  
  [testenv]
  commands = py.test -sv --doctest-modules my_project/__init__.py my_project/tests.py
  
  
  [testenv:py27]
  commands = coverage erase
             coverage run --source=my_project -m pytest
             coverage report -m
  deps = pytest
  </code></pre>
  
  <p>When I run in the command line <code>tox</code> the py.tests succeed, but the coverage tests fail with the following error:</p>
  
  <pre><code>============== test session starts ===================
  platform darwin -- Python 2.7.13, pytest-3.1.1, py-1.4.33, pluggy-0.4.0
  rootdir: /Users/xyz/Documents/ProjectGit/MyProject/my_project, inifile:
  plugins: cov-2.5.1
  collecting 10334 items / 167 errorsERROR: InvocationError: 
  '/Users/xyz/Documents/ProjectGit/MyProject/my_project/.tox/py27/bin/coverage run --source=my_project -m pytest'
  cov inst-nodeps: /Users/xyz/Documents/ProjectGit/MyProject/my_project/.tox/dist/my_project-0.0.1.zip
  cov installed: asn1crypto==0.22.0,attrs==17.2.0,Automat==0.6.0,bitarray==0.8.1,boto3==1.4.4,
  botocore==1.5.59,cffi==1.10.0,constantly==15.1.0,coverage==4.4.1,cryptography==1.9,
  cycler==0.10.0,docutils==0.13.1,enum34==1.1.6,Fabric==1.13.2,functools32==3.2.3.post2,
  future==0.16.0,futures==3.1.1,idna==2.5,incremental==17.5.0,ipaddress==1.0.18,jmespath==0.9.3,
  my_project==0.0.1,matplotlib==2.0.2,msgpack-python==0.4.8,numpy==1.12.1,paramiko==2.1.2,
  Paver==1.2.4,petlib==0.0.41,py==1.4.33,pyasn1==0.2.3,pybloom==1.1,pycparser==2.17,
  pyparsing==2.2.0,pytest==3.1.1,pytest-cov==2.5.1,python-dateutil==2.6.0,pytz==2017.2,
  s3transfer==0.1.10,scapy==2.3.3,scipy==0.19.0,six==1.10.0,sphinxmix==0.0.6,
  subprocess32==3.2.7,Twisted==17.1.0,zope.interface==4.4.1
  cov runtests: PYTHONHASHSEED='3903710496'
  cov runtests: commands[0] | py.test -sv --doctest-modules my_project/__init__.py my_project/tests.py
  </code></pre>
  
  <p>How can I fix my tox.ini file to make it work?</p>