title: ImportError: cannot import name <lib> python 2.7
body: <p>I'm trying to run <code>tox -e coverage</code> in <code>python2.7</code> for the <a href="https://github.com/warner/python-ecdsa" rel="nofollow noreferrer">ecdsa</a> library in Python.</p>
  <p>The <code>src/ecdsa/util.py</code> file looks like:</p>
  <pre><code>import os
  import math
  import sys
  import binascii
  from hashlib import sha256
  from . import der
  from .curves import orderlen
  </code></pre>
  <p>However, when I run <code>tox -e coverage</code> I'm getting the following error:</p>
  <pre><code>________________________________ ERROR collecting src/ecdsa/test_numbertheory.py _________________________________
  /usr/lib/python2.7/dist-packages/py/_path/local.py:650: in pyimport
      __import__(modname)
  src/ecdsa/__init__.py:1: in &lt;module&gt;
      from .keys import (
  src/ecdsa/keys.py:62: in &lt;module&gt;
      from . import ecdsa
  src/ecdsa/ecdsa.py:59: in &lt;module&gt;
      from .util import bit_length
  src/ecdsa/util.py:9: in &lt;module&gt;
      from . import der
  E   ImportError: cannot import name der
  </code></pre>
  <p>What's the correct way to import these libraries in <code>python2.7</code>?</p>
  <h1>Update:</h1>
  <p>So, inside <code>src/ecdsa/ecdsa.py</code> I have the following definition:</p>
  <pre><code># NIST Curve P-192:
  _p = 6277101735386680763835789423207666416083908700390324961279
  _r = 6277101735386680763835789423176059013767194773182842284081
  # s = 0x3045ae6fc8422f64ed579528d38120eae12196d5L
  # c = 0x3099d2bbbfcb2538542dcd5fb078b6ef5f3d6fe2c745de65L
  _b = 0x64210519E59C80E70FA7E9AB72243049FEB8DEECC146B9B1
  _Gx = 0x188DA80EB03090F67CBF20EB43A18800F4FF0AFD82FF1012
  _Gy = 0x07192B95FFC8DA78631011ED6B24CDD573F977A11E794811
  
  curve_192 = ellipticcurve.CurveFp(_p, -3, _b)
  generator_192 = ellipticcurve.Point(curve_192, _Gx, _Gy, _r)
  </code></pre>
  <p>and inside <code>src/ecdsa/curves.py</code> I have:</p>
  <pre><code>import der
  import ecdsa
  </code></pre>
  <p>When I run <code>tox -e coverage</code> I'm getting the following error:</p>
  <pre><code>____________________________________ ERROR collecting src/ecdsa/test_ecdsa.py ____________________________________
  /usr/lib/python2.7/dist-packages/py/_path/local.py:650: in pyimport
      __import__(modname)
  src/ecdsa/__init__.py:1: in &lt;module&gt;
      from keys import (
  src/ecdsa/keys.py:62: in &lt;module&gt;
      import ecdsa
  src/ecdsa/ecdsa.py:59: in &lt;module&gt;
      from util import bit_length
  src/ecdsa/util.py:10: in &lt;module&gt;
      from curves import orderlen
  src/ecdsa/curves.py:36: in &lt;module&gt;
      ecdsa.curve_192,
  E   AttributeError: 'module' object has no attribute 'curve_192'
  </code></pre>