Add --skip-existing flag to upload command

This new flag allows users to force twine to skip existing files when
uploading to PyPI. For example:

    ~/s/gh3.py git:develop ❯❯❯ twine upload --skip-existing dist/*
    Uploading distributions to https://pypi.python.org/pypi
    Uploading github3.py-1.0.0a2-py2.py3-none-any.whl
      Skipping github3.py-1.0.0a2-py2.py3-none-any.whl because it
    appears to already exist
    Uploading github3.py-1.0.0a2.tar.gz
      Skipping github3.py-1.0.0a2.tar.gz because it appears to
    already exist

Closes pypa#115

tests/test_upload.py

@@ -16,11 +16,15 @@
 import os
 import textwrap
 
+import mock
 import pytest
 
 from twine.commands import upload
 
 
+WHEEL_FIXTURE = 'tests/fixtures/twine-1.5.0-py2.py3-none-any.whl'
+
+
 def test_ensure_wheel_files_uploaded_first():
     files = upload.group_wheel_files_first(["twine/foo.py",
                                             "twine/first.whl",
@@ -72,11 +76,36 @@ def test_get_config_old_format(tmpdir):
     try:
         upload.upload(dists="foo", repository="pypi", sign=None, identity=None,
                       username=None, password=None, comment=None,
-                      sign_with=None, config_file=pypirc)
+                      sign_with=None, config_file=pypirc, skip_existing=False)
     except KeyError as err:
         assert err.args[0] == (
             "Missing 'pypi' section from the configuration file.\n"
             "Maybe you have a out-dated '{0}' format?\n"
             "more info: "
             "https://docs.python.org/distutils/packageindex.html#pypirc\n"
         ).format(pypirc)
+
+
+def test_skip_existing_skips_files_already_on_PyPI(monkeypatch):
+    response = mock.Mock(spec=upload.requests.models.Response)
+    session = mock.Mock(spec=upload.requests.sessions.Session)
+
+    monkeypatch.setattr(upload.requests, 'session', lambda: session)
+    monkeypatch.setattr(upload.os.path, 'exists', lambda args: True)
+
+    session.post.return_value = response
+    response.status_code = 400
+    response.is_redirect = False
+
+    upload.upload(
+        dists=[WHEEL_FIXTURE],
+        repository="pypi",
+        sign=None,
+        identity=None,
+        username='username',
+        password='password',
+        comment=None,
+        sign_with=None,
+        config_file='',
+        skip_existing=True
+    )

tox.ini

@@ -4,6 +4,7 @@ envlist = py26,py27,pypy,py32,py33,py34,docs,pep8
 [testenv]
 deps =
     coverage
+    mock
     pretend
     pytest
 commands =

twine/commands/upload.py

@@ -54,7 +54,7 @@ def find_dists(dists):
 
 
 def upload(dists, repository, sign, identity, username, password, comment,
-           sign_with, config_file):
+           sign_with, config_file, skip_existing):
     # Check that a nonsensical option wasn't given
     if not sign and identity:
         raise ValueError("sign must be given along with identity")
@@ -106,10 +106,15 @@ def upload(dists, repository, sign, identity, username, password, comment,
         if resp.is_redirect:
             raise exc.RedirectDetected(
                 ('"{0}" attempted to redirect to "{1}" during upload.'
-                 ' Aborting...').format(config["respository"],
+                 ' Aborting...').format(config["repository"],
                                         resp.headers["location"]))
+
         # Otherwise, raise an HTTPError based on the status code.
-        resp.raise_for_status()
+        if resp.status_code == 400 and skip_existing:
+            print("  Skipping {0} because it appears to already exist".format(
+                basefilename))
+        else:
+            resp.raise_for_status()
 
     # Bug 28. Try to silence a ResourceWarning by clearing the connection
     # pool.
@@ -155,6 +160,12 @@ def main(args):
         default="~/.pypirc",
         help="The .pypirc config file to use",
     )
+    parser.add_argument(
+        "--skip-existing",
+        default=False,
+        action="store_true",
+        help="Continue uploading files if one already exists",
+    )
     parser.add_argument(
         "dists",
         nargs="+",