Allow to manage project version with git tags

To avoid to manage textual version of a project allow poetry
to retrieve those informations directly from git tags.

To determine the current version number we currently retrieve
the highest available tag number.

If the version is not available in the pyproject.toml then
poetry try to retrieve version from git so project can continue
to manage these informations manually.

If the version is not available in the pyproject.toml and if
it's not a git repository then an exception is raised and we
retrieve the previous behavior of poetry.

poetry/poetry.py

@@ -21,6 +21,7 @@
 from .utils._compat import Path
 from .utils.helpers import get_http_basic_auth
 from .utils.toml_file import TomlFile
+from .vcs.git import version_from_tags
 
 
 class Poetry:
@@ -107,7 +108,7 @@ def create(cls, cwd):  # type: (Path) -> Poetry
 
         # Load package
         name = local_config["name"]
-        version = local_config["version"]
+        version = local_config.get("version", version_from_tags())
         package = ProjectPackage(name, version, version)
         package.root_dir = poetry_file.parent
 

poetry/vcs/git.py

@@ -1,4 +1,5 @@
 # -*- coding: utf-8 -*-
+import pkg_resources
 import re
 import subprocess
 
@@ -44,6 +45,9 @@ def config(self):  # type: () -> GitConfig
     def clone(self, repository, dest):  # type: (...) -> str
         return self.run("clone", repository, str(dest))
 
+    def tag(self):  # type: (...) -> list
+        return self.run("tag").split("\n")
+
     def checkout(self, rev, folder=None):  # type: (...) -> str
         args = []
         if folder is None and self._work_dir:
@@ -100,3 +104,15 @@ def run(self, *args):  # type: (...) -> str
         return decode(
             subprocess.check_output(["git"] + list(args), stderr=subprocess.STDOUT)
         )
+
+
+def version_from_tags():
+    from poetry.semver import Version
+
+    git = Git()
+    version = "0.0.0"
+    tags = git.tag()
+    if tags:
+        version = max(tags, key=pkg_resources.parse_version)
+        version = Version.parse(version)
+    return version