Add setuptools.get_version(path, field='__version__')

This allows people who use setuptools to maintain project versions
in a single place, for example as attributes in their Python source.

Closes pypa#1316
Closes pypa/pip#6004
Closes pypa/packaging.python.org#571

Author: techtonik

setup.cfg

@@ -25,5 +25,3 @@ universal = 1
 [metadata]
 license_file = LICENSE
 
-[bumpversion:file:setup.py]
-

setup.py

@@ -89,7 +89,7 @@ def pypi_link(pkg_filename):
 
 setup_params = dict(
     name="setuptools",
-    version="40.8.0",
+    version=setuptools.get_version("setup.cfg", field="current_version"),
     description=(
         "Easily download, build, install, upgrade, and uninstall "
         "Python packages"

setuptools/__init__.py

@@ -27,7 +27,7 @@
 __all__ = [
     'setup', 'Distribution', 'Feature', 'Command', 'Extension', 'Require',
     'SetuptoolsDeprecationWarning',
-    'find_packages'
+    'find_packages', 'get_version'
 ]
 
 if PY3:
@@ -224,5 +224,25 @@ def findall(dir=os.curdir):
     return list(files)
 
 
+def get_version(path, field='__version__'):
+    """
+    Get version information from a text file. Version is extracted
+    from "key = value" format with key matched at the beginning of
+    a line and specified by `field` parameter. Returns string.
+    """
+    version_file = os.path.abspath(path)
+    # Using binary matching to avoid possible encoding problems
+    # when reading arbitrary text files
+    if type(field) is not bytes:
+        field = field.encode('utf-8')
+    for line in open(version_file, 'rb'):
+        if line.startswith(field):
+            # __version__ = "0.9"
+            # current_version = 4.8.0
+            _, value = line.split(b'=')
+            version = value.strip(b' \'\"').decode()
+            return version
+
+
 # Apply monkey patches
 monkey.patch_all()