Support loading version from a file

changelog.d/1359.change.rst

+Support using "file:" to load a PEP 440-compliant package version
+from a text file.

docs/setuptools.txt

@@ -2421,7 +2421,7 @@ Metadata
 Key                             Aliases            Type
 ==============================  =================  =====
 name                                               str
-version                                            attr:, str
+version                                            attr:, file:, str
 url                             home-page          str
 download_url                    download-url       str
 project_urls                                       dict
@@ -2441,6 +2441,10 @@ requires                                           list-comma
 obsoletes                                          list-comma
 ==============================  =================  =====
 
+.. note::
+    A version loaded using the ``file:`` directive must comply with PEP 440.
+    It is easy to accidentally put something other than a valid version
+    string in such a file, so validation is stricter in this case.
 
 Options
 -------

setuptools/config.py

@@ -7,6 +7,7 @@ from functools import partial
 from importlib import import_module
 
 from distutils.errors import DistutilsOptionError, DistutilsFileError
+from setuptools.extern.packaging.version import LegacyVersion, parse
 from setuptools.extern.six import string_types
 
 
@@ -427,6 +428,18 @@ class ConfigMetadataHandler(ConfigHandler):
         :rtype: str
 
         """
+        version = self._parse_file(value)
+
+        if version != value:
+            version = version.strip()
+            # Be strict about versions loaded from file because it's easy to
+            # accidentally include newlines and other unintended content
+            if isinstance(parse(version), LegacyVersion):
+                raise DistutilsOptionError('Version loaded from %s does not comply with PEP 440: %s' % (
+                    value, version
+                ))
+            return version
+
         version = self._parse_attr(value)
 
         if callable(version):

setuptools/tests/test_config.py

@@ -268,6 +268,23 @@ class TestMetadata:
         with get_dist(tmpdir) as dist:
             assert dist.metadata.version == '2016.11.26'
 
+    def test_version_file(self, tmpdir):
+
+        _, config = fake_env(
+            tmpdir,
+            '[metadata]\n'
+            'version = file: fake_package/version.txt\n'
+        )
+        tmpdir.join('fake_package', 'version.txt').write('1.2.3\n')
+
+        with get_dist(tmpdir) as dist:
+            assert dist.metadata.version == '1.2.3'
+
+        tmpdir.join('fake_package', 'version.txt').write('1.2.3\n4.5.6\n')
+        with pytest.raises(DistutilsOptionError):
+            with get_dist(tmpdir) as dist:
+                _ = dist.metadata.version
+
     def test_unknown_meta_item(self, tmpdir):
 
         fake_env(