Fixed the RTD build issue due to version file not available

Details:

* Fixed the RTD build by using the version from the highest tag that is
  reachable from the checked out branch, if the version file _version_scm.py
  is not available.

Signed-off-by: Andreas Maier <maiera@de.ibm.com>

changes/1559.notshown.rst

@@ -0,0 +1,3 @@
+Fixed the RTD build by using the version from the highest tag that is
+reachable from the checked out branch, if the version file _version_scm.py
+is not available.

docs/conf.py

@@ -20,24 +20,70 @@
 
 import sys
 import os
+import subprocess
 
 
 def get_version():
     """
     Return the package version as a string.
 
-    The version file must have been generated by setuptools-scm.
+    This script is used both for local docs builds and by ReadTheDocs builds.
+
+    For local builds, the Makefile ensures that there is a _version_scm.py
+    file with the exact version. The version file must have been generated by
+    setuptools-scm.
+
+    For ReadTheDocs builds, the Makefile is not used and because the
+    _version_scm.py file is not in the repo (since it would need to be updated
+    in each PR) it is not available. In that case, the version is determined
+    from the highest tag reachable from the checked out branch (ie. shown by
+    'git tag -l --merged').
     """
     version_file = os.path.join('..', 'zhmcclient', '_version_scm.py')
     # pylint: disable=unspecified-encoding
-    with open(version_file) as fp:
-        version_source = fp.read()
-    _globals = {}
-    exec(version_source, _globals)  # pylint: disable=exec-used
-    try:
-        _version = _globals['__version__']
-    except KeyError:
-        _version = _globals['version']
+
+    if os.path.exists(version_file):
+
+        # Get version from version file
+        with open(version_file) as fp:
+            version_source = fp.read()
+        _globals = {}
+        exec(version_source, _globals)  # pylint: disable=exec-used
+        try:
+            _version = _globals['__version__']
+        except KeyError:
+            _version = _globals['version']
+        return _version
+
+    # Get version from highest tag
+
+    cmd = 'git tag -l --merged'
+    cp = subprocess.run(
+        cmd, shell=True, check=True, capture_output=True, timeout=30)
+    tags = cp.stdout.decode('utf-8').split('\n')
+
+    # Convert the version strings into lists of integers for sorting
+    tags2 = []  # list of tags represented as list of int
+    for tag in tags:
+        if tag != '':  # output ends with an empty line -> empty string
+            tag_parts = tag.split('.')
+            tag_parts2 = []
+            for tag_part in tag_parts:
+                # Handle start tags (e.g. '1.17.0a0')
+                if tag_part.endswith('a0'):
+                    tag_part = tag_part[:-2]
+                try:
+                    tag_part2 = int(tag_part)
+                except ValueError:
+                    # Should not happen, but in case it does we use the string
+                    tag_part2 = tag_part
+                tag_parts2.append(tag_part2)
+            tags2.append(tag_parts2)
+
+    # Sort the integer-based versions, take the highest and convert to string
+    highest_tag2 = sorted(tags2)[-1]
+    _version = '.'.join(map(str, highest_tag2))
+
     return _version