meta: fix tag check, clarify use of prerel numbers

lib/meta.py

@@ -113,14 +113,18 @@
 for part in (MAJOR, MINOR, PATCH, PREREL_NUM):
     assert isinstance(part, int) and part >= 0
 assert PREREL in {'', '-alpha', '-beta'}
+if PREREL == '-alpha':
+    assert PREREL_NUM == 0
 
 #: Base version string.
 #: This is required to match a tag in git for formal releases. However
 #: for pre-release (hyphenated) base versions, the formal version will
 #: be further decorated with the number of commits following the tag.
 MYPAINT_VERSION = '{major}.{minor}.{patch}{prerel}'.format(
     major=MAJOR, minor=MINOR, patch=PATCH,
-    prerel=PREREL and PREREL+'.'+str(PREREL_NUM)
+    prerel=PREREL and
+    # Prerelease numbers should only be used for beta releases
+    PREREL + ('.' + str(PREREL_NUM) if PREREL == '-beta' else '')
 )