Make tox -e generate easier to use

- auto detect python paths (which)
- make it possible to call `tox -e generate` w/o extra args
- update list of pythons to include currently supported versions

Change-Id: Ib727242c1aba8597389259d8dbacba7194f9f0d6

openstack_requirements/cmds/generate.py

@@ -15,6 +15,7 @@ import copy
 import hashlib
 import optparse
 import os.path
+import shutil
 import subprocess
 import sys
 
@@ -155,7 +156,7 @@ def _validate_options(options):
     if not options.pythons:
         raise Exception("No Pythons given - see -p.")
     for python in options.pythons:
-        if not os.path.exists(python):
+        if not shutil.which(python):
             raise Exception(
                 "Python %(python)s not found." % dict(python=python))
     if not options.requirements:
@@ -227,6 +228,8 @@ def main(argv=None, stdout=None):
         _freeze(options.requirements, python) for python in options.pythons]
     _clone_versions(freezes, options)
     blacklist = _parse_blacklist(options.blacklist)
-    frozen = sorted(_combine_freezes(freezes, blacklist), key=_make_sort_key)
+    frozen = [
+        "# generated using: tox -e generate\n",
+        *sorted(_combine_freezes(freezes, blacklist), key=_make_sort_key)]
     stdout.writelines(frozen)
     stdout.flush()

tox.ini

@@ -30,10 +30,11 @@ commands = {posargs}
 commands = update-requirements {posargs}
 
 [testenv:generate]
+description = Regenerates upper-constraints.txt
 # Generate needs an unconstrained install to get new dependencies
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
-commands = generate-constraints {posargs}
+commands = generate-constraints {posargs: -b blacklist.txt -r global-requirements.txt -p python3.6 -p python3.7 -p python3.8 -p python3.9 > upper-constraints.txt}
 
 [testenv:validate]
 commands =
@@ -49,6 +50,7 @@ whitelist_externals = {[testenv:linters]whitelist_externals}
 commands = {[testenv:linters]commands}
 
 [testenv:linters]
+description = Perform linting
 deps =
     hacking>=1.0.0
     bashate>=0.5.1