unparser: drop Python 2, fix testing bugs with newer Pythons (#38424)

The `unparser` that Spack uses for package hashing had several tweaks to ensure compatibility
with Python 2.7:
1. Currently, the unparser automatically moves `*` and `**` args to the end to preserve
   compatibility with `python@:3.4`
2. `print a, b, c` statements and single-tuple `print((a, b, c))` function calls were
   remapped to `print(a, b, c)` in the unparsed output for consistency across versions.

(1) is causing issues in our tests because a recent patch to the Python source code  
(https://github.com/python/cpython/pull/102953/files#diff-7972dffec6674d5f09410c71766ac6caacb95b9bccbf032061806ae304519c9bR813-R823) 
has a `**` arg before an named argument, and we round-trip the core python source code
as a test of our unparser.  This isn't actually a break with our consistent unpausing -- it's still
consistent, the python source just doesn't unparse to the same thing anymore. It does makes
it harder to test, so it's not worth maintaining the Python2-specific stuff anymore.

Since we only support `python@3.6:`, this PR removes (1) and (2) from the unparser, but keeps
one last tweak for unicode AST inconsistencies, as it's still needed for Python 3.5-3.7.

This fixes the CI error we've been seeing on `python@3.11.4` and `python@3.10.12`. Again, that
bug exists only in the test system and doesn't affect our canonical hashing of Python code.

lib/spack/spack/test/util/package_hash.py

@@ -337,15 +337,15 @@ def test_remove_complex_package_logic_filtered():
         ("grads", "rrlmwml3f2frdnqavmro3ias66h5b2ce"),
         ("llvm", "nufffum5dabmaf4l5tpfcblnbfjknvd3"),
         # has @when("@4.1.0") and raw unicode literals
-        ("mfem", "qtneutm6khd6epd2rhyuv2y6zavsxbed"),
+        ("mfem", "lbhr43gm5zdye2yhqznucxb4sg6vhryl"),
-        ("mfem@4.0.0", "qtneutm6khd6epd2rhyuv2y6zavsxbed"),
+        ("mfem@4.0.0", "lbhr43gm5zdye2yhqznucxb4sg6vhryl"),
-        ("mfem@4.1.0", "uit2ydzhra3b2mlvnq262qlrqqmuwq3d"),
+        ("mfem@4.1.0", "vjdjdgjt6nyo7ited2seki5epggw5gza"),
         # has @when("@1.5.0:")
         ("py-torch", "qs7djgqn7dy7r3ps4g7hv2pjvjk4qkhd"),
         ("py-torch@1.0", "qs7djgqn7dy7r3ps4g7hv2pjvjk4qkhd"),
         ("py-torch@1.6", "p4ine4hc6f2ik2f2wyuwieslqbozll5w"),
         # has a print with multiple arguments
-        ("legion", "sffy6vz3dusxnxeetofoomlaieukygoj"),
+        ("legion", "efpfd2c4pzhsbyc3o7plqcmtwm6b57yh"),
         # has nested `with when()` blocks and loops
         ("trilinos", "vqrgscjrla4hi7bllink7v6v6dwxgc2p"),
     ],

lib/spack/spack/util/unparse/unparser.py

@@ -87,28 +87,11 @@ def __init__(self, py_ver_consistent=False, _avoid_backslashes=False):
 
         Arguments:
             py_ver_consistent (bool): if True, generate unparsed code that is
-                consistent between Python 2.7 and 3.5-3.10.
+                consistent between Python versions 3.5-3.11.
-
-        Consistency is achieved by:
-            1. Ensuring that *args and **kwargs are always the last arguments,
-               regardless of the python version, because Python 2's AST does not
-               have sufficient information to reconstruct star-arg order.
-            2. Always unparsing print as a function.
-            3. Unparsing Python3 unicode literals the way Python 2 would.
-
-        Without these changes, the same source can generate different code for Python 2
-        and Python 3, depending on subtle AST differences.  The first of these two
-        causes this module to behave differently from Python 3.8+'s `ast.unparse()`
-
-        One place where single source will generate an inconsistent AST is with
-        multi-argument print statements, e.g.::
-
-            print("foo", "bar", "baz")
-
-        In Python 2, this prints a tuple; in Python 3, it is the print function with
-        multiple arguments.  Use ``from __future__ import print_function`` to avoid
-        this inconsistency.
 
+        For legacy reasons, consistency is achieved by unparsing Python3 unicode literals
+        the way Python 2 would. This preserved Spack package hash consistency during the
+        python2/3 transition
         """
         self.future_imports = []
         self._indent = 0
@@ -299,61 +282,6 @@ def visit_Exec(self, node):
             self.write(", ")
             self.dispatch(node.locals)
 
-    def visit_Print(self, node):
-        # Use print function so that python 2 unparsing is consistent with 3
-        if self._py_ver_consistent:
-            self.fill("print")
-            with self.delimit("(", ")"):
-                values = node.values
-
-                # Can't tell print(foo, bar, baz) and print((foo, bar, baz)) apart in
-                # python 2 and 3, so treat them the same to make hashes consistent.
-                # Single-tuple print are rare and unlikely to affect package hashes,
-                # esp. as they likely print to stdout.
-                if len(values) == 1 and isinstance(values[0], ast.Tuple):
-                    values = node.values[0].elts
-
-                do_comma = False
-                for e in values:
-                    if do_comma:
-                        self.write(", ")
-                    else:
-                        do_comma = True
-                    self.dispatch(e)
-
-                if not node.nl:
-                    if do_comma:
-                        self.write(", ")
-                    else:
-                        do_comma = True
-                    self.write("end=''")
-
-                if node.dest:
-                    if do_comma:
-                        self.write(", ")
-                    else:
-                        do_comma = True
-                    self.write("file=")
-                    self.dispatch(node.dest)
-
-        else:
-            # unparse Python 2 print statements
-            self.fill("print ")
-
-            do_comma = False
-            if node.dest:
-                self.write(">>")
-                self.dispatch(node.dest)
-                do_comma = True
-            for e in node.values:
-                if do_comma:
-                    self.write(", ")
-                else:
-                    do_comma = True
-                self.dispatch(e)
-            if not node.nl:
-                self.write(",")
-
     def visit_Global(self, node):
         self.fill("global ")
         interleave(lambda: self.write(", "), self.write, node.names)
@@ -962,25 +890,16 @@ def visit_Call(self, node):
         self.set_precedence(_Precedence.ATOM, node.func)
 
         args = node.args
-        if self._py_ver_consistent:
-            # make print(a, b, c) and print((a, b, c)) equivalent, since you can't
-            # tell them apart between Python 2 and 3. See _Print() for more details.
-            if getattr(node.func, "id", None) == "print":
-                if len(node.args) == 1 and isinstance(node.args[0], ast.Tuple):
-                    args = node.args[0].elts
-
         self.dispatch(node.func)
+
         with self.delimit("(", ")"):
             comma = False
 
-            # starred arguments last in Python 3.5+, for consistency w/earlier versions
+            # NOTE: this code is no longer compatible with python versions 2.7:3.4
-            star_and_kwargs = []
+            # If you run on python@:3.4, you will see instability in package hashes
-            move_stars_last = sys.version_info[:2] >= (3, 5)
+            # across python versions
 
             for e in args:
-                if move_stars_last and isinstance(e, ast.Starred):
-                    star_and_kwargs.append(e)
-                else:
                 if comma:
                     self.write(", ")
                 else:
@@ -988,40 +907,12 @@ def visit_Call(self, node):
                 self.dispatch(e)
 
             for e in node.keywords:
-                # starting from Python 3.5 this denotes a kwargs part of the invocation
-                if e.arg is None and move_stars_last:
-                    star_and_kwargs.append(e)
-                else:
                 if comma:
                     self.write(", ")
                 else:
                     comma = True
                 self.dispatch(e)
 
-            if move_stars_last:
-                for e in star_and_kwargs:
-                    if comma:
-                        self.write(", ")
-                    else:
-                        comma = True
-                    self.dispatch(e)
-
-            if sys.version_info[:2] < (3, 5):
-                if node.starargs:
-                    if comma:
-                        self.write(", ")
-                    else:
-                        comma = True
-                    self.write("*")
-                    self.dispatch(node.starargs)
-                if node.kwargs:
-                    if comma:
-                        self.write(", ")
-                    else:
-                        comma = True
-                    self.write("**")
-                    self.dispatch(node.kwargs)
-
     def visit_Subscript(self, node):
         self.set_precedence(_Precedence.ATOM, node.value)
         self.dispatch(node.value)