Warn when no "language_level" is specified to prepare the transition to language_level=3.

Closes #2593.

Cython/Compiler/Main.py

@@ -67,9 +67,10 @@ class Context(object):
     #  language_level        int     currently 2 or 3 for Python 2/3
 
     cython_scope = None
+    language_level = None  # warn when not set but default to Py2
 
     def __init__(self, include_directories, compiler_directives, cpp=False,
-                 language_level=2, options=None):
+                 language_level=None, options=None):
         # cython_scope is a hack, set to False by subclasses, in order to break
         # an infinite loop.
         # Better code organization would fix it.
@@ -91,7 +92,8 @@ class Context(object):
             os.path.join(os.path.dirname(__file__), os.path.pardir, 'Includes')))
         self.include_directories = include_directories + [standard_include_path]
 
-        self.set_language_level(language_level)
+        if language_level is not None:
+            self.set_language_level(language_level)
 
         self.gdb_debug_outputwriter = None
 
@@ -548,9 +550,10 @@ class CompilationOptions(object):
                 ', '.join(unknown_options))
             raise ValueError(message)
 
+        directive_defaults = Options.get_directive_defaults()
         directives = dict(options['compiler_directives'])  # copy mutable field
         # check for invalid directives
-        unknown_directives = set(directives) - set(Options.get_directive_defaults())
+        unknown_directives = set(directives) - set(directive_defaults)
         if unknown_directives:
             message = "got unknown compiler directive%s: %s" % (
                 's' if len(unknown_directives) > 1 else '',
@@ -562,7 +565,9 @@ class CompilationOptions(object):
             warnings.warn("C++ mode forced when in Pythran mode!")
             options['cplus'] = True
         if 'language_level' in directives and 'language_level' not in kw:
-            options['language_level'] = int(directives['language_level'])
+            options['language_level'] = directives['language_level']
+        elif not options.get('language_level'):
+            options['language_level'] = directive_defaults.get('language_level')
         if 'formal_grammar' in directives and 'formal_grammar' not in kw:
             options['formal_grammar'] = directives['formal_grammar']
         if options['cache'] is True:
@@ -824,7 +829,7 @@ default_options = dict(
     emit_linenums = False,
     relative_path_in_code_position_comments = True,
     c_line_in_traceback = True,
-    language_level = 2,
+    language_level = None,  # warn but default to 2
     formal_grammar = False,
     gdb_debug = False,
     compile_time_env = None,

Cython/Compiler/Options.py

@@ -197,7 +197,7 @@ _directive_defaults = {
     'autotestdict': True,
     'autotestdict.cdef': False,
     'autotestdict.all': False,
-    'language_level': 2,
+    'language_level': None,
     'fast_getattr': False,  # Undocumented until we come up with a better way to handle this everywhere.
     'py2_import': False,  # For backward compatibility of Cython's source code in Py3 source mode
     'preliminary_late_includes_cy28': False,  # Temporary directive in 0.28, to be removed in a later version (see GH#2079).
@@ -299,6 +299,7 @@ def normalise_encoding_name(option_name, encoding):
 
 # Override types possibilities above, if needed
 directive_types = {
+    'language_level': int,  # values can be None/2/3, where None == 2+warning
     'auto_pickle': bool,
     'final' : bool,  # final cdef classes and methods
     'internal' : bool,  # cdef class visibility in the module dict

Cython/Compiler/Parsing.py

@@ -3662,6 +3662,17 @@ def p_module(s, pxd, full_module_name, ctx=Ctx):
     directive_comments = p_compiler_directive_comments(s)
     s.parse_comments = False
 
+    if s.context.language_level is None:
+        s.context.set_language_level(2)
+        if pos[0].filename:
+            import warnings
+            warnings.warn(
+                "Cython directive 'language_level' not set, using 2 for now (Py2). "
+                "This will change in a later release! File: %s" % pos[0].filename,
+                FutureWarning,
+                stacklevel=1 if cython.compiled else 2,
+            )
+
     doc = p_doc_string(s)
     if pxd:
         level = 'module_pxd'

Cython/Compiler/Scanning.py

@@ -147,6 +147,8 @@ class SourceDescriptor(object):
     """
     A SourceDescriptor should be considered immutable.
     """
+    filename = None
+
     _file_type = 'pyx'
 
     _escaped_description = None
@@ -274,8 +276,6 @@ class StringSourceDescriptor(SourceDescriptor):
     Instances of this class can be used instead of a filenames if the
     code originates from a string object.
     """
-    filename = None
-
     def __init__(self, name, code):
         self.name = name
         #self.set_file_type_from_name(name)

Cython/Compiler/TreeFragment.py

@@ -29,7 +29,8 @@ class StringParseContext(Main.Context):
             include_directories = []
         if compiler_directives is None:
             compiler_directives = {}
-        Main.Context.__init__(self, include_directories, compiler_directives, cpp=cpp)
+        # TODO: see if "language_level=3" also works for our internal code here.
+        Main.Context.__init__(self, include_directories, compiler_directives, cpp=cpp, language_level=2)
         self.module_name = name
 
     def find_module(self, module_name, relative_to=None, pos=None, need_pxd=1, absolute_fallback=True):

runtests.py

@@ -2295,8 +2295,8 @@ def runtests(options, cmd_args, coverage=None):
         sys.stderr.write("Disabling forked testing to support XML test output\n")
         options.fork = False
 
-    if WITH_CYTHON and options.language_level == 3:
-        sys.stderr.write("Using Cython language level 3.\n")
+    if WITH_CYTHON:
+        sys.stderr.write("Using Cython language level %d.\n" % options.language_level)
 
     test_bugs = False
     if options.tickets:

setup.py

@@ -156,8 +156,9 @@ def compile_cython_modules(profile=False, compile_more=False, cython_with_refnan
         extensions[-1].sources[0] = pyx_source_file
 
     from Cython.Distutils.build_ext import new_build_ext
+    from Cython.Compiler.Options import get_directive_defaults
+    get_directive_defaults()['language_level'] = 2
     if profile:
-        from Cython.Compiler.Options import get_directive_defaults
         get_directive_defaults()['profile'] = True
         sys.stderr.write("Enabled profiling for the Cython binary modules\n")