Allow Sage to work with a system Python 3.6.

Currently sage-the-distribution is tested against a minimum of Python
3.7, but we can support more system Pythons by supporting down to 3.6
with some minimal fixes to tests.

Author: embray

build/bin/sage-spkg

@@ -68,7 +68,9 @@
 #*****************************************************************************
 
 # Avoid surprises with character ranges [a-z] in regular expressions
-export LC_ALL=C
+# See Trac #15791; some locales can produce different results for
+# character ranges (use C.UTF-8 to ensure UTF-8 default encoding in Python)
+export LC_ALL=C.UTF-8
 
 usage()
 {

build/pkgs/python3/spkg-configure.m4

@@ -10,14 +10,14 @@ SAGE_SPKG_CONFIGURE([python3], [
         dnl Using Python 3 for Sage.  Check if we can do venv with a system python3
         dnl instead of building our own copy.
         check_modules="sqlite3, ctypes, math, hashlib, crypt, readline, socket, zlib, distutils.core"
-        AC_CACHE_CHECK([for python3 >= 3.7.3, < 3.8 with modules $check_modules], [ac_cv_path_PYTHON3], [
+        AC_CACHE_CHECK([for python3 >= 3.6, < 3.8 with modules $check_modules], [ac_cv_path_PYTHON3], [
             AC_MSG_RESULT([])
-            AC_PATH_PROGS_FEATURE_CHECK([PYTHON3], [python3.7 python3], [
+            AC_PATH_PROGS_FEATURE_CHECK([PYTHON3], [python3.7 python3.6 python3], [
                 AC_MSG_CHECKING([... whether $ac_path_PYTHON3 is good])
                 python3_version=`"$ac_path_PYTHON3" --version 2>&1 \
                     | $SED -n -e 's/\([[0-9]]*\.[[0-9]]*\.[[0-9]]*\).*/\1/p'`
                 AS_IF([test -n "$python3_version"], [
-                    AX_COMPARE_VERSION([$python3_version], [ge], [3.7.3], [
+                    AX_COMPARE_VERSION([$python3_version], [ge], [3.6.0], [
                         AX_COMPARE_VERSION([$python3_version], [lt], [3.8.0], [
                             dnl Because the system python is not used directly but rather in a venv without site-packages,
                             dnl we test whether the module will be available in a venv.
@@ -118,7 +118,7 @@ EOF
                                         ac_path_PYTHON3_found=:
                                         AC_MSG_RESULT([yes])
                                         dnl introduction for AC_MSG_RESULT printed by AC_CACHE_CHECK
-                                        AC_MSG_CHECKING([for python3 >= 3.7.3, < 3.8 with modules $check_modules])
+                                        AC_MSG_CHECKING([for python3 >= 3.6, < 3.8 with modules $check_modules])
                                     ], [
                                         AC_MSG_RESULT([no, the version is in the supported range, and the modules can be imported, but distutils cannot build a C++ 11 extension])
                                     ])

src/sage/all.py

@@ -22,7 +22,7 @@
     ....:     'IPython', 'prompt_toolkit', 'jedi',     # sage dependencies
     ....:     'threading', 'multiprocessing',  # doctest dependencies
     ....:     '__main__', 'sage.doctest',      # doctesting
-    ....:     'signal', 'enum',                # may appear in Python 3
+    ....:     'signal', 'enum', 'types'        # may appear in Python 3
     ....: ]
     sage: def is_not_allowed(frame):
     ....:     module = inspect.getmodule(frame)

src/sage/combinat/subset.py

@@ -357,7 +357,22 @@ def cardinality(self):
         """
         return Integer(1) << self._s.cardinality()
 
-    __len__ = cardinality
+    def __len__(self):
+        r"""
+        Equivalent to ``self.cardinality()``.
+
+        TESTS::
+
+            ``__len__`` should return a Python int; in Python 3.7+ this happens
+            automatically, but not on Python 3.6.
+
+            sage: S = Subsets(Set([1,2,3]))
+            sage: len(S)
+            8
+            sage: type(len(S)) is int
+            True
+        """
+        return int(self.cardinality())
 
     def first(self):
         """

src/sage/graphs/views.pyx

@@ -611,6 +611,7 @@ cdef class EdgesView:
         elif i < 0:
             return list(self)[i]
         else:
+            i = int(i)  # For Python < 3.7 where islice doesn't support non-int
             try:
                 return next(islice(self, i, i + 1, 1))
             except StopIteration:

src/sage/misc/sagedoc.py

@@ -1064,7 +1064,7 @@ def search_src(string, extra1='', extra2='', extra3='', extra4='',
         sage: print(search_src(" fetch(", "def", interact=False)) # py3
         Traceback (most recent call last):
         ...
-        re.error: missing ), unterminated subpattern at position 6
+        error: missing ), unterminated subpattern at position 6
 
     To fix this, *escape* the parenthesis with a backslash::
 

src/sage/misc/sageinspect.py

@@ -1724,10 +1724,8 @@ def sage_formatargspec(args, varargs=None, varkw=None, defaults=None,
         sage: defaults = [3]
         sage: sage_formatargspec(args, defaults=defaults)
         '(a, b, c=3)'
-        sage: formatargspec(args, defaults=defaults) == sage_formatargspec(args, defaults=defaults) # py2
-        True
-        sage: formatargspec(args, defaults=defaults) == sage_formatargspec(args, defaults=defaults) # py3
-        doctest:...: DeprecationWarning: `formatargspec` is deprecated since Python 3.5. Use `signature` and the `Signature` object directly
+        sage: import warnings; warnings.simplefilter('ignore')  # py3: ignore DeprecationWarning
+        sage: formatargspec(args, defaults=defaults) == sage_formatargspec(args, defaults=defaults)
         True
     """
     def formatargandannotation(arg):

src/sage/symbolic/expression.pyx

@@ -5869,14 +5869,10 @@ cdef class Expression(CommutativeRingElement):
 
         Indexing directly with ``t[1]`` causes problems with numpy types.
 
-            sage: t[1] # py2
+            sage: t[1]
             Traceback (most recent call last):
             ...
-            TypeError: 'sage.symbolic.expression.Expression' object does not support indexing
-            sage: t[1] # py3
-            Traceback (most recent call last):
-            ...
-            TypeError: 'sage.symbolic.expression.Expression' object is not subscriptable
+            TypeError: 'sage.symbolic.expression.Expression' object ...
         """
         if (is_a_symbol(self._gobj) or is_a_constant(self._gobj) or
             is_a_numeric(self._gobj)):