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.

build/bin/sage-spkg

@@ -64,7 +64,9 @@
 #*****************************************************************************
 
 # 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

@@ -1,14 +1,14 @@
 SAGE_SPKG_CONFIGURE([python3], [
     SAGE_SPKG_DEPCHECK([sqlite libpng bzip2 xz libffi], [
         check_modules="sqlite3, ctypes, math, hashlib, crypt, readline, socket, zlib, distutils.core"
-        AC_CACHE_CHECK([for python3 >= 3.7, < 3.8 with sqlite3 module], [ac_cv_path_PYTHON3], [
+        AC_CACHE_CHECK([for python3 >= 3.6, < 3.8 with sqlite3 module], [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], [3.7.0], [
+                    AX_COMPARE_VERSION([$python3_version], [ge], [3.6.0], [
                         AX_COMPARE_VERSION([$python3_version], [lt], [3.8.0], [
                             AS_IF(["$ac_path_PYTHON3" -S -c "import $check_modules"], [
                                 AC_LANG_PUSH([C])
@@ -51,7 +51,7 @@ 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, < 3.8 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 an extension])
                                 ])

src/bin/sage-env-config.in

@@ -38,6 +38,14 @@ fi
 
 export SAGE_SYSTEM_PYTHON="@SAGE_SYSTEM_PYTHON@"
 
+if [ -n "$SAGE_SYSTEM_PYTHON" ]; then
+    # Force LC_CTYPE to a UTF-8 locale to ensure that Python uses UTF-8 by
+    # default (on Python 3.7+ this is not necessary as UTF-8 is always the
+    # default encoding, so we only need this if using a system Python that
+    # is less than 3.7, since Sage's Python is 3.7+)
+    export LC_CTYPE="C.UTF-8"
+fi
+
 # This is usually blank if the system GMP is used, or $SAGE_LOCAL otherwise
 export SAGE_GMP_PREFIX="@SAGE_GMP_PREFIX@"
 export SAGE_GMP_INCLUDE="@SAGE_GMP_INCLUDE@"

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

@@ -359,7 +359,22 @@ 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

@@ -601,6 +601,7 @@ 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

@@ -1035,7 +1035,7 @@ 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

@@ -1752,10 +1752,8 @@ 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

@@ -5879,14 +5879,10 @@ 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)):