more careful handling of the recursion limit

Author: mezzarobba

src/sage/functions/transcendental.py

@@ -17,6 +17,7 @@
 # ****************************************************************************
 import sys
 
+from sage.misc.misc import increase_recursion_limit
 from sage.rings.integer_ring import ZZ
 from sage.rings.real_mpfr import RR
 from sage.rings.real_double import RDF
@@ -545,9 +546,8 @@ def _eval_(self, x):
             max = x.parent()(1.1)*x + 10
             abs_prec = (-self.approximate(max).log2() + rel_prec + 2*max.log2()).ceil()
             self._f = {}
-            if sys.getrecursionlimit() < max + 10:
-                sys.setrecursionlimit(int(max) + 10)
-            self._compute_power_series(max.floor(), abs_prec, cache_ring=x.parent())
+            with increase_recursion_limit(int(max)):
+                self._compute_power_series(max.floor(), abs_prec, cache_ring=x.parent())
         return self._f[n](2*(x-n-x.parent()(0.5)))
 
     def power_series(self, n, abs_prec):

src/sage/misc/misc.py

@@ -38,9 +38,11 @@
 #                  https://www.gnu.org/licenses/
 # ****************************************************************************
 
+import contextlib
 import functools
 import os
 import pdb
+import sys
 import warnings
 
 from .lazy_string import lazy_string
@@ -1167,3 +1169,34 @@ def wrapper(*args, **kwargs):
             return result
     wrapper.has_run = False
     return wrapper
+
+
+@contextlib.contextmanager
+def increase_recursion_limit(increment):
+    r"""
+    Context manager to temporarily change the Python maximum recursion depth.
+
+    INPUT:
+
+    - `increment`: increment to add to the current limit
+
+    EXAMPLES::
+
+        sage: from sage.misc.misc import increase_recursion_limit
+        sage: def rec(n): None if n == 0 else rec(n-1)
+        sage: rec(10000)
+        Traceback (most recent call last):
+        ...
+        RecursionError: maximum recursion depth exceeded...
+        sage: with increase_recursion_limit(10000): rec(10000)
+        sage: rec(10000)
+        Traceback (most recent call last):
+        ...
+        RecursionError: maximum recursion depth exceeded...
+    """
+    old_limit = sys.getrecursionlimit()
+    sys.setrecursionlimit(old_limit + increment)
+    try:
+        yield
+    finally:
+        sys.setrecursionlimit(old_limit)

src/sage/rings/qqbar.py

@@ -560,6 +560,7 @@
 from sage.misc.fast_methods import Singleton
 from sage.misc.cachefunc import cached_method
 from sage.misc.lazy_string import lazy_string
+from sage.misc.misc import increase_recursion_limit
 from sage.structure.coerce import parent_is_numerical, parent_is_real_numerical
 from sage.structure.sage_object import SageObject
 from sage.structure.richcmp import (richcmp, richcmp_method,
@@ -8543,9 +8544,7 @@ def exactify(self):
             sage: sys.setrecursionlimit(old_recursion_limit)
         """
         import sys
-        old_recursion_limit = sys.getrecursionlimit()
-        sys.setrecursionlimit(old_recursion_limit + 10)
-        try:
+        with increase_recursion_limit(10):
             left = self._left
             right = self._right
             left.exactify()
@@ -8560,8 +8559,7 @@ def exactify(self):
                 return ANRational(value)
             else:
                 return ANExtensionElement(gen, value)
-        finally:
-            sys.setrecursionlimit(old_recursion_limit)
+
 
 # These are the functions used to add, subtract, multiply, and divide
 # algebraic numbers. Basically, we try to compute exactly if both