Trac #27680: MR11: Lazy initialization of libgap

E. Madison Bray ([https://gitlab.com/embray @embray]) opened a merge
request at https://gitlab.com/sagemath/sage/merge_requests/11:
----
{{{
#!markdown
Previously the `Gap` class that implements the (effectively singleton)
`libgap` instance variable in `sage.libs.gap.libgap` called
`initialize()` (the function which initializes libgap for the process)
in its `__init__` method.

Because of this, it was impossible to import it without causing libgap
to be initialized, creating slowdown during Sage initialization.  The
tradeoff is that most code in Sage that uses the libgap interface has to
awkwardly use inline imports of `from sage.libs.gap.libgap import
libgap` all over the place.

I would like to do away with that, especially for work on
[#18267](https://trac.sagemath.org/ticket/18267).

This removes `initialize()` from `Gap.__init__()` and instead carefully
places calls to `initialize()` just in the few places where it's
absolutely crucial to ensure libgap is initialized first (specifically
in code paths that users and developers are actually intended to use
directly; it is not added directly to every single function that uses
GAP objects).

This sacrifices some simplicity in implementation of the libgap
interface for simplicity in using it, which I think is crucial for
updating more code in Sage to use it over the pexpect interface.
}}}

URL: https://trac.sagemath.org/27680
Reported by: galois
Ticket author(s): Erik Bray
Reviewer(s): Dima Pasechnik

src/sage/libs/gap/element.pyx

@@ -22,6 +22,7 @@ from cpython.object cimport Py_EQ, Py_NE, Py_LE, Py_GE, Py_LT, Py_GT
 from cysignals.signals cimport sig_on, sig_off
 
 from .gap_includes cimport *
+from .libgap import libgap
 from .util cimport *
 from .util import GAPError
 from sage.cpython.string cimport str_to_bytes, char_to_str
@@ -73,7 +74,6 @@ cdef Obj make_gap_list(sage_list) except NULL:
 
     The list of the elements in ``a`` as a Gap ``Obj``.
     """
-    from sage.libs.gap.libgap import libgap
     cdef GapElement l = libgap.eval('[]')
     cdef GapElement elem
     for x in sage_list:
@@ -104,7 +104,6 @@ cdef Obj make_gap_matrix(sage_list, gap_ring) except NULL:
 
     The list of the elements in ``sage_list`` as a Gap ``Obj``.
     """
-    from sage.libs.gap.libgap import libgap
     cdef GapElement l = libgap.eval('[]')
     cdef GapElement elem
     cdef GapElement one
@@ -170,7 +169,6 @@ cdef Obj make_gap_record(sage_dict) except NULL:
         sage: libgap({'a': 1, 'b':123})   # indirect doctest
         rec( a := 1, b := 123 )
     """
-    from sage.libs.gap.libgap import libgap
     data = [ (str(key), libgap(value)) for key, value in sage_dict.iteritems() ]
 
     cdef Obj rec
@@ -578,7 +576,6 @@ cdef class GapElement(RingElement):
             ...
             GAPError: Error, no method found! Error, no 1st choice method found for `in' on 2 arguments
         """
-        from sage.libs.gap.libgap import libgap
         GAP_IN = libgap.eval(r'\in')
         return GAP_IN(other, self).sage()
 

src/sage/libs/gap/libgap.pyx

@@ -228,17 +228,6 @@ from sage.misc.randstate cimport current_randstate
 from sage.misc.superseded import deprecated_function_alias, deprecation
 
 
-############################################################################
-### Debugging ##############################################################
-############################################################################
-
-from sage.misc.lazy_import import is_during_startup
-if is_during_startup():
-    import sys, traceback
-    print('Importing libgap during startup!')
-    traceback.print_stack(None, None, sys.stdout)
-
-
 ############################################################################
 ### Gap  ###################################################################
 ############################################################################
@@ -312,6 +301,7 @@ class Gap(Parent):
             [ 0.333333, 0.8, 3. ]
 
         """
+        initialize()
         if isinstance(x, GapElement):
             return x
         elif isinstance(x, (list, tuple, Vector)):
@@ -412,6 +402,7 @@ class Gap(Parent):
         if not isinstance(gap_command, basestring):
             gap_command = str(gap_command._gap_init_())
 
+        initialize()
         elem = make_any_gap_element(self, gap_eval(gap_command))
 
         # If the element is NULL just return None instead
@@ -445,6 +436,7 @@ class Gap(Parent):
             sage: libgap.function_factory('Print')
             <Gap function "Print">
         """
+        initialize()
         return make_GapElement_Function(self, gap_eval(function_name))
 
     def set_global(self, variable, value):
@@ -552,6 +544,7 @@ class Gap(Parent):
             1
         """
         from sage.libs.gap.context_managers import GlobalVariableContext
+        initialize()
         return GlobalVariableContext(variable, value)
 
     def set_seed(self, seed=None):
@@ -630,8 +623,6 @@ class Gap(Parent):
             sage: type(libgap._get_object())
             <class 'sage.libs.gap.libgap.Gap'>
         """
-        initialize()
-        from sage.rings.integer_ring import ZZ
         Parent.__init__(self, base=ZZ)
 
     def __repr__(self):
@@ -690,9 +681,11 @@ class Gap(Parent):
         from sage.libs.gap.gap_functions import common_gap_functions
         from sage.libs.gap.gap_globals import common_gap_globals
         if name in common_gap_functions:
+            initialize()
             g = make_GapElement_Function(self, gap_eval(name))
             assert g.is_function()
         elif name in common_gap_globals:
+            initialize()
             g = make_any_gap_element(self, gap_eval(name))
         else:
             raise AttributeError(f'No such attribute: {name}.')
@@ -785,6 +778,7 @@ class Gap(Parent):
             sage: del a
             sage: libgap.collect()
         """
+        initialize()
         rc = CollectBags(0, 1)
         if rc != 1:
             raise RuntimeError('Garbage collection failed.')

src/sage/tests/startup.py

@@ -1,11 +1,6 @@
 r"""
 Ensure that certain modules are not loaded on startup.
 
-EXAMPLES::
-
-    sage: 'sage.libs.gap.libgap' in sys.modules
-    False
-
 Check that IPython is not imported at startup (:trac:`18726`). It is
 imported by the doctest framework, so the simple test like above would
 not work. Instead, we test this by starting a new Python process::