Issue #13150: sysconfig no longer parses the Makefile and config.h files

when imported, instead doing it at build time.  This makes importing
sysconfig faster and reduces Python startup time by 20%.

.hgignore

@@ -49,6 +49,7 @@ libpython*.so*
 *.pyd
 *.cover
 *~
+Lib/_sysconfigdata.py
 Lib/lib2to3/*.pickle
 Lib/test/data/*
 Misc/*.wpu

Lib/sysconfig.py

@@ -319,9 +319,11 @@ def get_makefile_filename():
         config_dir_name = 'config'
     return os.path.join(get_path('stdlib'), config_dir_name, 'Makefile')
 
-
-def _init_posix(vars):
-    """Initialize the module as appropriate for POSIX systems."""
+def _generate_posix_vars():
+    """Generate the Python module containing build-time variables."""
+    import pprint
+    vars = {}
+    destfile = os.path.join(os.path.dirname(__file__), '_sysconfigdata.py')
     # load the installed Makefile:
     makefile = get_makefile_filename()
     try:
@@ -346,7 +348,15 @@ def _init_posix(vars):
     # the scripts are in another directory.
     if _PYTHON_BUILD:
         vars['LDSHARED'] = vars['BLDSHARED']
+    with open(destfile, 'w', encoding='utf8') as f:
+        f.write('build_time_vars = ')
+        pprint.pprint(vars, stream=f)
 
+def _init_posix(vars):
+    """Initialize the module as appropriate for POSIX systems."""
+    # _sysconfigdata is generated at build time, see _generate_posix_vars()
+    from _sysconfigdata import build_time_vars
+    vars.update(build_time_vars)
 
 def _init_non_posix(vars):
     """Initialize the module as appropriate for NT"""
@@ -753,6 +763,9 @@ def _print_dict(title, data):
 
 def _main():
     """Display all information sysconfig detains."""
+    if '--generate-posix-vars' in sys.argv:
+        _generate_posix_vars()
+        return
     print('Platform: "%s"' % get_platform())
     print('Python version: "%s"' % get_python_version())
     print('Current installation scheme: "%s"' % _get_default_scheme())

Makefile.pre.in

@@ -396,7 +396,7 @@ LIBRARY_OBJS=	\
 
 # Default target
 all:		build_all
-build_all:	$(BUILDPYTHON) oldsharedmods sharedmods gdbhooks Modules/_testembed
+build_all:	$(BUILDPYTHON) sysconfig oldsharedmods sharedmods gdbhooks Modules/_testembed
 
 # Compile a binary with gcc profile guided optimization.
 profile-opt:
@@ -429,12 +429,15 @@ coverage:
 $(BUILDPYTHON):	Modules/python.o $(LIBRARY) $(LDLIBRARY) $(PY3LIBRARY)
 	$(LINKCC) $(PY_LDFLAGS) $(LINKFORSHARED) -o $@ Modules/python.o $(BLDLIBRARY) $(LIBS) $(MODLIBS) $(SYSLIBS) $(LDLAST)
 
-platform: $(BUILDPYTHON)
+platform: $(BUILDPYTHON) sysconfig
 	$(RUNSHARED) ./$(BUILDPYTHON) -E -c 'import sys ; from sysconfig import get_platform ; print(get_platform()+"-"+sys.version[0:3])' >platform
 
+# Generate the sysconfig build-time data
+sysconfig: $(BUILDPYTHON)
+	$(RUNSHARED) ./$(BUILDPYTHON) -SE -m sysconfig --generate-posix-vars
 
 # Build the shared modules
-sharedmods: $(BUILDPYTHON)
+sharedmods: $(BUILDPYTHON) sysconfig
 	@case $$MAKEFLAGS in \
 	*s*) $(RUNSHARED) CC='$(CC)' LDSHARED='$(BLDSHARED)' OPT='$(OPT)' ./$(BUILDPYTHON) -E $(srcdir)/setup.py -q build;; \
 	*) $(RUNSHARED) CC='$(CC)' LDSHARED='$(BLDSHARED)' OPT='$(OPT)' ./$(BUILDPYTHON) -E $(srcdir)/setup.py build;; \
@@ -1379,7 +1382,7 @@ patchcheck:
 Python/thread.o: @THREADHEADERS@
 
 # Declare targets that aren't real files
-.PHONY: all build_all sharedmods oldsharedmods test quicktest
+.PHONY: all build_all sysconfig sharedmods oldsharedmods test quicktest
 .PHONY: install altinstall oldsharedinstall bininstall altbininstall
 .PHONY: maninstall libinstall inclinstall libainstall sharedinstall
 .PHONY: frameworkinstall frameworkinstallframework frameworkinstallstructure

Misc/NEWS

@@ -315,6 +315,10 @@ Core and Builtins
 Library
 -------
 
+- Issue #13150: sysconfig no longer parses the Makefile and config.h files
+  when imported, instead doing it at build time.  This makes importing
+  sysconfig faster and reduces Python startup time by 20%.
+
 - Issue #12448: smtplib now flushes stdout while running ``python -m smtplib``
   in order to display the prompt correctly.