STINNER Victor added the comment: Serhiy: "It looks to me that this optimization was added to avoid spending executing time for docstrings."
Hum, let me dig Mercurial history. ---- changeset: 39364:8ef3f8cf90e1 branch: legacy-trunk user: Neal Norwitz <nnorw...@gmail.com> date: Fri Aug 04 05:09:28 2006 +0000 files: Lib/test/test_code.py Misc/NEWS Python/compile.c description: Bug #1333982: string/number constants were inappropriately stored in the byte code and co_consts even if they were not used, ie immediately popped off the stack. --- https://bugs.python.org/issue1333982#msg26659: "For reference, an optimization that got lost: (...) 'a' is the docstring, but the 'b' previously did not show up anywhere in the code object. Now there is the LOAD_CONST/POP_TOP pair." Ah, it was a regression introduced by the new AST compiler. But the change introduced a new optimization: numbers are now also ignored. In Python 2.4 (before AST), numbers were not ignored: --- >>> def f(): ... "a" ... 1 ... "b" ... >>> import dis; dis.dis(f) 3 0 LOAD_CONST 1 (1) 3 POP_TOP 4 LOAD_CONST 2 (None) 7 RETURN_VALUE --- If we continue to dig deeper, before AST, I found: --- changeset: 4991:8276916e1ea8 branch: legacy-trunk user: Guido van Rossum <gu...@python.org> date: Fri Jan 17 21:04:03 1997 +0000 files: Python/compile.c description: Add co_stacksize field to codeobject structure, and stacksize argument to PyCode_New() argument list. Move MAXBLOCKS constant to conpile.h. Added accurate calculation of the actual stack size needed by the generated code. Also commented out all fprintf statements (except for a new one to diagnose stack underflow, and one in #ifdef'ed out code), and added some new TO DO suggestions (now that the stacksize is taken of the TO DO list). --- This patch added the following code to com_expr_stmt() in Python/compile.c: + /* Forget it if we have just a doc string here */ + if (NCH(n) == 1 && get_rawdocstring(n) != NULL) + return; I'm unable to find the exact part of the compiler which ignores strings in statement expressions. ---------- _______________________________________ Python tracker <rep...@bugs.python.org> <http://bugs.python.org/issue26204> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
