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Commitabfa16b

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GH-114847: Speed upposixpath.realpath() (#114848)
Apply the following optimizations to `posixpath.realpath()`:- Remove use of recursion- Construct child paths directly rather than using `join()`- Use `os.getcwd[b]()` rather than `abspath()`- Use `startswith(sep)` rather than `isabs()`- Use slicing rather than `split()`Co-authored-by: Petr Viktorin <encukou@gmail.com>
1 parent9ceaee7 commitabfa16b

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3 files changed

+64
-34
lines changed

3 files changed

+64
-34
lines changed

‎Lib/posixpath.py‎

Lines changed: 54 additions & 34 deletions
Original file line numberDiff line numberDiff line change
@@ -403,55 +403,66 @@ def realpath(filename, *, strict=False):
403403
"""Return the canonical path of the specified filename, eliminating any
404404
symbolic links encountered in the path."""
405405
filename=os.fspath(filename)
406-
path,ok=_joinrealpath(filename[:0],filename,strict, {})
407-
returnabspath(path)
408-
409-
# Join two paths, normalizing and eliminating any symbolic links
410-
# encountered in the second path.
411-
# Two leading slashes are replaced by a single slash.
412-
def_joinrealpath(path,rest,strict,seen):
413-
ifisinstance(path,bytes):
406+
ifisinstance(filename,bytes):
414407
sep=b'/'
415408
curdir=b'.'
416409
pardir=b'..'
410+
getcwd=os.getcwdb
417411
else:
418412
sep='/'
419413
curdir='.'
420414
pardir='..'
415+
getcwd=os.getcwd
416+
417+
# The stack of unresolved path parts. When popped, a special value of None
418+
# indicates that a symlink target has been resolved, and that the original
419+
# symlink path can be retrieved by popping again. The [::-1] slice is a
420+
# very fast way of spelling list(reversed(...)).
421+
rest=filename.split(sep)[::-1]
422+
423+
# The resolved path, which is absolute throughout this function.
424+
# Note: getcwd() returns a normalized and symlink-free path.
425+
path=sepiffilename.startswith(sep)elsegetcwd()
421426

422-
ifrest.startswith(sep):
423-
rest=rest[1:]
424-
path=sep
427+
# Mapping from symlink paths to *fully resolved* symlink targets. If a
428+
# symlink is encountered but not yet resolved, the value is None. This is
429+
# used both to detect symlink loops and to speed up repeated traversals of
430+
# the same links.
431+
seen= {}
432+
433+
# Whether we're calling lstat() and readlink() to resolve symlinks. If we
434+
# encounter an OSError for a symlink loop in non-strict mode, this is
435+
# switched off.
436+
querying=True
425437

426438
whilerest:
427-
name,_,rest=rest.partition(sep)
439+
name=rest.pop()
440+
ifnameisNone:
441+
# resolved symlink target
442+
seen[rest.pop()]=path
443+
continue
428444
ifnotnameorname==curdir:
429445
# current dir
430446
continue
431447
ifname==pardir:
432448
# parent dir
433-
ifpath:
434-
parent,name=split(path)
435-
ifname==pardir:
436-
# ../..
437-
path=join(path,pardir)
438-
else:
439-
# foo/bar/.. -> foo
440-
path=parent
441-
else:
442-
# ..
443-
path=pardir
449+
path=path[:path.rindex(sep)]orsep
450+
continue
451+
ifpath==sep:
452+
newpath=path+name
453+
else:
454+
newpath=path+sep+name
455+
ifnotquerying:
456+
path=newpath
444457
continue
445-
newpath=join(path,name)
446458
try:
447459
st=os.lstat(newpath)
460+
ifnotstat.S_ISLNK(st.st_mode):
461+
path=newpath
462+
continue
448463
exceptOSError:
449464
ifstrict:
450465
raise
451-
is_link=False
452-
else:
453-
is_link=stat.S_ISLNK(st.st_mode)
454-
ifnotis_link:
455466
path=newpath
456467
continue
457468
# Resolve the symbolic link
@@ -467,14 +478,23 @@ def _joinrealpath(path, rest, strict, seen):
467478
os.stat(newpath)
468479
else:
469480
# Return already resolved part + rest of the path unchanged.
470-
returnjoin(newpath,rest),False
481+
path=newpath
482+
querying=False
483+
continue
471484
seen[newpath]=None# not resolved symlink
472-
path,ok=_joinrealpath(path,os.readlink(newpath),strict,seen)
473-
ifnotok:
474-
returnjoin(path,rest),False
475-
seen[newpath]=path# resolved symlink
485+
target=os.readlink(newpath)
486+
iftarget.startswith(sep):
487+
# Symlink target is absolute; reset resolved path.
488+
path=sep
489+
# Push the symlink path onto the stack, and signal its specialness by
490+
# also pushing None. When these entries are popped, we'll record the
491+
# fully-resolved symlink target in the 'seen' mapping.
492+
rest.append(newpath)
493+
rest.append(None)
494+
# Push the unresolved symlink target parts onto the stack.
495+
rest.extend(target.split(sep)[::-1])
476496

477-
returnpath,True
497+
returnpath
478498

479499

480500
supports_unicode_filenames= (sys.platform=='darwin')

‎Lib/test/test_posixpath.py‎

Lines changed: 9 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -456,6 +456,15 @@ def test_realpath_relative(self):
456456
finally:
457457
os_helper.unlink(ABSTFN)
458458

459+
@os_helper.skip_unless_symlink
460+
@skip_if_ABSTFN_contains_backslash
461+
deftest_realpath_missing_pardir(self):
462+
try:
463+
os.symlink(os_helper.TESTFN+"1",os_helper.TESTFN)
464+
self.assertEqual(realpath("nonexistent/../"+os_helper.TESTFN),ABSTFN+"1")
465+
finally:
466+
os_helper.unlink(os_helper.TESTFN)
467+
459468
@os_helper.skip_unless_symlink
460469
@skip_if_ABSTFN_contains_backslash
461470
deftest_realpath_symlink_loops(self):
Lines changed: 1 addition & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -0,0 +1 @@
1+
Speed up:func:`os.path.realpath` on non-Windows platforms.

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