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/Lib/os.py

http://unladen-swallow.googlecode.com/
Python | 759 lines | 759 code | 0 blank | 0 comment | 2 complexity | 7bd77f5baa16a87d5265d4300606a05a MD5 | raw file
  1r"""OS routines for Mac, NT, or Posix depending on what system we're on.
  2
  3This exports:
  4  - all functions from posix, nt, os2, or ce, e.g. unlink, stat, etc.
  5  - os.path is one of the modules posixpath, or ntpath
  6  - os.name is 'posix', 'nt', 'os2', 'ce' or 'riscos'
  7  - os.curdir is a string representing the current directory ('.' or ':')
  8  - os.pardir is a string representing the parent directory ('..' or '::')
  9  - os.sep is the (or a most common) pathname separator ('/' or ':' or '\\')
 10  - os.extsep is the extension separator ('.' or '/')
 11  - os.altsep is the alternate pathname separator (None or '/')
 12  - os.pathsep is the component separator used in $PATH etc
 13  - os.linesep is the line separator in text files ('\r' or '\n' or '\r\n')
 14  - os.defpath is the default search path for executables
 15  - os.devnull is the file path of the null device ('/dev/null', etc.)
 16
 17Programs that import and use 'os' stand a better chance of being
 18portable between different platforms.  Of course, they must then
 19only use functions that are defined by all platforms (e.g., unlink
 20and opendir), and leave all pathname manipulation to os.path
 21(e.g., split and join).
 22"""
 23
 24#'
 25
 26import sys, errno
 27
 28_names = sys.builtin_module_names
 29
 30# Note:  more names are added to __all__ later.
 31__all__ = ["altsep", "curdir", "pardir", "sep", "extsep", "pathsep", "linesep",
 32           "defpath", "name", "path", "devnull",
 33           "SEEK_SET", "SEEK_CUR", "SEEK_END"]
 34
 35def _get_exports_list(module):
 36    try:
 37        return list(module.__all__)
 38    except AttributeError:
 39        return [n for n in dir(module) if n[0] != '_']
 40
 41if 'posix' in _names:
 42    name = 'posix'
 43    linesep = '\n'
 44    from posix import *
 45    try:
 46        from posix import _exit
 47    except ImportError:
 48        pass
 49    import posixpath as path
 50
 51    import posix
 52    __all__.extend(_get_exports_list(posix))
 53    del posix
 54
 55elif 'nt' in _names:
 56    name = 'nt'
 57    linesep = '\r\n'
 58    from nt import *
 59    try:
 60        from nt import _exit
 61    except ImportError:
 62        pass
 63    import ntpath as path
 64
 65    import nt
 66    __all__.extend(_get_exports_list(nt))
 67    del nt
 68
 69elif 'os2' in _names:
 70    name = 'os2'
 71    linesep = '\r\n'
 72    from os2 import *
 73    try:
 74        from os2 import _exit
 75    except ImportError:
 76        pass
 77    if sys.version.find('EMX GCC') == -1:
 78        import ntpath as path
 79    else:
 80        import os2emxpath as path
 81        from _emx_link import link
 82
 83    import os2
 84    __all__.extend(_get_exports_list(os2))
 85    del os2
 86
 87elif 'ce' in _names:
 88    name = 'ce'
 89    linesep = '\r\n'
 90    from ce import *
 91    try:
 92        from ce import _exit
 93    except ImportError:
 94        pass
 95    # We can use the standard Windows path.
 96    import ntpath as path
 97
 98    import ce
 99    __all__.extend(_get_exports_list(ce))
100    del ce
101
102elif 'riscos' in _names:
103    name = 'riscos'
104    linesep = '\n'
105    from riscos import *
106    try:
107        from riscos import _exit
108    except ImportError:
109        pass
110    import riscospath as path
111
112    import riscos
113    __all__.extend(_get_exports_list(riscos))
114    del riscos
115
116else:
117    raise ImportError, 'no os specific module found'
118
119sys.modules['os.path'] = path
120from os.path import (curdir, pardir, sep, pathsep, defpath, extsep, altsep,
121    devnull)
122
123del _names
124
125# Python uses fixed values for the SEEK_ constants; they are mapped
126# to native constants if necessary in posixmodule.c
127SEEK_SET = 0
128SEEK_CUR = 1
129SEEK_END = 2
130
131#'
132
133# Super directory utilities.
134# (Inspired by Eric Raymond; the doc strings are mostly his)
135
136def makedirs(name, mode=0777):
137    """makedirs(path [, mode=0777])
138
139    Super-mkdir; create a leaf directory and all intermediate ones.
140    Works like mkdir, except that any intermediate path segment (not
141    just the rightmost) will be created if it does not exist.  This is
142    recursive.
143
144    """
145    head, tail = path.split(name)
146    if not tail:
147        head, tail = path.split(head)
148    if head and tail and not path.exists(head):
149        try:
150            makedirs(head, mode)
151        except OSError, e:
152            # be happy if someone already created the path
153            if e.errno != errno.EEXIST:
154                raise
155        if tail == curdir:           # xxx/newdir/. exists if xxx/newdir exists
156            return
157    mkdir(name, mode)
158
159def removedirs(name):
160    """removedirs(path)
161
162    Super-rmdir; remove a leaf directory and all empty intermediate
163    ones.  Works like rmdir except that, if the leaf directory is
164    successfully removed, directories corresponding to rightmost path
165    segments will be pruned away until either the whole path is
166    consumed or an error occurs.  Errors during this latter phase are
167    ignored -- they generally mean that a directory was not empty.
168
169    """
170    rmdir(name)
171    head, tail = path.split(name)
172    if not tail:
173        head, tail = path.split(head)
174    while head and tail:
175        try:
176            rmdir(head)
177        except error:
178            break
179        head, tail = path.split(head)
180
181def renames(old, new):
182    """renames(old, new)
183
184    Super-rename; create directories as necessary and delete any left
185    empty.  Works like rename, except creation of any intermediate
186    directories needed to make the new pathname good is attempted
187    first.  After the rename, directories corresponding to rightmost
188    path segments of the old name will be pruned way until either the
189    whole path is consumed or a nonempty directory is found.
190
191    Note: this function can fail with the new directory structure made
192    if you lack permissions needed to unlink the leaf directory or
193    file.
194
195    """
196    head, tail = path.split(new)
197    if head and tail and not path.exists(head):
198        makedirs(head)
199    rename(old, new)
200    head, tail = path.split(old)
201    if head and tail:
202        try:
203            removedirs(head)
204        except error:
205            pass
206
207__all__.extend(["makedirs", "removedirs", "renames"])
208
209def walk(top, topdown=True, onerror=None, followlinks=False):
210    """Directory tree generator.
211
212    For each directory in the directory tree rooted at top (including top
213    itself, but excluding '.' and '..'), yields a 3-tuple
214
215        dirpath, dirnames, filenames
216
217    dirpath is a string, the path to the directory.  dirnames is a list of
218    the names of the subdirectories in dirpath (excluding '.' and '..').
219    filenames is a list of the names of the non-directory files in dirpath.
220    Note that the names in the lists are just names, with no path components.
221    To get a full path (which begins with top) to a file or directory in
222    dirpath, do os.path.join(dirpath, name).
223
224    If optional arg 'topdown' is true or not specified, the triple for a
225    directory is generated before the triples for any of its subdirectories
226    (directories are generated top down).  If topdown is false, the triple
227    for a directory is generated after the triples for all of its
228    subdirectories (directories are generated bottom up).
229
230    When topdown is true, the caller can modify the dirnames list in-place
231    (e.g., via del or slice assignment), and walk will only recurse into the
232    subdirectories whose names remain in dirnames; this can be used to prune
233    the search, or to impose a specific order of visiting.  Modifying
234    dirnames when topdown is false is ineffective, since the directories in
235    dirnames have already been generated by the time dirnames itself is
236    generated.
237
238    By default errors from the os.listdir() call are ignored.  If
239    optional arg 'onerror' is specified, it should be a function; it
240    will be called with one argument, an os.error instance.  It can
241    report the error to continue with the walk, or raise the exception
242    to abort the walk.  Note that the filename is available as the
243    filename attribute of the exception object.
244
245    By default, os.walk does not follow symbolic links to subdirectories on
246    systems that support them.  In order to get this functionality, set the
247    optional argument 'followlinks' to true.
248
249    Caution:  if you pass a relative pathname for top, don't change the
250    current working directory between resumptions of walk.  walk never
251    changes the current directory, and assumes that the client doesn't
252    either.
253
254    Example:
255
256    import os
257    from os.path import join, getsize
258    for root, dirs, files in os.walk('python/Lib/email'):
259        print root, "consumes",
260        print sum([getsize(join(root, name)) for name in files]),
261        print "bytes in", len(files), "non-directory files"
262        if 'CVS' in dirs:
263            dirs.remove('CVS')  # don't visit CVS directories
264    """
265
266    from os.path import join, isdir, islink
267
268    # We may not have read permission for top, in which case we can't
269    # get a list of the files the directory contains.  os.path.walk
270    # always suppressed the exception then, rather than blow up for a
271    # minor reason when (say) a thousand readable directories are still
272    # left to visit.  That logic is copied here.
273    try:
274        # Note that listdir and error are globals in this module due
275        # to earlier import-*.
276        names = listdir(top)
277    except error, err:
278        if onerror is not None:
279            onerror(err)
280        return
281
282    dirs, nondirs = [], []
283    for name in names:
284        if isdir(join(top, name)):
285            dirs.append(name)
286        else:
287            nondirs.append(name)
288
289    if topdown:
290        yield top, dirs, nondirs
291    for name in dirs:
292        path = join(top, name)
293        if followlinks or not islink(path):
294            for x in walk(path, topdown, onerror, followlinks):
295                yield x
296    if not topdown:
297        yield top, dirs, nondirs
298
299__all__.append("walk")
300
301# Make sure os.environ exists, at least
302try:
303    environ
304except NameError:
305    environ = {}
306
307def execl(file, *args):
308    """execl(file, *args)
309
310    Execute the executable file with argument list args, replacing the
311    current process. """
312    execv(file, args)
313
314def execle(file, *args):
315    """execle(file, *args, env)
316
317    Execute the executable file with argument list args and
318    environment env, replacing the current process. """
319    env = args[-1]
320    execve(file, args[:-1], env)
321
322def execlp(file, *args):
323    """execlp(file, *args)
324
325    Execute the executable file (which is searched for along $PATH)
326    with argument list args, replacing the current process. """
327    execvp(file, args)
328
329def execlpe(file, *args):
330    """execlpe(file, *args, env)
331
332    Execute the executable file (which is searched for along $PATH)
333    with argument list args and environment env, replacing the current
334    process. """
335    env = args[-1]
336    execvpe(file, args[:-1], env)
337
338def execvp(file, args):
339    """execp(file, args)
340
341    Execute the executable file (which is searched for along $PATH)
342    with argument list args, replacing the current process.
343    args may be a list or tuple of strings. """
344    _execvpe(file, args)
345
346def execvpe(file, args, env):
347    """execvpe(file, args, env)
348
349    Execute the executable file (which is searched for along $PATH)
350    with argument list args and environment env , replacing the
351    current process.
352    args may be a list or tuple of strings. """
353    _execvpe(file, args, env)
354
355__all__.extend(["execl","execle","execlp","execlpe","execvp","execvpe"])
356
357def _execvpe(file, args, env=None):
358    if env is not None:
359        func = execve
360        argrest = (args, env)
361    else:
362        func = execv
363        argrest = (args,)
364        env = environ
365
366    head, tail = path.split(file)
367    if head:
368        func(file, *argrest)
369        return
370    if 'PATH' in env:
371        envpath = env['PATH']
372    else:
373        envpath = defpath
374    PATH = envpath.split(pathsep)
375    saved_exc = None
376    saved_tb = None
377    for dir in PATH:
378        fullname = path.join(dir, file)
379        try:
380            func(fullname, *argrest)
381        except error, e:
382            tb = sys.exc_info()[2]
383            if (e.errno != errno.ENOENT and e.errno != errno.ENOTDIR
384                and saved_exc is None):
385                saved_exc = e
386                saved_tb = tb
387    if saved_exc:
388        raise error, saved_exc, saved_tb
389    raise error, e, tb
390
391# Change environ to automatically call putenv() if it exists
392try:
393    # This will fail if there's no putenv
394    putenv
395except NameError:
396    pass
397else:
398    # Fake unsetenv() for Windows
399    # not sure about os2 here but
400    # I'm guessing they are the same.
401
402    if name in ('os2', 'nt'):
403        def unsetenv(key):
404            putenv(key, "")
405
406    if name == "riscos":
407        # On RISC OS, all env access goes through getenv and putenv
408        from riscosenviron import _Environ
409    elif name in ('os2', 'nt'):  # Where Env Var Names Must Be UPPERCASE
410        # But we store them as upper case
411        import UserDict
412        class _Environ(UserDict.IterableUserDict):
413            def __init__(self, environ):
414                UserDict.UserDict.__init__(self)
415                data = self.data
416                for k, v in environ.items():
417                    data[k.upper()] = v
418            def __setitem__(self, key, item):
419                putenv(key, item)
420                self.data[key.upper()] = item
421            def __getitem__(self, key):
422                return self.data[key.upper()]
423            try:
424                unsetenv
425            except NameError:
426                def __delitem__(self, key):
427                    del self.data[key.upper()]
428            else:
429                def __delitem__(self, key):
430                    unsetenv(key)
431                    del self.data[key.upper()]
432                def clear(self):
433                    for key in self.data.keys():
434                        unsetenv(key)
435                        del self.data[key]
436                def pop(self, key, *args):
437                    unsetenv(key)
438                    return self.data.pop(key.upper(), *args)
439            def has_key(self, key):
440                return key.upper() in self.data
441            def __contains__(self, key):
442                return key.upper() in self.data
443            def get(self, key, failobj=None):
444                return self.data.get(key.upper(), failobj)
445            def update(self, dict=None, **kwargs):
446                if dict:
447                    try:
448                        keys = dict.keys()
449                    except AttributeError:
450                        # List of (key, value)
451                        for k, v in dict:
452                            self[k] = v
453                    else:
454                        # got keys
455                        # cannot use items(), since mappings
456                        # may not have them.
457                        for k in keys:
458                            self[k] = dict[k]
459                if kwargs:
460                    self.update(kwargs)
461            def copy(self):
462                return dict(self)
463
464    else:  # Where Env Var Names Can Be Mixed Case
465        class _Environ(dict):
466            def __setitem__(self, key, item):
467                putenv(key, item)
468                super(_Environ, self).__setitem__(key, item)
469            def update(self,  dict=None, **kwargs):
470                if dict:
471                    try:
472                        keys = dict.keys()
473                    except AttributeError:
474                        # List of (key, value)
475                        for k, v in dict:
476                            self[k] = v
477                    else:
478                        # got keys
479                        # cannot use items(), since mappings
480                        # may not have them.
481                        for k in keys:
482                            self[k] = dict[k]
483                if kwargs:
484                    self.update(kwargs)
485            try:
486                unsetenv
487            except NameError:
488                pass
489            else:
490                def __delitem__(self, key):
491                    unsetenv(key)
492                    super(_Environ, self).__delitem__(key)
493                def clear(self):
494                    for key in self.keys():
495                        unsetenv(key)
496                        del self[key]
497                def pop(self, key, *args):
498                    unsetenv(key)
499                    return super(_Environ, self).pop(key, *args)
500            def copy(self):
501                return dict(self)
502
503
504    environ = _Environ(environ)
505
506def getenv(key, default=None):
507    """Get an environment variable, return None if it doesn't exist.
508    The optional second argument can specify an alternate default."""
509    return environ.get(key, default)
510__all__.append("getenv")
511
512def _exists(name):
513    try:
514        eval(name)
515        return True
516    except NameError:
517        return False
518
519# Supply spawn*() (probably only for Unix)
520if _exists("fork") and not _exists("spawnv") and _exists("execv"):
521
522    P_WAIT = 0
523    P_NOWAIT = P_NOWAITO = 1
524
525    # XXX Should we support P_DETACH?  I suppose it could fork()**2
526    # and close the std I/O streams.  Also, P_OVERLAY is the same
527    # as execv*()?
528
529    def _spawnvef(mode, file, args, env, func):
530        # Internal helper; func is the exec*() function to use
531        pid = fork()
532        if not pid:
533            # Child
534            try:
535                if env is None:
536                    func(file, args)
537                else:
538                    func(file, args, env)
539            except:
540                _exit(127)
541        else:
542            # Parent
543            if mode == P_NOWAIT:
544                return pid # Caller is responsible for waiting!
545            while 1:
546                wpid, sts = waitpid(pid, 0)
547                if WIFSTOPPED(sts):
548                    continue
549                elif WIFSIGNALED(sts):
550                    return -WTERMSIG(sts)
551                elif WIFEXITED(sts):
552                    return WEXITSTATUS(sts)
553                else:
554                    raise error, "Not stopped, signaled or exited???"
555
556    def spawnv(mode, file, args):
557        """spawnv(mode, file, args) -> integer
558
559Execute file with arguments from args in a subprocess.
560If mode == P_NOWAIT return the pid of the process.
561If mode == P_WAIT return the process's exit code if it exits normally;
562otherwise return -SIG, where SIG is the signal that killed it. """
563        return _spawnvef(mode, file, args, None, execv)
564
565    def spawnve(mode, file, args, env):
566        """spawnve(mode, file, args, env) -> integer
567
568Execute file with arguments from args in a subprocess with the
569specified environment.
570If mode == P_NOWAIT return the pid of the process.
571If mode == P_WAIT return the process's exit code if it exits normally;
572otherwise return -SIG, where SIG is the signal that killed it. """
573        return _spawnvef(mode, file, args, env, execve)
574
575    # Note: spawnvp[e] is't currently supported on Windows
576
577    def spawnvp(mode, file, args):
578        """spawnvp(mode, file, args) -> integer
579
580Execute file (which is looked for along $PATH) with arguments from
581args in a subprocess.
582If mode == P_NOWAIT return the pid of the process.
583If mode == P_WAIT return the process's exit code if it exits normally;
584otherwise return -SIG, where SIG is the signal that killed it. """
585        return _spawnvef(mode, file, args, None, execvp)
586
587    def spawnvpe(mode, file, args, env):
588        """spawnvpe(mode, file, args, env) -> integer
589
590Execute file (which is looked for along $PATH) with arguments from
591args in a subprocess with the supplied environment.
592If mode == P_NOWAIT return the pid of the process.
593If mode == P_WAIT return the process's exit code if it exits normally;
594otherwise return -SIG, where SIG is the signal that killed it. """
595        return _spawnvef(mode, file, args, env, execvpe)
596
597if _exists("spawnv"):
598    # These aren't supplied by the basic Windows code
599    # but can be easily implemented in Python
600
601    def spawnl(mode, file, *args):
602        """spawnl(mode, file, *args) -> integer
603
604Execute file with arguments from args in a subprocess.
605If mode == P_NOWAIT return the pid of the process.
606If mode == P_WAIT return the process's exit code if it exits normally;
607otherwise return -SIG, where SIG is the signal that killed it. """
608        return spawnv(mode, file, args)
609
610    def spawnle(mode, file, *args):
611        """spawnle(mode, file, *args, env) -> integer
612
613Execute file with arguments from args in a subprocess with the
614supplied environment.
615If mode == P_NOWAIT return the pid of the process.
616If mode == P_WAIT return the process's exit code if it exits normally;
617otherwise return -SIG, where SIG is the signal that killed it. """
618        env = args[-1]
619        return spawnve(mode, file, args[:-1], env)
620
621
622    __all__.extend(["spawnv", "spawnve", "spawnl", "spawnle",])
623
624
625if _exists("spawnvp"):
626    # At the moment, Windows doesn't implement spawnvp[e],
627    # so it won't have spawnlp[e] either.
628    def spawnlp(mode, file, *args):
629        """spawnlp(mode, file, *args) -> integer
630
631Execute file (which is looked for along $PATH) with arguments from
632args in a subprocess with the supplied environment.
633If mode == P_NOWAIT return the pid of the process.
634If mode == P_WAIT return the process's exit code if it exits normally;
635otherwise return -SIG, where SIG is the signal that killed it. """
636        return spawnvp(mode, file, args)
637
638    def spawnlpe(mode, file, *args):
639        """spawnlpe(mode, file, *args, env) -> integer
640
641Execute file (which is looked for along $PATH) with arguments from
642args in a subprocess with the supplied environment.
643If mode == P_NOWAIT return the pid of the process.
644If mode == P_WAIT return the process's exit code if it exits normally;
645otherwise return -SIG, where SIG is the signal that killed it. """
646        env = args[-1]
647        return spawnvpe(mode, file, args[:-1], env)
648
649
650    __all__.extend(["spawnvp", "spawnvpe", "spawnlp", "spawnlpe",])
651
652
653# Supply popen2 etc. (for Unix)
654if _exists("fork"):
655    if not _exists("popen2"):
656        def popen2(cmd, mode="t", bufsize=-1):
657            """Execute the shell command 'cmd' in a sub-process.  On UNIX, 'cmd'
658            may be a sequence, in which case arguments will be passed directly to
659            the program without shell intervention (as with os.spawnv()).  If 'cmd'
660            is a string it will be passed to the shell (as with os.system()). If
661            'bufsize' is specified, it sets the buffer size for the I/O pipes.  The
662            file objects (child_stdin, child_stdout) are returned."""
663            import warnings
664            msg = "os.popen2 is deprecated.  Use the subprocess module."
665            warnings.warn(msg, DeprecationWarning, stacklevel=2)
666
667            import subprocess
668            PIPE = subprocess.PIPE
669            p = subprocess.Popen(cmd, shell=isinstance(cmd, basestring),
670                                 bufsize=bufsize, stdin=PIPE, stdout=PIPE,
671                                 close_fds=True)
672            return p.stdin, p.stdout
673        __all__.append("popen2")
674
675    if not _exists("popen3"):
676        def popen3(cmd, mode="t", bufsize=-1):
677            """Execute the shell command 'cmd' in a sub-process.  On UNIX, 'cmd'
678            may be a sequence, in which case arguments will be passed directly to
679            the program without shell intervention (as with os.spawnv()).  If 'cmd'
680            is a string it will be passed to the shell (as with os.system()). If
681            'bufsize' is specified, it sets the buffer size for the I/O pipes.  The
682            file objects (child_stdin, child_stdout, child_stderr) are returned."""
683            import warnings
684            msg = "os.popen3 is deprecated.  Use the subprocess module."
685            warnings.warn(msg, DeprecationWarning, stacklevel=2)
686
687            import subprocess
688            PIPE = subprocess.PIPE
689            p = subprocess.Popen(cmd, shell=isinstance(cmd, basestring),
690                                 bufsize=bufsize, stdin=PIPE, stdout=PIPE,
691                                 stderr=PIPE, close_fds=True)
692            return p.stdin, p.stdout, p.stderr
693        __all__.append("popen3")
694
695    if not _exists("popen4"):
696        def popen4(cmd, mode="t", bufsize=-1):
697            """Execute the shell command 'cmd' in a sub-process.  On UNIX, 'cmd'
698            may be a sequence, in which case arguments will be passed directly to
699            the program without shell intervention (as with os.spawnv()).  If 'cmd'
700            is a string it will be passed to the shell (as with os.system()). If
701            'bufsize' is specified, it sets the buffer size for the I/O pipes.  The
702            file objects (child_stdin, child_stdout_stderr) are returned."""
703            import warnings
704            msg = "os.popen4 is deprecated.  Use the subprocess module."
705            warnings.warn(msg, DeprecationWarning, stacklevel=2)
706
707            import subprocess
708            PIPE = subprocess.PIPE
709            p = subprocess.Popen(cmd, shell=isinstance(cmd, basestring),
710                                 bufsize=bufsize, stdin=PIPE, stdout=PIPE,
711                                 stderr=subprocess.STDOUT, close_fds=True)
712            return p.stdin, p.stdout
713        __all__.append("popen4")
714
715import copy_reg as _copy_reg
716
717def _make_stat_result(tup, dict):
718    return stat_result(tup, dict)
719
720def _pickle_stat_result(sr):
721    (type, args) = sr.__reduce__()
722    return (_make_stat_result, args)
723
724try:
725    _copy_reg.pickle(stat_result, _pickle_stat_result, _make_stat_result)
726except NameError: # stat_result may not exist
727    pass
728
729def _make_statvfs_result(tup, dict):
730    return statvfs_result(tup, dict)
731
732def _pickle_statvfs_result(sr):
733    (type, args) = sr.__reduce__()
734    return (_make_statvfs_result, args)
735
736try:
737    _copy_reg.pickle(statvfs_result, _pickle_statvfs_result,
738                     _make_statvfs_result)
739except NameError: # statvfs_result may not exist
740    pass
741
742if not _exists("urandom"):
743    def urandom(n):
744        """urandom(n) -> str
745
746        Return a string of n random bytes suitable for cryptographic use.
747
748        """
749        try:
750            _urandomfd = open("/dev/urandom", O_RDONLY)
751        except (OSError, IOError):
752            raise NotImplementedError("/dev/urandom (or equivalent) not found")
753        try:
754            bs = b""
755            while n - len(bs) >= 1:
756                bs += read(_urandomfd, n - len(bs))
757        finally:
758            close(_urandomfd)
759        return bs