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# 27.1. `sys` — System-specific parameters and functions

This module provides access to some variables used or maintained by the interpreter and to functions that interact strongly with the interpreter. It is always available.

`sys.``argv`

  The list of command line arguments passed to a Python script. `argv[0]` is the script name (it is operating system dependent whether this is a full pathname or not). If the command was executed using the -c command line option to the interpreter, `argv[0]` is set to the string `'-c'`. If no script name was passed to the Python interpreter, `argv[0]` is the empty string.

  To loop over the standard input, or the list of files given on the command line, see the `fileinput` module.

`sys.``byteorder`
  An indicator of the native byte order. This will have the value `'big'` on big-endian (most-significant byte first) platforms, and `'little'` on little-endian (least-significant byte first) platforms.

`sys.``subversion`
  A triple (repo, branch, version) representing the Subversion information of the Python interpreter. repo is the name of the repository, `'CPython'`. branch is a string of one of the forms `'trunk'`, `'branches/name'` or `'tags/name'`. version is the output of `svnversion`, if the interpreter was built from a Subversion checkout; it contains the revision number (range) and possibly a trailing ‘M’ if there were local modifications. If the tree was exported (or svnversion was not available), it is the revision of `Include/patchlevel.h` if the branch is a tag. Otherwise, it is `None`.

`sys.``builtin_module_names`
  A tuple of strings giving the names of all modules that are compiled into this Python interpreter. (This information is not available in any other way — `modules.keys()` only lists the imported modules.)

`sys.``copyright`
  A string containing the copyright pertaining to the Python interpreter.

`sys.``_clear_type_cache`()

  Clear the internal type cache. The type cache is used to speed up attribute and method lookups. Use the function only to drop unnecessary references during reference leak debugging.

  This function should be used for internal and specialized purposes only.

`sys.``_current_frames`()

  Return a dictionary mapping each thread’s identifier to the topmost stack frame currently active in that thread at the time the function is called. Note that functions in the `traceback` module can build the call stack given such a frame.

  This is most useful for debugging deadlock: this function does not require the deadlocked threads’ cooperation, and such threads’ call stacks are frozen for as long as they remain deadlocked. The frame returned for a non-deadlocked thread may bear no relationship to that thread’s current activity by the time calling code examines the frame.

  This function should be used for internal and specialized purposes only.

`sys.``dllhandle`
  Integer specifying the handle of the Python DLL. Availability: Windows.

`sys.``displayhook`(value)

  If value is not `None`, this function prints it to `sys.stdout`, and saves it in `builtins._`.

  `sys.displayhook` is called on the result of evaluating an expression entered in an interactive Python session. The display of these values can be customized by assigning another one-argument function to `sys.displayhook`.

`sys.``excepthook`(type, value, traceback)

  This function prints out a given traceback and exception to `sys.stderr`.

  When an exception is raised and uncaught, the interpreter calls `sys.excepthook` with three arguments, the exception class, exception instance, and a traceback object. In an interactive session this happens just before control is returned to the prompt; in a Python program this happens just before the program exits. The handling of such top-level exceptions can be customized by assigning another three-argument function to `sys.excepthook`.

`sys.``__displayhook__`
`sys.``__excepthook__`
  These objects contain the original values of `displayhook` and `excepthook` at the start of the program. They are saved so that `displayhook` and `excepthook` can be restored in case they happen to get replaced with broken objects.

`sys.``exc_info`()

  This function returns a tuple of three values that give information about the exception that is currently being handled. The information returned is specific both to the current thread and to the current stack frame. If the current stack frame is not handling an exception, the information is taken from the calling stack frame, or its caller, and so on until a stack frame is found that is handling an exception. Here, “handling an exception” is defined as “executing an except clause.” For any stack frame, only information about the exception being currently handled is accessible.

  If no exception is being handled anywhere on the stack, a tuple containing three `None` values is returned. Otherwise, the values returned are `(type, value, traceback)`. Their meaning is: type gets the exception type of the exception being handled (a class object); value gets the exception parameter (its associated value or the second argument to `raise`, which is always a class instance if the exception type is a class object); traceback gets a traceback object (see the Reference Manual) which encapsulates the call stack at the point where the exception originally occurred.

  Warning

  Assigning the traceback return value to a local variable in a function that is handling an exception will cause a circular reference. Since most functions don’t need access to the traceback, the best solution is to use something like `exctype, value = sys.exc_info()[:2]` to extract only the exception type and value. If you do need the traceback, make sure to delete it after use (best done with a `try` ... `finally` statement) or to call `exc_info()` in a function that does not itself handle an exception.

  Such cycles are normally automatically reclaimed when garbage collection is enabled and they become unreachable, but it remains more efficient to avoid creating cycles.

`sys.``exec_prefix`
  A string giving the site-specific directory prefix where the platform-dependent Python files are installed; by default, this is also `'/usr/local'`. This can be set at build time with the --exec-prefix argument to the configure script. Specifically, all configuration files (e.g. the `pyconfig.h` header file) are installed in the directory `exec_prefix + '/lib/pythonversion/config'`, and shared library modules are installed in `exec_prefix + '/lib/pythonversion/lib-dynload'`, where version is equal to `version[:3]`.

`sys.``executable`
  A string giving the name of the executable binary for the Python interpreter, on systems where this makes sense.

`sys.``exit`([arg])
  Exit from Python. This is implemented by raising the `SystemExit` exception, so cleanup actions specified by finally clauses of `try` statements are honored, and it is possible to intercept the exit attempt at an outer level. The optional argument arg can be an integer giving the exit status (defaulting to zero), or another type of object. If it is an integer, zero is considered “successful termination” and any nonzero value is considered “abnormal termination” by shells and the like. Most systems require it to be in the range 0-127, and produce undefined results otherwise. Some systems have a convention for assigning specific meanings to specific exit codes, but these are generally underdeveloped; Unix programs generally use 2 for command line syntax errors and 1 for all other kind of errors. If another type of object is passed, `None` is equivalent to passing zero, and any other object is printed to `sys.stderr` and results in an exit code of 1. In particular, `sys.exit("some error message")` is a quick way to exit a program when an error occurs.

`sys.``flags`

  The struct sequence flags exposes the status of command line flags. The attributes are read only.

  attribute | flag
  --- | ---
  `debug` | -d
  `py3k_warning` | -3
  `division_warning` | -Q
  `division_new` | -Qnew
  `inspect` | -i
  `interactive` | -i
  `optimize` | -O or -OO
  `dont_write_bytecode` | -B
  `no_site` | -S
  `ignore_environment` | -E
  `verbose` | -v
  `unicode` | -U

`sys.``float_info`

  A structseq holding information about the float type. It contains low level information about the precision and internal representation. Please study your system’s `float.h` for more information.

  attribute | explanation
  --- | ---
  `epsilon` | Difference between 1 and the next representable floating point number
  `dig` | digits (see `float.h`)
  `mant_dig` | mantissa digits (see `float.h`)
  `max` | maximum representable finite float
  `max_exp` | maximum int e such that radix**(e-1) is in the range of finite representable floats
  `max_10_exp` | maximum int e such that 10**e is in the range of finite representable floats
  `min` | Minimum positive normalizer float
  `min_exp` | minimum int e such that radix**(e-1) is a normalized float
  `min_10_exp` | minimum int e such that 10**e is a normalized float
  `radix` | radix of exponent
  `rounds` | addition rounds (see `float.h`)

  Note

  The information in the table is simplified.

`sys.``float_repr_style`

  A string indicating how the `repr()` function behaves for floats. If the string has value `'short'` then for a finite float `x`, `repr(x)` aims to produce a short string with the property that `float(repr(x)) == x`. This is the usual behaviour in Python 3.1 and later. Otherwise, `float_repr_style` has value `'legacy'` and `repr(x)` behaves in the same way as it did in versions of Python prior to 3.1.

  New in version 3.1.

`sys.``getcheckinterval`()
  Return the interpreter’s “check interval”; see `setcheckinterval()`.

`sys.``getdefaultencoding`()
  Return the name of the current default string encoding used by the Unicode implementation.

`sys.``getdlopenflags`()
  Return the current value of the flags that are used for `dlopen` calls. The flag constants are defined in the `ctypes` and `DLFCN` modules. Availability: Unix.

`sys.``getfilesystemencoding`()

  Return the name of the encoding used to convert Unicode filenames into system file names, or `None` if the system default encoding is used. The result value depends on the operating system:

  - On Windows 9x, the encoding is “mbcs”.
  - On Mac OS X, the encoding is “utf-8”.
  - On Unix, the encoding is the user’s preference according to the result of nl_langinfo(CODESET), or `None` if the `nl_langinfo(CODESET)` failed.
  - On Windows NT+, file names are Unicode natively, so no conversion is performed. `getfilesystemencoding()` still returns `'mbcs'`, as this is the encoding that applications should use when they explicitly want to convert Unicode strings to byte strings that are equivalent when used as file names.

`sys.``getrefcount`(object)
  Return the reference count of the object. The count returned is generally one higher than you might expect, because it includes the (temporary) reference as an argument to `getrefcount()`.

`sys.``getrecursionlimit`()
  Return the current value of the recursion limit, the maximum depth of the Python interpreter stack. This limit prevents infinite recursion from causing an overflow of the C stack and crashing Python. It can be set by `setrecursionlimit()`.

`sys.``getsizeof`(object[, default])

  Return the size of an object in bytes. The object can be any type of object. All built-in objects will return correct results, but this does not have to hold true for third-party extensions as it is implementation specific.

  The default argument allows to define a value which will be returned if the object type does not provide means to retrieve the size and would cause a TypeError.

  func:getsizeof calls the object’s __sizeof__ method and adds an additional garbage collector overhead if the object is managed by the garbage collector.

`sys.``_getframe`([depth])

  Return a frame object from the call stack. If optional integer depth is given, return the frame object that many calls below the top of the stack. If that is deeper than the call stack, `ValueError` is raised. The default for depth is zero, returning the frame at the top of the call stack.

  This function should be used for internal and specialized purposes only.

`sys.``getprofile`()

  Get the profiler function as set by `setprofile()`.

`sys.``gettrace`()

  Get the trace function as set by `settrace()`.

  Note

  The `gettrace()` function is intended only for implementing debuggers, profilers, coverage tools and the like. Its behavior is part of the implementation platform, rather than part of the language definition, and thus may not be available in all Python implementations.

`sys.``getwindowsversion`()

  Return a tuple containing five components, describing the Windows version currently running. The elements are major, minor, build, platform, and text. text contains a string while all other values are integers.

  platform may be one of the following values:

  Constant | Platform
  --- | ---
  `0 (VER_PLATFORM_WIN32s)` | Win32s on Windows 3.1
  `1 (VER_PLATFORM_WIN32_WINDOWS)` | Windows 95/98/ME
  `2 (VER_PLATFORM_WIN32_NT)` | Windows NT/2000/XP/x64
  `3 (VER_PLATFORM_WIN32_CE)` | Windows CE

  This function wraps the Win32 `GetVersionEx` function; see the Microsoft documentation for more information about these fields.

  Availability: Windows.

`sys.``hexversion`

  The version number encoded as a single integer. This is guaranteed to increase with each version, including proper support for non-production releases. For example, to test that the Python interpreter is at least version 1.5.2, use:

      if sys.hexversion >= 0x010502F0:
          # use some advanced feature
          ...
      else:
          # use an alternative implementation or warn the user
          ...

  This is called `hexversion` since it only really looks meaningful when viewed as the result of passing it to the built-in `hex()` function. The `version_info` value may be used for a more human-friendly encoding of the same information.

`sys.``int_info`

  A struct sequence that holds information about Python’s internal representation of integers. The attributes are read only.

  attribute | explanation
  --- | ---
  `bits_per_digit` | number of bits held in each digit. Python integers are stored internally in base `2**int_info.bits_per_digit`
  `sizeof_digit` | size in bytes of the C type used to represent a digit

  New in version 3.1.

`sys.``intern`(string)

  Enter string in the table of “interned” strings and return the interned string – which is string itself or a copy. Interning strings is useful to gain a little performance on dictionary lookup – if the keys in a dictionary are interned, and the lookup key is interned, the key comparisons (after hashing) can be done by a pointer compare instead of a string compare. Normally, the names used in Python programs are automatically interned, and the dictionaries used to hold module, class or instance attributes have interned keys.

  Interned strings are not immortal; you must keep a reference to the return value of `intern()` around to benefit from it.

`sys.``last_type`
`sys.``last_value`
`sys.``last_traceback`

  These three variables are not always defined; they are set when an exception is not handled and the interpreter prints an error message and a stack traceback. Their intended use is to allow an interactive user to import a debugger module and engage in post-mortem debugging without having to re-execute the command that caused the error. (Typical use is `import pdb; pdb.pm()` to enter the post-mortem debugger; see chapter pdb — The Python Debugger for more information.)

  The meaning of the variables is the same as that of the return values from `exc_info()` above. (Since there is only one interactive thread, thread-safety is not a concern for these variables, unlike for `exc_type` etc.)

`sys.``maxsize`
  An integer giving the maximum value a variable of type `Py_ssize_t` can take. It’s usually `2**31 - 1` on a 32-bit platform and `2**63 - 1` on a 64-bit platform.

`sys.``maxunicode`
  An integer giving the largest supported code point for a Unicode character. The value of this depends on the configuration option that specifies whether Unicode characters are stored as UCS-2 or UCS-4.

`sys.``meta_path`

  A list of finder objects that have their `find_module()` methods called to see if one of the objects can find the module to be imported. The `find_module()` method is called at least with the absolute name of the module being imported. If the module to be imported is contained in package then the parent package’s `__path__` attribute is passed in as a second argument. The method returns `None` if the module cannot be found, else returns a loader.

  `sys.meta_path` is searched before any implicit default finders or `sys.path`.

  See PEP 302 for the original specification.

`sys.``modules`
  This is a dictionary that maps module names to modules which have already been loaded. This can be manipulated to force reloading of modules and other tricks.

`sys.``path`

  A list of strings that specifies the search path for modules. Initialized from the environment variable PYTHONPATH, plus an installation-dependent default.

  As initialized upon program startup, the first item of this list, `path[0]`, is the directory containing the script that was used to invoke the Python interpreter. If the script directory is not available (e.g. if the interpreter is invoked interactively or if the script is read from standard input), `path[0]` is the empty string, which directs Python to search modules in the current directory first. Notice that the script directory is inserted before the entries inserted as a result of PYTHONPATH.

  A program is free to modify this list for its own purposes.

  See also

  Module `site` This describes how to use .pth files to extend `sys.path`.

`sys.``path_hooks`

  A list of callables that take a path argument to try to create a finder for the path. If a finder can be created, it is to be returned by the callable, else raise `ImportError`.

  Originally specified in PEP 302.

`sys.``path_importer_cache`

  A dictionary acting as a cache for finder objects. The keys are paths that have been passed to `sys.path_hooks` and the values are the finders that are found. If a path is a valid file system path but no explicit finder is found on `sys.path_hooks` then `None` is stored to represent the implicit default finder should be used. If the path is not an existing path then `imp.NullImporter` is set.

  Originally specified in PEP 302.

`sys.``platform`

  This string contains a platform identifier that can be used to append platform-specific components to `sys.path`, for instance.

  For Unix systems, this is the lowercased OS name as returned by `uname -s` with the first part of the version as returned by `uname -r` appended, e.g. `'sunos5'` or `'linux2'`, at the time when Python was built. For other systems, the values are:

  System | `platform` value
  --- | ---
  Windows | `'win32'`
  Windows/Cygwin | `'cygwin'`
  Mac OS X | `'darwin'`
  OS/2 | `'os2'`
  OS/2 EMX | `'os2emx'`
  AtheOS | `'atheos'`

`sys.``prefix`
  A string giving the site-specific directory prefix where the platform independent Python files are installed; by default, this is the string `'/usr/local'`. This can be set at build time with the --prefix argument to the configure script. The main collection of Python library modules is installed in the directory `prefix + '/lib/pythonversion'` while the platform independent header files (all except `pyconfig.h`) are stored in `prefix + '/include/pythonversion'`, where version is equal to `version[:3]`.

`sys.``ps1`
`sys.``ps2`

  Strings specifying the primary and secondary prompt of the interpreter. These are only defined if the interpreter is in interactive mode. Their initial values in this case are `'>>> '` and `'... '`. If a non-string object is assigned to either variable, its `str()` is re-evaluated each time the interpreter prepares to read a new interactive command; this can be used to implement a dynamic prompt.

`sys.``dont_write_bytecode`
  If this is true, Python won’t try to write `.pyc` or `.pyo` files on the import of source modules. This value is initially set to `True` or `False` depending on the `-B` command line option and the `PYTHONDONTWRITEBYTECODE` environment variable, but you can set it yourself to control bytecode file generation.

`sys.``setcheckinterval`(interval)
  Set the interpreter’s “check interval”. This integer value determines how often the interpreter checks for periodic things such as thread switches and signal handlers. The default is `100`, meaning the check is performed every 100 Python virtual instructions. Setting it to a larger value may increase performance for programs using threads. Setting it to a value `<=` 0 checks every virtual instruction, maximizing responsiveness as well as overhead.

`sys.``setdefaultencoding`(name)
  Set the current default string encoding used by the Unicode implementation. If name does not match any available encoding, `LookupError` is raised. This function is only intended to be used by the `site` module implementation and, where needed, by `sitecustomize`. Once used by the `site` module, it is removed from the `sys` module’s namespace.

`sys.``setdlopenflags`(n)
  Set the flags used by the interpreter for `dlopen` calls, such as when the interpreter loads extension modules. Among other things, this will enable a lazy resolving of symbols when importing a module, if called as `sys.setdlopenflags(0)`. To share symbols across extension modules, call as `sys.setdlopenflags(ctypes.RTLD_GLOBAL)`. Symbolic names for the flag modules can be either found in the `ctypes` module, or in the `DLFCN` module. If `DLFCN` is not available, it can be generated from `/usr/include/dlfcn.h` using the h2py script. Availability: Unix.

`sys.``setfilesystemencoding`(enc)
  Set the encoding used when converting Python strings to file names to enc. By default, Python tries to determine the encoding it should use automatically on Unix; on Windows, it avoids such conversion completely. This function can be used when Python’s determination of the encoding needs to be overwritten, e.g. when not all file names on disk can be decoded using the encoding that Python had chosen.

`sys.``setprofile`(profilefunc)

  Set the system’s profile function, which allows you to implement a Python source code profiler in Python. See chapter The Python Profilers for more information on the Python profiler. The system’s profile function is called similarly to the system’s trace function (see `settrace()`), but it isn’t called for each executed line of code (only on call and return, but the return event is reported even when an exception has been set). The function is thread-specific, but there is no way for the profiler to know about context switches between threads, so it does not make sense to use this in the presence of multiple threads. Also, its return value is not used, so it can simply return `None`.

`sys.``setrecursionlimit`(limit)

  Set the maximum depth of the Python interpreter stack to limit. This limit prevents infinite recursion from causing an overflow of the C stack and crashing Python.

  The highest possible limit is platform-dependent. A user may need to set the limit higher when she has a program that requires deep recursion and a platform that supports a higher limit. This should be done with care, because a too-high limit can lead to a crash.

`sys.``settrace`(tracefunc)

  Set the system’s trace function, which allows you to implement a Python source code debugger in Python. The function is thread-specific; for a debugger to support multiple threads, it must be registered using `settrace()` for each thread being debugged.

  Trace functions should have three arguments: frame, event, and arg. frame is the current stack frame. event is a string: `'call'`, `'line'`, `'return'`, `'exception'`, `'c_call'`, `'c_return'`, or `'c_exception'`. arg depends on the event type.

  The trace function is invoked (with event set to `'call'`) whenever a new local scope is entered; it should return a reference to a local trace function to be used that scope, or `None` if the scope shouldn’t be traced.

  The local trace function should return a reference to itself (or to another function for further tracing in that scope), or `None` to turn off tracing in that scope.

  The events have the following meaning:

  `'call'`
    A function is called (or some other code block entered). The global trace function is called; arg is `None`; the return value specifies the local trace function.
  `'line'`
    The interpreter is about to execute a new line of code (sometimes multiple line events on one line exist). The local trace function is called; arg is `None`; the return value specifies the new local trace function.
  `'return'`
    A function (or other code block) is about to return. The local trace function is called; arg is the value that will be returned. The trace function’s return value is ignored.
  `'exception'`
    An exception has occurred. The local trace function is called; arg is a tuple `(exception, value, traceback)`; the return value specifies the new local trace function.
  `'c_call'`
    A C function is about to be called. This may be an extension function or a builtin. arg is the C function object.
  `'c_return'`
    A C function has returned. arg is `None`.
  `'c_exception'`
    A C function has thrown an exception. arg is `None`.

  Note that as an exception is propagated down the chain of callers, an `'exception'` event is generated at each level.

  For more information on code and frame objects, refer to The standard type hierarchy.

  Note

  The `settrace()` function is intended only for implementing debuggers, profilers, coverage tools and the like. Its behavior is part of the implementation platform, rather than part of the language definition, and thus may not be available in all Python implementations.

`sys.``settscdump`(on_flag)
  Activate dumping of VM measurements using the Pentium timestamp counter, if on_flag is true. Deactivate these dumps if on_flag is off. The function is available only if Python was compiled with --with-tsc. To understand the output of this dump, read `Python/ceval.c` in the Python sources.

`sys.``stdin`
`sys.``stdout`
`sys.``stderr`

  File objects corresponding to the interpreter’s standard input, output and error streams. `stdin` is used for all interpreter input except for scripts but including calls to `input()`. `stdout` is used for the output of `print()` and expression statements and for the prompts of `input()`. The interpreter’s own prompts and (almost all of) its error messages go to `stderr`. `stdout` and `stderr` needn’t be built-in file objects: any object is acceptable as long as it has a `write()` method that takes a string argument. (Changing these objects doesn’t affect the standard I/O streams of processes executed by `os.popen()`, `os.system()` or the `exec*()` family of functions in the `os` module.)

  The standard streams are in text mode by default. To write or read binary data to these, use the underlying binary buffer. For example, to write bytes to `stdout`, use `sys.stdout.buffer.write(b'abc')`. Using `io.TextIOBase.detach()` streams can be made binary by default. For example, this function sets all the standard streams to binary:

      def make_streams_binary():
          sys.stdin = sys.stdin.detach()
          sys.stdout = sys.stdout.detach()
          sys.stderr = sys.stderr.detach()

`sys.``__stdin__`
`sys.``__stdout__`
`sys.``__stderr__`

  These objects contain the original values of `stdin`, `stderr` and `stdout` at the start of the program. They are used during finalization, and could be useful to print to the actual standard stream no matter if the `sys.std*` object has been redirected.

  It can also be used to restore the actual files to known working file objects in case they have been overwritten with a broken object. However, the preferred way to do this is to explicitly save the previous stream before replacing it, and restore the saved object.

  Note

  Under some conditions `stdin`, `stdout` and `stderr` as well as the original values `__stdin__`, `__stdout__` and `__stderr__` can be None. It is usually the case for Windows GUI apps that aren’t connected to a console and Python apps started with pythonw.

`sys.``tracebacklimit`
  When this variable is set to an integer value, it determines the maximum number of levels of traceback information printed when an unhandled exception occurs. The default is `1000`. When set to `0` or less, all traceback information is suppressed and only the exception type and value are printed.

`sys.``version`

  A string containing the version number of the Python interpreter plus additional information on the build number and compiler used. It has a value of the form `'version (#build_number, build_date, build_time) [compiler]'`. The first three characters are used to identify the version in the installation directories (where appropriate on each platform). An example:

      >>> import sys
      >>> sys.version
      '1.5.2 (#0 Apr 13 1999, 10:51:12) [MSC 32 bit (Intel)]'

`sys.``api_version`
  The C API version for this interpreter. Programmers may find this useful when debugging version conflicts between Python and extension modules.

`sys.``version_info`

  A tuple containing the five components of the version number: major, minor, micro, releaselevel, and serial. All values except releaselevel are integers; the release level is `'alpha'`, `'beta'`, `'candidate'`, or `'final'`. The `version_info` value corresponding to the Python version 2.0 is `(2, 0, 0, 'final', 0)`. The components can also be accessed by name, so `sys.version_info[0]` is equivalent to `sys.version_info.major` and so on.

  Changed in version 3.1: Added named component attributes

`sys.``warnoptions`
  This is an implementation detail of the warnings framework; do not modify this value. Refer to the `warnings` module for more information on the warnings framework.

`sys.``winver`
  The version number used to form registry keys on Windows platforms. This is stored as string resource 1000 in the Python DLL. The value is normally the first three characters of `version`. It is provided in the `sys` module for informational purposes; modifying this value has no effect on the registry keys used by Python. Availability: Windows.

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