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17.1.2 Defining restricted environments

The `RExec` class has the following class attributes, which are used by the `__init__()` method. Changing them on an existing instance won't have any effect; instead, create a subclass of `RExec` and assign them new values in the class definition. Instances of the new class will then use those new values. All these attributes are tuples of strings.

`nok_builtin_names`
  Contains the names of built-in functions which will not be available to programs running in the restricted environment. The value for `RExec` is `('open', 'reload', '__import__')`. (This gives the exceptions, because by far the majority of built-in functions are harmless. A subclass that wants to override this variable should probably start with the value from the base class and concatenate additional forbidden functions -- when new dangerous built-in functions are added to Python, they will also be added to this module.)

`ok_builtin_modules`
  Contains the names of built-in modules which can be safely imported. The value for `RExec` is `('audioop', 'array', 'binascii', 'cmath', 'errno', 'imageop', 'marshal', 'math', 'md5', 'operator', 'parser', 'regex', 'rotor', 'select', 'sha', '_sre', 'strop', 'struct', 'time')`. A similar remark about overriding this variable applies -- use the value from the base class as a starting point.

`ok_path`
  Contains the directories which will be searched when an `import` is performed in the restricted environment. The value for `RExec` is the same as `sys.path` (at the time the module is loaded) for unrestricted code.

`ok_posix_names`
  Contains the names of the functions in the `os` module which will be available to programs running in the restricted environment. The value for `RExec` is `('error', 'fstat', 'listdir', 'lstat', 'readlink', 'stat', 'times', 'uname', 'getpid', 'getppid', 'getcwd', 'getuid', 'getgid', 'geteuid', 'getegid')`.

`ok_sys_names`
  Contains the names of the functions and variables in the `sys` module which will be available to programs running in the restricted environment. The value for `RExec` is `('ps1', 'ps2', 'copyright', 'version', 'platform', 'exit', 'maxint')`.

`ok_file_types`
  Contains the file types from which modules are allowed to be loaded. Each file type is an integer constant defined in the `imp` module. The meaningful values are `PY_SOURCE`, `PY_COMPILED`, and `C_EXTENSION`. The value for `RExec` is `(C_EXTENSION, PY_SOURCE)`. Adding `PY_COMPILED` in subclasses is not recommended; an attacker could exit the restricted execution mode by putting a forged byte-compiled file (.pyc) anywhere in your file system, for example by writing it to /tmp or uploading it to the /incoming directory of your public FTP server.