# 12.1 Standard Module `rexec`

This module contains the `RExec` class, which supports `r_exec()`, `r_eval()`, `r_execfile()`, and `r_import()` methods, which are restricted versions of the standard Python functions `exec()`, `eval()`, `execfile()`, and the `import` statement. Code executed in this restricted environment will only have access to modules and functions that are deemed safe; you can subclass `RExec` to add or remove capabilities as desired.

Note: The `RExec` class can prevent code from performing unsafe operations like reading or writing disk files, or using TCP/IP sockets. However, it does not protect against code using extremely large amounts of memory or CPU time.

`RExec` ([hooks[, verbose]])
  Returns an instance of the `RExec` class.

  hooks is an instance of the `RHooks` class or a subclass of it. If it is omitted or `None`, the default `RHooks` class is instantiated. Whenever the RExec module searches for a module (even a built-in one) or reads a module's code, it doesn't actually go out to the file system itself. Rather, it calls methods of an RHooks instance that was passed to or created by its constructor. (Actually, the RExec object doesn't make these calls--they are made by a module loader object that's part of the RExec object. This allows another level of flexibility, e.g. using packages.)

  By providing an alternate RHooks object, we can control the file system accesses made to import a module, without changing the actual algorithm that controls the order in which those accesses are made. For instance, we could substitute an RHooks object that passes all filesystem requests to a file server elsewhere, via some RPC mechanism such as ILU. Grail's applet loader uses this to support importing applets from a URL for a directory.

  If verbose is true, additional debugging output may be sent to standard output.

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',` `'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')`.

RExec instances support the following methods:

`r_eval` (code)
  code must either be a string containing a Python expression, or a compiled code object, which will be evaluated in the restricted environment's `__main__` module. The value of the expression or code object will be returned.

`r_exec` (code)
  code must either be a string containing one or more lines of Python code, or a compiled code object, which will be executed in the restricted environment's `__main__` module.

`r_execfile` (filename)
  Execute the Python code contained in the file filename in the restricted environment's `__main__` module.

Methods whose names begin with `s_` are similar to the functions beginning with `r_`, but the code will be granted access to restricted versions of the standard I/O streans `sys.stdin`, `sys.stderr`, and `sys.stdout`.

`s_eval` (code)
  code must be a string containing a Python expression, which will be evaluated in the restricted environment.

`s_exec` (code)
  code must be a string containing one or more lines of Python code, which will be executed in the restricted environment.

`s_execfile` (code)
  Execute the Python code contained in the file filename in the restricted environment.

`RExec` objects must also support various methods which will be implicitly called by code executing in the restricted environment. Overriding these methods in a subclass is used to change the policies enforced by a restricted environment.

`r_import` (modulename[, globals, locals, fromlist])
  Import the module modulename, raising an `ImportError` exception if the module is considered unsafe.

`r_open` (filename[, mode[, bufsize]])
  Method called when `open()` is called in the restricted environment. The arguments are identical to those of `open()`, and a file object (or a class instance compatible with file objects) should be returned. `RExec`'s default behaviour is allow opening any file for reading, but forbidding any attempt to write a file. See the example below for an implementation of a less restrictive `r_open()`.

`r_reload` (module)
  Reload the module object module, re-parsing and re-initializing it.

`r_unload` (module)
  Unload the module object module (i.e., remove it from the restricted environment's `sys.modules` dictionary).

And their equivalents with access to restricted standard I/O streams:

`s_import` (modulename[, globals, locals, fromlist])
  Import the module modulename, raising an `ImportError` exception if the module is considered unsafe.

`s_reload` (module)
  Reload the module object module, re-parsing and re-initializing it.

`s_unload` (module)
  Unload the module object module.

- 12.1.1 An example