# 27.2 `codeop` -- Compile Python code

The `codeop` module provides utilities upon which the Python read-eval-print loop can be emulated, as is done in the `code` module. As a result, you probably don't want to use the module directly; if you want to include such a loop in your program you probably want to use the `code` module instead.

There are two parts to this job:

1. Being able to tell if a line of input completes a Python statement: in short, telling whether to print ``>>>` ' or ``...` ' next.
2. Remembering which future statements the user has entered, so subsequent input can be compiled with these in effect.

The `codeop` module provides a way of doing each of these things, and a way of doing them both.

To do just the former:

`compile_command`( | source[, filename[, symbol]])

  Tries to compile source, which should be a string of Python code and return a code object if source is valid Python code. In that case, the filename attribute of the code object will be filename, which defaults to `'<input>'`. Returns `None` if source is not valid Python code, but is a prefix of valid Python code.

  If there is a problem with source, an exception will be raised. `SyntaxError` is raised if there is invalid Python syntax, and `OverflowError` or `ValueError` if there is an invalid literal.

  The symbol argument determines whether source is compiled as a statement (`'single'`, the default) or as an expression (`'eval'`). Any other value will cause `ValueError` to be raised.

  Caveat: It is possible (but not likely) that the parser stops parsing with a successful outcome before reaching the end of the source; in this case, trailing symbols may be ignored instead of causing an error. For example, a backslash followed by two newlines may be followed by arbitrary garbage. This will be fixed once the API for the parser is better.

class `Compile`( | )

  Instances of this class have `__call__()` methods identical in signature to the built-in function `compile()`, but with the difference that if the instance compiles program text containing a `__future__` statement, the instance 'remembers' and compiles all subsequent program texts with the statement in force.

class `CommandCompiler`( | )

  Instances of this class have `__call__()` methods identical in signature to `compile_command()`; the difference is that if the instance compiles program text containing a `__future__` statement, the instance 'remembers' and compiles all subsequent program texts with the statement in force.

A note on version compatibility: the `Compile` and `CommandCompiler` are new in Python 2.2. If you want to enable the future-tracking features of 2.2 but also retain compatibility with 2.1 and earlier versions of Python you can either write

    try:
        from codeop import CommandCompiler
        compile_command = CommandCompiler()
        del CommandCompiler
    except ImportError:
        from codeop import compile_command

which is a low-impact change, but introduces possibly unwanted global state into your program, or you can write:

    try:
        from codeop import CommandCompiler
    except ImportError:
        def CommandCompiler():
            from codeop import compile_command
            return compile_command

and then call `CommandCompiler` every time you need a fresh compiler object.