def decorator(caller, _func=None):
    """decorator(caller) converts a caller function into a decorator"""
    if _func is not None:  # return a decorated function
        # this is obsolete behavior; you should use decorate instead
        return decorate(_func, caller)
    # else return a decorator function
    defaultargs, defaults = '', ()
    if inspect.isclass(caller):
        name = caller.__name__.lower()
        doc = 'decorator(%s) converts functions/generators into ' \
            'factories of %s objects' % (caller.__name__, caller.__name__)
    elif inspect.isfunction(caller):
        if caller.__name__ == '<lambda>':
            name = '_lambda_'
        else:
            name = caller.__name__
        doc = caller.__doc__
        nargs = caller.__code__.co_argcount
        ndefs = len(caller.__defaults__ or ())
        defaultargs = ', '.join(caller.__code__.co_varnames[nargs-ndefs:nargs])
        if defaultargs:
            defaultargs += ','
        defaults = caller.__defaults__
    else:  # assume caller is an object with a __call__ method
        name = caller.__class__.__name__.lower()
        doc = caller.__call__.__doc__
    evaldict = dict(_call=caller, _decorate_=decorate)
    dec = FunctionMaker.create(
        '%s(%s func)' % (name, defaultargs),
        'if func is None: return lambda func:  _decorate_(func, _call, (%s))\n'
        'return _decorate_(func, _call, (%s))' % (defaultargs, defaultargs),
        evaldict, doc=doc, module=caller.__module__, __wrapped__=caller)
    if defaults:
        dec.__defaults__ = defaults + (None,)
    return dec
decorator(caller) converts a caller function into a decorator
pyinstrument/vendor/decorator.py