IPython/core/async_helpers.py

"""
Async helper function that are invalid syntax on Python 3.5 and below.

This code is best effort, and may have edge cases not behaving as expected. In
particular it contain a number of heuristics to detect whether code is
effectively async and need to run in an event loop or not.

Some constructs (like top-level `return`, or `yield`) are taken care of
explicitly to actually raise a SyntaxError and stay as close as possible to
Python semantics.
"""


import ast
import sys
from textwrap import dedent, indent


class _AsyncIORunner:

    def __call__(self, coro):
        """
        Handler for asyncio autoawait
        """
        import asyncio

        return asyncio.get_event_loop().run_until_complete(coro)

    def __str__(self):
        return 'asyncio'

_asyncio_runner = _AsyncIORunner()


def _curio_runner(coroutine):
    """
    handler for curio autoawait
    """
    import curio

    return curio.run(coroutine)


def _trio_runner(async_fn):
    import trio

    async def loc(coro):
        """
        We need the dummy no-op async def to protect from
        trio's internal. See https://github.com/python-trio/trio/issues/89
        """
        return await coro

    return trio.run(loc, async_fn)


def _pseudo_sync_runner(coro):
    """
    A runner that does not really allow async execution, and just advance the coroutine.

    See discussion in https://github.com/python-trio/trio/issues/608,

    Credit to Nathaniel Smith

    """
    try:
        coro.send(None)
    except StopIteration as exc:
        return exc.value
    else:
        # TODO: do not raise but return an execution result with the right info.
        raise RuntimeError(
            "{coro_name!r} needs a real async loop".format(coro_name=coro.__name__)
        )


def _asyncify(code: str) -> str:
    """wrap code in async def definition.

    And setup a bit of context to run it later.
    """
    res = dedent(
        """
    async def __wrapper__():
        try:
    {usercode}
        finally:
            locals()
    """
    ).format(usercode=indent(code, " " * 8))
    return res


class _AsyncSyntaxErrorVisitor(ast.NodeVisitor):
    """
    Find syntax errors that would be an error in an async repl, but because
    the implementation involves wrapping the repl in an async function, it
    is erroneously allowed (e.g. yield or return at the top level)
    """

    def generic_visit(self, node):
        func_types = (ast.FunctionDef, ast.AsyncFunctionDef)
        invalid_types = (ast.Return, ast.Yield, ast.YieldFrom)

        if isinstance(node, func_types):
            return  # Don't recurse into functions
        elif isinstance(node, invalid_types):
            raise SyntaxError()
        else:
            super().generic_visit(node)


def _async_parse_cell(cell: str) -> ast.AST:
    """
    This is a compatibility shim for pre-3.7 when async outside of a function
    is a syntax error at the parse stage.

    It will return an abstract syntax tree parsed as if async and await outside
    of a function were not a syntax error.
    """
    if sys.version_info < (3, 7):
        # Prior to 3.7 you need to asyncify before parse
        wrapped_parse_tree = ast.parse(_asyncify(cell))
        return wrapped_parse_tree.body[0].body[0]
    else:
        return ast.parse(cell)


def _should_be_async(cell: str) -> bool:
    """Detect if a block of code need to be wrapped in an `async def`

    Attempt to parse the block of code, it it compile we're fine.
    Otherwise we  wrap if and try to compile.

    If it works, assume it should be async. Otherwise Return False.

    Not handled yet: If the block of code has a return statement as  the top
    level, it will be seen as async. This is a know limitation.
    """

    try:
        # we can't limit ourself to ast.parse, as it __accepts__ to parse on
        # 3.7+, but just does not _compile_
        compile(cell, "<>", "exec")
        return False
    except SyntaxError:
        try:
            parse_tree = _async_parse_cell(cell)

            # Raise a SyntaxError if there are top-level return or yields
            v = _AsyncSyntaxErrorVisitor()
            v.visit(parse_tree)

        except SyntaxError:
            return False
        return True
    return False