`FunctionMaker.create` raises unexpected SyntaxError when return is present as substring for async function

[pengzhengyi]

About

In current FunctionMaker.create, if the function to create from is an async function and if the function body contains any occurrence of return, it will blindly replace it with return await. This will raise a SyntaxError in edge cases where return is present not as the keyword return but a substring.

Reproduce

from decorator import FunctionMaker


async def IDENTITY(return_value):
    return return_value

FunctionMaker.create('WILL_RAISE_SYNTAX_ERROR()', 'return IDENTITY(return_value=actual_return_value)', dict(IDENTITY=IDENTITY, _call_=IDENTITY, actual_return_value=10), addsource=True)

Error Message

Error in generated code:
async def WILL_RAISE_SYNTAX_ERROR():
    return await IDENTITY(return await_value=actual_return await_value)

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/opt/conda/lib/python3.9/site-packages/decorator.py", line 196, in create
    return self.make(body, evaldict, addsource, **attrs)
  File "/opt/conda/lib/python3.9/site-packages/decorator.py", line 159, in make
    code = compile(src, filename, 'single')
  File "<decorator-gen-0>", line 2
    return await IDENTITY(return await_value=actual_return await_value)
                          ^
SyntaxError: invalid syntax

Analysis

FunctionMaker.create has the following:

if caller and iscoroutinefunction(caller):
    body = ('async def %(name)s(%(signature)s):\n' + ibody).replace(
        'return', 'return await')

However, this replacement is blind, if a parameter is named like return_value, it will be also replaced as it contains return (like the above example).

Moreover, the following lines can produce similar errors:

• return_value = ... A simple assignment statement
• "This is a long message containing a return word"

Fix

To fix, we need to match return as a keyword rather than as any substring. A straightforward fix looks like the following (not fully tested):

if caller and iscoroutinefunction(caller):
    rawbody = "async def %(name)s(%(signature)s):\n" + ibody
    body = re.sub(r"(^|\s)return(\s)", r"\1return await\2", rawbody)