[New-bugs-announce] [issue39166] Python 3.9.0a2 changed how "async for" traces its final iteration

[Ned Batchelder]

New submission from Ned Batchelder <ned at nedbatchelder.com>:

3.9.0a2 changed how the final iteration of "async for" is traced.  The body of the loop is traced when the body is not executed.  Standard "for" loops don't show the same effect.

In the output below, notice that 3.9.0a2 and 3.9.0a2+ both show one last execution of line 32, but that line is not actually executed (there is no output).  The standard for loop doesn't show line 27 doing that in any version.

--- 8< ----------------------------------------------------
import linecache, sys

def trace(frame, event, arg):
    # The weird globals here is to avoid a NameError on shutdown...
    if frame.f_code.co_filename == globals().get("__file__"):
        lineno = frame.f_lineno
        print("{} {}: {}".format(event[:4], lineno, linecache.getline(__file__, lineno).rstrip()))
    return trace

import asyncio

class AsyncIteratorWrapper:
    def __init__(self, obj):
        self._it = iter(obj)

    def __aiter__(self):
        return self

    async def __anext__(self):
        try:
            return next(self._it)
        except StopIteration:
            raise StopAsyncIteration

def doit_sync():
    for letter in "ab":
        print(letter)
    print(".")

async def doit_async():
    async for letter in AsyncIteratorWrapper("ab"):
        print(letter)
    print(".")

print(sys.version)
sys.settrace(trace)

doit_sync()

loop = asyncio.new_event_loop()
loop.run_until_complete(doit_async())
loop.close()
--- 8< ----------------------------------------------------



$ /usr/local/pythonz/pythons/CPython-3.9.0a1/bin/python3.9 /tmp/bpo2.py
3.9.0a1 (default, Nov 20 2019, 18:52:14)
[Clang 10.0.0 (clang-1000.10.44.4)]
call 25: def doit_sync():
line 26:     for letter in "ab":
line 27:         print(letter)
a
line 26:     for letter in "ab":
line 27:         print(letter)
b
line 26:     for letter in "ab":
line 28:     print(".")
.
retu 28:     print(".")
call 30: async def doit_async():
line 31:     async for letter in AsyncIteratorWrapper("ab"):
call 13:     def __init__(self, obj):
line 14:         self._it = iter(obj)
retu 14:         self._it = iter(obj)
call 16:     def __aiter__(self):
line 17:         return self
retu 17:         return self
call 19:     async def __anext__(self):
line 20:         try:
line 21:             return next(self._it)
retu 21:             return next(self._it)
exce 31:     async for letter in AsyncIteratorWrapper("ab"):
line 32:         print(letter)
a
line 31:     async for letter in AsyncIteratorWrapper("ab"):
call 19:     async def __anext__(self):
line 20:         try:
line 21:             return next(self._it)
retu 21:             return next(self._it)
exce 31:     async for letter in AsyncIteratorWrapper("ab"):
line 32:         print(letter)
b
line 31:     async for letter in AsyncIteratorWrapper("ab"):
call 19:     async def __anext__(self):
line 20:         try:
line 21:             return next(self._it)
exce 21:             return next(self._it)
line 22:         except StopIteration:
line 23:             raise StopAsyncIteration
exce 23:             raise StopAsyncIteration
retu 23:             raise StopAsyncIteration
exce 31:     async for letter in AsyncIteratorWrapper("ab"):
line 33:     print(".")
.
retu 33:     print(".")

$ /usr/local/pythonz/pythons/CPython-3.9.0a2/bin/python3.9 /tmp/bpo2.py
3.9.0a2 (default, Dec 19 2019, 08:42:29)
[Clang 10.0.0 (clang-1000.10.44.4)]
call 25: def doit_sync():
line 26:     for letter in "ab":
line 27:         print(letter)
a
line 26:     for letter in "ab":
line 27:         print(letter)
b
line 26:     for letter in "ab":
line 28:     print(".")
.
retu 28:     print(".")
call 30: async def doit_async():
line 31:     async for letter in AsyncIteratorWrapper("ab"):
call 13:     def __init__(self, obj):
line 14:         self._it = iter(obj)
retu 14:         self._it = iter(obj)
call 16:     def __aiter__(self):
line 17:         return self
retu 17:         return self
call 19:     async def __anext__(self):
line 20:         try:
line 21:             return next(self._it)
retu 21:             return next(self._it)
exce 31:     async for letter in AsyncIteratorWrapper("ab"):
line 32:         print(letter)
a
line 31:     async for letter in AsyncIteratorWrapper("ab"):
call 19:     async def __anext__(self):
line 20:         try:
line 21:             return next(self._it)
retu 21:             return next(self._it)
exce 31:     async for letter in AsyncIteratorWrapper("ab"):
line 32:         print(letter)
b
line 31:     async for letter in AsyncIteratorWrapper("ab"):
call 19:     async def __anext__(self):
line 20:         try:
line 21:             return next(self._it)
exce 21:             return next(self._it)
line 22:         except StopIteration:
line 23:             raise StopAsyncIteration
exce 23:             raise StopAsyncIteration
retu 23:             raise StopAsyncIteration
exce 31:     async for letter in AsyncIteratorWrapper("ab"):
line 32:         print(letter)
line 33:     print(".")
.
retu 33:     print(".")

$ /usr/local/cpython/bin/python3.9 /tmp/bpo2.py
3.9.0a2+ (heads/master:89aa7f0ede, Dec 30 2019, 07:52:33)
[Clang 10.0.0 (clang-1000.10.44.4)]
call 25: def doit_sync():
line 26:     for letter in "ab":
line 27:         print(letter)
a
line 26:     for letter in "ab":
line 27:         print(letter)
b
line 26:     for letter in "ab":
line 28:     print(".")
.
retu 28:     print(".")
call 30: async def doit_async():
line 31:     async for letter in AsyncIteratorWrapper("ab"):
call 13:     def __init__(self, obj):
line 14:         self._it = iter(obj)
retu 14:         self._it = iter(obj)
call 16:     def __aiter__(self):
line 17:         return self
retu 17:         return self
call 19:     async def __anext__(self):
line 20:         try:
line 21:             return next(self._it)
retu 21:             return next(self._it)
exce 31:     async for letter in AsyncIteratorWrapper("ab"):
line 32:         print(letter)
a
line 31:     async for letter in AsyncIteratorWrapper("ab"):
call 19:     async def __anext__(self):
line 20:         try:
line 21:             return next(self._it)
retu 21:             return next(self._it)
exce 31:     async for letter in AsyncIteratorWrapper("ab"):
line 32:         print(letter)
b
line 31:     async for letter in AsyncIteratorWrapper("ab"):
call 19:     async def __anext__(self):
line 20:         try:
line 21:             return next(self._it)
exce 21:             return next(self._it)
line 22:         except StopIteration:
line 23:             raise StopAsyncIteration
exce 23:             raise StopAsyncIteration
retu 23:             raise StopAsyncIteration
exce 31:     async for letter in AsyncIteratorWrapper("ab"):
line 32:         print(letter)
line 33:     print(".")
.
retu 33:     print(".")

$

----------
keywords: 3.9regression
messages: 359040
nosy: Mark.Shannon, nedbat
priority: normal
severity: normal
status: open
title: Python 3.9.0a2 changed how "async for" traces its final iteration

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue39166>
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