2to3 mishandles indented imports

[galunguy]

BPO	39331
Nosy	@csabella
Superseder	bpo-38681: 2to3 Conversion Result using BlankLine() can be Syntactically Incorrect
Files	ptypes.py

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2020-01-18.03:15:58.085>
created_at = <Date 2020-01-14.15:47:38.329>
labels = ['expert-2to3', '3.9', 'type-crash']
title = '2to3 mishandles indented imports'
updated_at = <Date 2020-01-18.03:15:58.082>
user = 'https://bugs.python.org/galunguy'

bugs.python.org fields:

activity = <Date 2020-01-18.03:15:58.082>
actor = 'cheryl.sabella'
assignee = 'none'
closed = True
closed_date = <Date 2020-01-18.03:15:58.085>
closer = 'cheryl.sabella'
components = ['2to3 (2.x to 3.x conversion tool)']
creation = <Date 2020-01-14.15:47:38.329>
creator = 'galun.guy'
dependencies = []
files = ['48839']
hgrepos = []
issue_num = 39331
keywords = []
message_count = 2.0
messages = ['359978', '360230']
nosy_count = 2.0
nosy_names = ['cheryl.sabella', 'galun.guy']
pr_nums = []
priority = 'normal'
resolution = 'duplicate'
stage = 'resolved'
status = 'closed'
superseder = '38681'
type = 'crash'
url = 'https://bugs.python.org/issue39331'
versions = ['Python 3.9']

[galunguy]

When encountering an import that should be removed in Python 3 (e.g. "from itertools import izip"), 2to3 changes it a blank line, which may cause a runtime error if that import was indented:

error: module importing failed: expected an indented block (ptypes.py, line 10)
File "temp.py", line 1, in <module>
File "./lldbmacros/xnu.py", line 771, in <module>
from memory import *
File "./lldbmacros/memory.py", line 11, in <module>
import macho
File "./lldbmacros/macho.py", line 3, in <module>
from macholib import MachO as macho
File "./lldbmacros/macholib/MachO.py", line 10, in <module>
from .mach_o import MH_FILETYPE_SHORTNAMES, LC_DYSYMTAB, LC_SYMTAB
File "./lldbmacros/macholib/mach_o.py", line 16, in <module>
from macholib.ptypes import p_uint32, p_uint64, Structure, p_long, pypackable

Relevant section before 2to3:

try:
from itertools import izip, imap
except ImportError:
izip, imap = zip, map
from itertools import chain, starmap

And after 2to3:

try:

except ImportError:
izip, imap = zip, map
from itertools import chain, starmap

• Side note:
  This specific case may only be problematic with scripts that are partially aware of Python 3, otherwise they wouldn't try-catch that import.

• Proposed solution:
  In case of that kind of import being the single line of an indented block, change it to "pass" instead of a blank line.

[csabella]

Thank you for the report. This is a duplicate of bpo-38681.