"if else" condition generates "uncompyle6.semantics.pysource.SourceWalkerError: Deparsing stopped due to parse error"

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Assessment

Difficulty
3/5
Estimated time
1-2 days
Newbie friendliness
45/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Stale
Tech stack
python
Domain
reverse-engineering

Research direction

Reproduce the issue by compiling the shown test.py with py_compile and running uncompyle6.decompile_file on the resulting Python 3.7 .pyc. Start by tracing the handling of the nested if/else bytecode around the COME_FROM instruction and SourceWalkerError. Done means this example decompiles without a parse error while retaining the conditional branches.

Written by the indexing model from the issue text.

Description

Control Flow Python 3.7 Volunteer wanted Works on decompyle3

If you compile this test.py into a .pyc (with import py_compile; py_compile.compile("test.py")):

def f():
    L = []
    if len(L) > 1:
        if 1:
            pass
        else:
            pass
    else:
        pass

then decompiling it with uncompyle6 with

with open('test_decompiled.py', 'w') as f:
    uncompyle6.decompile_file(r'__pycache__\test.cpython-37.pyc', f)

generates this error:

uncompyle6.semantics.pysource.SourceWalkerError: Deparsing stopped due to parse error

Here is the full error:

# uncompyle6 version 3.7.1
# Python bytecode 3.7 (3394)
# Decompiled from: Python 3.7.6 (tags/v3.7.6:43364a7ae0, Dec 19 2019, 00:42:30) [MSC v.1916 64 bit (AMD64)]
# Embedded file name: test-class.py
# Compiled at: 2020-06-16 19:00:25
# Size of source mod 2**32: 131 bytes


def f--- This code section failed: ---

 L.   2         0  BUILD_LIST_0          0 
                2  STORE_FAST               'L'

 L.   3         4  LOAD_GLOBAL              len
                6  LOAD_FAST                'L'
                8  CALL_FUNCTION_1       1  '1 positional argument'
               10  LOAD_CONST               1
               12  COMPARE_OP               >
               14  POP_JUMP_IF_FALSE    18  'to 18'

 L.   5        16  JUMP_FORWARD         18  'to 18'
             18_0  COME_FROM            16  '16'
             18_1  COME_FROM            14  '14'

Parse error at or near `COME_FROM' instruction at offset 18_0
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