Import from `past` causes traceback

[gazpachoking]

There seems to be an issue (on some python installations) when importing from the past package. One of our users is reporting this traceback:

from past.builtins import basestring
File "/usr/lib/python2.7/site-packages/past/__init__.py", line 88, in 
from past.translation import install_hooks as autotranslate
File "/usr/lib/python2.7/site-packages/past/translation/__init__.py", line 41, in 
from lib2to3.pgen2.parse import ParseError
ImportError: No module named lib2to3.pgen2.parse

Any ideas what's going on here? Downstream ticket, for referece: Flexget/Flexget#1093

[jayvdb]

the only time i see this type of problem is when an ImportError is caught and ignored, and them the same name is imported again somewhere else afterwards

[stevezau]

Hi @jayvdb can you eloborate on that a little bit more? An importerror relating to python future??

[stevezau]

@jayvdb this error is thrown on NAS type devices as they won't have 2to3 compiled into python2..

Is it possible to maybe check for the existance for lib2to3 via a try catch block and disable the functions that use it if it does not exist?

[msj33]

Yes, can we please reopen this - lib2to3 is clearly broken on NAS/Router devices.

[gazpachoking]

Is it possible to maybe check for the existance for lib2to3 via a try catch block and disable the functions that use it if it does not exist?

This seems like a good idea to me. Can someone comment on what these imports are actually doing? We are getting more reports of users that do not have the lib2to3 in their stdlib.

[programatix]

Hi,

I just received a reply on my question on lib2to3 over at Entware/Entware-ng#268 (comment). I believe the answer actually state that lib2to3 is not built into our python is because it's version 2. In my case, it's version 2.7. lib2to3 is a module which convert python 2 script to python 3 automatically (https://docs.python.org/2.7/library/2to3.html). So, it make sense that our python 2 doesn't include lib2to3.

From what I see, by requiring lib2to3, you guys are removing support for python 2 automatically. Wasn't the purpose of lib2to3 is to support python 2 script in python 3? So, shouldn't a version check be done before importing lib2to3? Something like this perhaps?

if sys.version_info >= (3, 0):
    from lib2to3.pgen2.parse import ParseError

Btw, I'm not a python coder, so the above code is just my assumption.

[edschofield]

Thanks for reporting this, Chase!

@programatix : lib2to3 is a standard package on Python 2 systems.

@gazpachoking, @stevezau: Yes, it would be possible for the past.translation package to provide an alternative autotranslate() method that raises an ImportError on some NAS-type devices that don't seem to install lib2to3. If one of you would like to contribute a pull request, I can aim to incorporate it in the next version of future (v0.16).

[blubberdiblub]

For reference, as pyglet fails to run under Python 3.6 due to this problem, I worked around it using the following change in past/translation/__init__.py (in addition to commenting out the import in line 2 in libfuturize/fixes/__init__.py):

--- past/translation/__init__.py.orig	2017-09-24 08:23:47.646644743 +0200
+++ past/translation/__init__.py	2018-01-01 10:31:36.584576652 +0100
@@ -38,8 +38,32 @@
 import os
 import sys
 import copy
-from lib2to3.pgen2.parse import ParseError
-from lib2to3.refactor import RefactoringTool
+
+try:
+    from lib2to3.pgen2.parse import ParseError
+
+except ImportError:
+
+    class ParseError(SyntaxError):
+        pass
+
+try:
+    from lib2to3.refactor import RefactoringTool
+
+except ImportError:
+
+    class RefactoringTool:
+
+        def __init__(self, *args, **kwargs):
+            self._args = [repr(a) for a in args]
+            self._args += ["{}={!r}".format(k, v) for k, v in kwargs.items()]
+
+        def __repr__(self):
+            return "{}({})".format(self.__class__.__name__, ", ".join(self._args))
+
+        def refactor_string(self, *args, **kwargs):
+            raise NotImplementedError("dummy RefactoringTool")
+
 
 from libfuturize import fixes