Python 2.5 fails where 2.4 works when running external program

[Heikki Toivonen]

We have successfully used a script to run external programs for several
years. Now we upgraded our Python to 2.5, and are hitting a mysterious
error.

The expected output from the sample script (see below) with 2.4 looks
like this:

ret ['5\n']
else
********************
ExternalCommandErrorWithOutputList 1 ['Traceback (most recent call
last):\n', ' File "<string>", line 1, in ?\n', 'ZeroDivisionError:
integer division or modulo by zero\n']
********************
ret ['6\n', '7\n', '8\n']
else
********************

With 2.5 we get:

ret ['5\n']
else
********************
Exception 'int' object is not iterable
Traceback (most recent call last):
File "...test.py", line 43, in <module>
ret = executeCommandReturnOutput(cmd)
File "...test.py", line 6, in __init__
self.args = args[0]
TypeError: 'int' object is not iterable

********************
ret ['6\n', '7\n', '8\n']
else
********************

What is going on? How do we fix this? We'd like to be able to run with
both python 2.4 and 2.5.

And here is the script:

---CLIP---
import os, traceback, sys

class ExternalCommandErrorWithOutputList(Exception):
def __init__(self,args=None):
if args:
self.args = args[0]
self.outputList = args[1]
else:
self.args = args
self.outputList = []


def executeCommandReturnOutput(args):
args_str = ' '.join(args)

if os.name not in ['nt', 'os2']:
import popen2
p = popen2.Popen4(args_str)
p.tochild.close()
outputList = p.fromchild.readlines()
exitCode = p.wait()
if exitCode == 0:
exitCode = None
else:
exitCode >>= 8
else:
i,k = os.popen4(args_str)
i.close()
outputList = k.readlines()
exitCode = k.close()

if exitCode is not None:
raise ExternalCommandErrorWithOutputList, [exitCode, outputList]

return outputList


if __name__ == "__main__":
for cmd in [['python', '-c', '"print 5"'],
['python', '-c', '"1/0"'],
['python', '-c', '"print 6;import
sys;sys.stdout.flush();print >>sys.stderr, 7;print 8"'],
]:
try:
ret = executeCommandReturnOutput(cmd)
print 'ret', ret
except ExternalCommandErrorWithOutputList, e:
print 'ExternalCommandErrorWithOutputList', e, e.outputList
except Exception, e:
print 'Exception', e
type, value, stack = sys.exc_info()
print ''.join(traceback.format_exception(type, value, stack))
except:
print 'except'
type, value, stack = sys.exc_info()
print ''.join(traceback.format_exception(type, value, stack))
else:
print 'else'
print '*' * 20
---CLIP---

 

[Peter Otten]

We have successfully used a script to run external programs for several
years. Now we upgraded our Python to 2.5, and are hitting a mysterious
error.

The expected output from the sample script (see below) with 2.4 looks
like this:

ret ['5\n']
else
********************
ExternalCommandErrorWithOutputList 1 ['Traceback (most recent call
last):\n', ' File "<string>", line 1, in ?\n', 'ZeroDivisionError:
integer division or modulo by zero\n']
********************
ret ['6\n', '7\n', '8\n']
else
********************

With 2.5 we get:

ret ['5\n']
else
********************
Exception 'int' object is not iterable
Traceback (most recent call last):
File "...test.py", line 43, in <module>
ret = executeCommandReturnOutput(cmd)
File "...test.py", line 6, in __init__
self.args = args[0]
TypeError: 'int' object is not iterable

********************
ret ['6\n', '7\n', '8\n']
else
********************

What is going on? How do we fix this? We'd like to be able to run with
both python 2.4 and 2.5.

I think Exception.args always was supposed to be a tuple. Starting with 2.5
Python enforces that constraint:

[Python2.4]1

[Python 2.5]Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: 'int' object is not iterable

The quick fix is of course

e.args = [1]

but also note that according to the tutorial

http://docs.python.org/tut/node10.html#SECTION0010300000000000000000
"""
But use of .args is discouraged. Instead, the preferred use is to pass a
single argument to an exception (which can be a tuple if multiple arguments
are needed) and have it bound to the message attribute.
"""

Peter

 

[Heikki Toivonen]

I think Exception.args always was supposed to be a tuple. Starting with 2.5
Python enforces that constraint: [...]
http://docs.python.org/tut/node10.html#SECTION0010300000000000000000
"""
But use of .args is discouraged. Instead, the preferred use is to pass a
single argument to an exception (which can be a tuple if multiple arguments
are needed) and have it bound to the message attribute.
"""

Excellent, thank you!