Bugs item #1453145, was opened at 2006-03-18 14:02
Message generated for change (Comment added) made by loewis
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Category: Python Interpreter Core
Group: Python 2.4
>Status: Closed
>Resolution: Invalid
Priority: 5
Submitted By: Ali Gholami Rudi (aligrudi)
Assigned to: Nobody/Anonymous (nobody)
Summary: Unexpected module reloading
Initial Comment:
I'll demonstrate the problem:
Consider the following package hierarchy:
p/
__all__.py
m1.py
m2.py
The contents of m1 and m2 modules are:
-m1.py
import m2
import p.m2
--
-m2.py
print 'Loading m2 module'
--
Running the m1 module would yield the output
Loading m2 module
Loading m2 module
. As it is obvious from the output the module m2 is
loaded twice. The problem arrises when you want to do
things such as implementing a singleton class:
-Alternate m2.py-
class Singleton(object):
_instance = None
@staticmethod
def getInstance():
if Singleton._instance is None:
Singleton._instance = Singleton()
return _instace
-
-Alternate m1.py-
import m2
import p.m2
print m2.Singleton.getInstance()
print p.m2.Singleton.getInstance()
-
If you run m1 module, the output shows that the two
instaces are not the same objects. That is m2.Singleton
and p.m2.Singleton are not the same classes.
I think this is a bug.
--
>Comment By: Martin v. Löwis (loewis)
Date: 2006-03-19 11:44
Message:
Logged In: YES
user_id=21627
At first, I could reproduce the problem; look at this
transcript to see what I did.
[EMAIL PROTECTED]:~/tmp$ mkdir p
[EMAIL PROTECTED]:~/tmp$ echo >p/__init__.py
[EMAIL PROTECTED]:~/tmp$ cat >p/m1.py
import m2
import p.m2
[EMAIL PROTECTED]:~/tmp$ cat >p/m2.py
print 'Loading m2 module'
[EMAIL PROTECTED]:~/tmp$ python
Python 2.3.5 (#2, Mar 6 2006, 10:12:24)
[GCC 4.0.3 20060304 (prerelease) (Debian 4.0.2-10)] on linux2
Type "help", "copyright", "credits" or "license" for more
information.
py> import p.m1
Loading m2 module
py>
As you can see, the "Loading" output is printed only once.
This might happen if you run p/m1.py as the main program,
but I cannot reproduce it:
[EMAIL PROTECTED]:~/tmp$ python p/m1.py
Loading m2 module
Traceback (most recent call last):
File "p/m1.py", line 2, in ?
import p.m2
ImportError: No module named p.m2
As the current directory is not on sys.path, it won't find
the package p.
Now, if you also change that (e.g. by setting PYTHONPATH), I get
[EMAIL PROTECTED]:~/tmp$ export PYTHONPATH=`pwd`
[EMAIL PROTECTED]:~/tmp$ python p/m1.py
Loading m2 module
Loading m2 module
This is not a bug: Now *both* the current directory, and the
directory ~/tmp/p are on sys.path (print sys.path inside m1
to see what I mean).
When you do "import m2", it searches for a module named m2
on sys.path, and it finds p/m2.py. When you then import
p.m2, it searches for a package p on sys.path, finds the
package, and then imports the module m2. It happens that
both modules have the same source file - yet they are
different modules.
If you change the print statement to
print 'Loading', __name__, 'module'
you get
Loading m2 module
Loading p.m2 module
So in short: this is not a bug. Don't try running a module
in a package as the main program.
--
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